Exercise : Solution of Questions on page Number : 73
Answer 1:
Two properties of magnetic field lines:
1. They originate from the north pole of a magnet and end at the south pole.
2. They do not intersect one another.
Answer 2:
We can trace the magnetic field pattern of a bar magnet in the following two ways:
1. Using iron filings: In this process, iron filings show the shape of magnetic field produced by a bar magnet by aligning with magnetic field lines.
2. Using a compass: With the movement of compass from one position to other, magnetic field lines leave the north pole of the magnet and enter the south pole.
Answer 3:
Magnetic field of a magnet is strongest at the place where magnetic field lines are close to one another.
Answer 5:
The needle of a compass is a magnet. When the compass is placed in a magnetic field, a magnetic force acts on the needle and it gets deflected from its usual north–south position.
Answer 6:
Magnetic strips are used in refrigerator doors.
In the absence of magnet strip, the door will not close completely. This will allow warm air from outside to enter into the refrigerator. Further, the cool air from the refrigerator will also escape.
Answer 7:
(a) Magnetic field lines leave the north pole of a bar magnet and enter at its south pole.
(b) The earth’s magnetic field is rather like that of a bar magnet with its south pole in the northern hemisphere.
Answer 8:
Magnetic field lines around a bar magnet:
Answer 9:
Magnetic field is defined as the space surrounding a magnet in which magnetic force is exerted.
The direction of magnetic field lines at a place can be determined by finding the direction of magnetic force on the north pole of the magnet.
Answer 10:
If two magnetic field lines intersect each other, the resultant force on the north pole, placed at the point of intersection, will be along two directions. This is not possible. Therefore, two magnetic field lines cannot intersect each other.
Answer 11:
When an electric current is passed through a wire, it is true that a magnetic field is produced around it. However, such magnetic fields are very weak.
The magnetic field produced by an electric iron connecting cable is very weak and is not enough to attract nearby objects. Moreover, metallic wires of cables are shielded with a cover that prevents magnetic field to show its effect outside the cable.
Answer 12:
(a) Magnetic field lines:
Magnetic field is the space around a magnet wherein magnetic force can be felt.
The magnetic field lines of a bar magnet can be mapped using a magnetic compass. The compass can be moved from one pole of the bar magnet to another pole. We can trace the field lines by moving the compass gradually and drawing a corresponding line which the compass would trace while following a particular line of force.
In this activity, we will get a magnetic field pattern around the magnet. This also allows us to see the magnetic field lines leaving the north pole of the magnet and enter its south pole.
(b) A freely suspended magnet always points towards geographical north and geographical south directions because of the magnetism of the earth. It is assumed that a huge bar magnet is placed at the centre of the earth that causes a freely suspended magnet to align in the directions of north and south poles.
Answer 13:
(d) in all planes around the magnet
If a strong bar magnet is placed vertically above a horizontal wooden board, the magnetic lines of force will be along all planes around the magnet.
Answer 14:
(c) originate from the north pole and end at its south pole
Magnetic field lines produced by a bar magnet originate from the north pole and end at its south pole.
Answer 15:
(c) brass
Brass is not a magnetic material.
Answer 16:
(c) do not cross one another
The magnetic field lines do not cross one another because the resultant force at any point on the north pole can only be in one direction. This is impossible if the lines intersect.
Answer 17:
(a) geographical south
The north pole of earth’s magnet is in the geographical south because it attracts the south pole of a freely suspended magnet.
Answer 18:
(b) 15⁰
The axis of earth’s magnetic field is inclined with the geographical axis at an angle of about 15⁰.
Exercise : Solution of Questions on page Number : 74
Answer 19:
(d) bar magnet
The shape of the earth’s magnetic field resembles that of an imaginary bar magnet.
Answer 20:
(d) iron and steel
Magnet only attracts magnetic materials. Iron and steel are magnetic materials.
Answer 21:
(c) point towards the south pole
If a plotting compass is placed near the south pole of a bar magnet, this pole attracts the north pole of the compass needle. This causes the tip of the compass to point towards the south pole.
Answer 23:
(c) If magnetic field lines are parallel and equidistant, they represent zero field strength.
If magnetic field lines are parallel and equidistant, they represent a uniform magnetic field.
Answer 24:
The magnetic field lines leave the north pole of a magnet and enter its south pole.
Answer 25:
(a)
(b) X is the north pole.
Answer 26:
1st diagram: Magnetic field lines are beginning from A and B. The lines are repulsive also. So, A and B are north poles.
2nd diagram: Magnetic field lines are ending on C and D. The lines are repulsive also. So, C and D are south poles.
3rd diagram: Magnetic field lines are beginning from E and ending on F. So, E and F are north and south poles, respectively.
Answer 27:
(i) Magnet 1 S-N and Magnet 2 N-S
(ii) Magnet 2 is weaker because its lines are bending more. In other words, its lines are experiencing more force exerted by Magnet 1.
Exercise : Solution of Questions on page Number : 81
Answer 1:
The magnetic effect of current can be utilised in detecting a current carrying wire concealed in a wall.
Answer 2:
The deflection of compass needle by a current-carrying wire shows that the wire produces a magnetic field around it.
Exercise : Solution of Questions on page Number : 82
Answer 3:
The magnetic effect of current was discovered by Oersted in 1820.
Answer 4:
If an iron core is inserted into a current carrying solenoid, it acts as a magnet. If we switch off the current in the solenoid, magnetism of the soft iron core disappears.
Answer 5:
The direction of magnetic field produced by a straight current-carrying conductor can be obtained by using Maxwell’s right-hand thumb rule.
Answer 6:
Magnetic field lines around a straight current-carrying conductor are in the form of concentric circles with centre at the axis of the conductor.
Answer 7:
The other name of Maxwell’s right-hand thumb rule is Maxwell’s corkscrew rule.
Answer 8:
True.
The magnetic field inside a long circular coil carrying current will be in the form of parallel straight lines.
Answer 9:
The magnetic field produced by a current-carrying solenoid is similar to the magnetic field produced by a bar magnet. Therefore, the shape of the given current-carrying conductor is solenoid.
Answer 10:
Strength of an electromagnet can be increased in the following three ways:
1. By increasing the number of turns in the coil of the electromagnet
2. By increasing the flow of current in the coil of the electromagnet
3. By reducing the length of air gap between the poles of the electromagnet
Answer 11:
(a) The lines of magnetic field round a straight current-carrying conductor are in the shape of concentric circles.
(b) For a current-carrying solenoid, the magnetic field is like that of a bar magnet.
(c) The magnetic effect of a coil can be increased by increasing the number of turns increasing the current or inserting an iron core.
(d) If a coil is viewed from one end and the current flows in an anticlockwise direction, then this end is a north pole.
(e) If a coil is viewed from one end, and the current flows in a clockwise direction, then this end is a south pole.
Answer 12:
We know that a current-carrying wire produces magnetic field. Therefore, we can locate a current-carrying wire concealed in a wall by detecting the magnetic field produced by the wire. It is done with the help of a compass.
Answer 13:
We take a thick insulated wire and fix it in north-south direction. Now, we place a plotting compass under the wire. The two ends of the wire are connected to a battery through a switch.
Initially, let us fix the compass needle parallel to the wire, i.e., north-south direction, if no current is flowing through the wire. If we pass a current through the wire by pressing the switch, we observe that the compass needle gets deflected from its north-south position. Again, if current is switched off, the compass needle returns to its original position.
In this experiment, the deflection of compass needle by a current-carrying wire shows that an electric current produces a magnetic field around it. In other words, a magnetic field is associated with an electric current.
Answer 14:
(a) The magnetic field lines due to a current-carrying straight conductor are concentric circles whose centres lie on the wire.
(b) Maxwell’s right-hand thumb rule is used to determine the direction of magnetic field around a straight current-carrying conductor.
According to this rule, if we grasp a current-carrying wire in our right hand so that our thumb points in the direction of the current, the direction in which our fingers encircle the wire will give the direction of magnetic field lines around the wire.
When thumb points upwards, the curled fingers are anticlockwise. So, the direction of magnetic field lines is anticlockwise.
When thumb points downwards, the curled fingers are clockwise. So, the direction of magnetic field lines is clockwise.
Answer 15:
Maxwell’s right-hand thumb rule indicates the direction of magnetic field if the direction of current is known.
According to this rule, if we grasp the current-carrying wire in our right hand so that our thumb points in the direction of current, the direction in which our fingers encircle the wire will give the direction of magnetic field lines around the wire.
When thumb points upwards, the curled fingers are anticlockwise. So, the direction of magnetic field is anticlockwise.
When thumb points downwards, the curled fingers are clockwise. So, the direction of magnetic field is clockwise.
Answer 16:
Maxwell’s corkscrew rule:
If we imagine driving a corkscrew in the direction of a current, the direction in which we turn its handle is the direction of magnetic field.
This rule is used to find the direction of magnetic field if the direction of current is known.
When electric current flows vertically upwards, the direction of magnetic field produced is anticlockwise. Similarly, when electric current flows vertically downwards, the direction of magnetic field is clockwise.
Answer 17:
(a)
When current passes through a circular wire, magnetic field is produced around it. The magnetic field lines are circular near the wire. At the centre, these lines are straight.
(b) Ways to increase the strength of magnetic field produced by a current-carrying circular coil:
1. By increasing the current passing through the coil
2. By decreasing the radius of the coil
3. By increasing the number of turns of wire in the coil
Answer 18:
Clock face rule:
This rule is used to determine the polarity of two faces of a current-carrying circular coil.
According to clock face rule, if we look at one face of the coil through which a current is passing, we have following conclusions:
1. If the current around the face of the coil flows in clockwise direction, that face of the coil will be south pole.
2. If the current around the face of the coil flows in anticlockwise direction, that face of the coil will be north pole.
Answer 19:
The strength of magnetic field produced by a current-carrying solenoid depends on:
(i) The number of turns in the solenoid:
If the number of turns in the solenoid is large, they will produce greater magnetism.
(ii) The nature of core material used in making the solenoid:
If a soft iron rod is used as core in the solenoid, it produces strongest magnetism.
Answer 20:
(a) A soft iron piece can be transformed into an electromagnet by winding an insulated wire around it and passing current through the wire.
(b)
In this process of separation, a large electromagnet is attached to a crane and moved over the scrap yard. It picks up the iron objects from the mixture and carries them to other location. Further, iron objects is dropped by switching off the current of the electromagnet coil. In this way, we will be left with copper objects in the scrap yard.
Answer 21:
(a) Differences between an electromagnet and a permanent magnet:
| Electromagnet | Permanent magnet | ||
| 1. | It is a temporary magnet. | 1. | As the name suggests, it is a permanent magnet |
| 2. | It can produce very strong magnetic force. | 2. | It produces a comparatively weak magnetic field. |
| 3. | Its polarity can be changed by changing the direction of current in the coil | 3. | Its polarity is fixed and cannot be changed |
| 4. | Its strength can be changed by changing the number of turns or by changing the current. | 4. | Its strength cannot be changed.(b) Uses of electromagnet |
(b) Uses of electromagnet:
1. In MRI scanners
2. In magnetic levitation train
Uses of permanent magnet:
1. In microphones
2. In electric clocks
Answer 22:
(a) Solenoid:
A solenoid is a long coil that contains large number of close turns of insulated copper wire.
The magnetic field pattern produced by a current-carrying solenoid is similar to the magnetic field produced by a bar magnet.
(b) The magnetic field pattern of a current-carrying solenoid resembles with that of a bar magnet.
(c) The magnetic field lines inside a solenoid are in the form of parallel straight lines. This pattern of field lines indicates that the strength of magnetic field is same at all points inside the solenoid.
(d) Ways to increase the magnetic field strength of a current-carrying solenoid:
1. By increasing the number of turns in the solenoid
2. By increasing the flow of current passing through the solenoid
3. By using a soft iron rod as core in the solenoid
(e) A soft iron rod should be put inside a current-carrying solenoid to make an electromagnet.
Answer 23:
(a) Electromagnet:
It is a magnet that works on the principle of magnetic effect of current.
A temporary magnet consisting of a long coil of insulated copper wire wrapped around a soft iron core is called an electromagnet.
Construction and working:
We take a rod NS of soft iron and wind coil C of insulated copper wire around it. When we connect the two ends of the copper coil to a battery, an electromagnet is formed. The iron rod inside the coil becomes a strong electromagnet on passing a current. The magnetic field produced by an electromagnet is very strong.
(b) An electromagnet is called a temporary magnet because its magnetism stays only for the duration of current passing through it. All the magnetism disappears as soon as the current is switched off.
(c) The core of an electromagnet must be of soft iron because soft iron loses all its magnetism when current in the coil is switched off.
But if steel is used, it does not lose all its magnetism when current is switched off. Contrarily, it becomes a permanent magnet.
Therefore, steel cannot be used for making the core of electromagnets.
(d) Strength of an electromagnet depends on following factors:
1. Number of turns in the coil:
By increasing the number of turns in the coil, the strength of electromagnet can be increased.
2. Current flowing through the coil:
By increasing the flow of current in the coil, the strength of electromagnet can be increased.
3. Length of air gaps between poles:
By reducing the length of air gaps between the poles, the strength of electromagnet can be increased.
(e) Uses of electromagnet:
1. Electromagnets are used by doctors in MRI scanners.
2. It is used for treating knee pain by using pulsing electromagnetic field (PEMF).
3. It is used in magnetic levitation trains.
Exercise : Solution of Questions on page Number : 83
Answer 24:
(b) direction of current in electromagnet winding were reversed
In other options, the strength of magnetic field can change because the strength of magnetic field produced by a solenoid depends on number of turns, current and material of the core of the electromagnet.
Answer 25:
(b) a circular coil
The diagram represents a magnetic field caused by a current-carrying conductor which is a circular coil.
Answer 26:
(d) is the same at all points
The magnetic field inside a long straight solenoid carrying current is same at all points.
Answer 27:
(d) The field consists of concentric circles centred on the wire.
The magnetic field near a long straight wire consists of concentric circles centred on the wire.
Answer 28:
(c) Clock face rule.
The north–south polarities of an electromagnet can be found easily by using clock face rule.
Answer 29:
(c) south pole
If the direction of current in the coil at one end of an electromagnet is clockwise, this end of the electromagnet will be south pole.
Answer 30:
(c) north pole
If the direction of electric current in a solenoid when viewed from a particular end is anticlockwise, this end of the solenoid will be north pole.
Answer 31:
(a) soft iron
The most suitable material for making the core of an electromagnet is soft iron because its use as a core in a solenoid produces strongest magnetism.
Answer 32:
(c) Oersted
Oersted first discovered that current-carrying wire produces magnetic field around it.
Answer 33:
(b) will increase
If a soft iron bar is inserted inside a current-carrying solenoid, the magnetic field inside the solenoid will increase.
Answer 34:
(c) parallel to the axis of the tube
The magnetic field lines in the middle of a current-carrying solenoid are parallel to the axis of the tube because magnetic field is uniform inside a solenoid.
Answer 35:
(c) anticlockwiseAnswer 35:
Because anticlockwise flow of current induces north polarity.
Exercise : Solution of Questions on page Number : 84
Answer 36:
(b) clockwise
Because clockwise flow of current in a circular loop behaves like south pole.
Answer 37:
We will use right-hand thumb rule to get the answer.
(a) In the straight wire A, current is flowing in vertically downwards direction; therefore, the direction of magnetic field around it will be clockwise.
(b) In the straight wire B, current is flowing in vertically upwards direction; therefore, the direction of magnetic field around it will be anticlockwise.
Answer 38:
End A of the solenoid will be S pole because the direction of flow of current at this end is clockwise.
Answer 39:
It is given that the current-carrying straight wire is held in vertical position. If current passes through this wire in vertically upward direction, magnetic field produced by it will be in anticlockwise direction. Right hand thumb rule will be used to find the direction of magnetic field.
Answer 40:
When the switch is pressed,
(a) the current will flow in the clockwise direction at end A; therefore, this end will behave like the south pole.
(b) The other end of the coil will behave like the north pole, and because of the repulsive force at the north pole, the compass (or point of the compass) will move away from the coil.
Answer 41:
If current flows downwards in a wire that passes vertically through a table top, the magnetic field lines around it will go clockwise when viewed from above the table. This is concluded by applying right-hand thumb rule.
Answer 42:
(a) The polarity of end X is south because current is flowing in clockwise direction at this end.
(b) The polarity of end Y is north because current is flowing in anticlockwise direction at this end.
(c) We have used clock face rule to determine the polarities.
Answer 43:
It is given that the magnetic field associated with a current-carrying straight conductor is in anticlockwise direction. Further, the conductor was held along east–west direction; therefore, according to right hand thumb rule, the direction of current through it will be from east to west.
Answer 44:
(a) from top towards bottom
It is given that the current-carrying conductor is held in exactly vertical direction. In order to produce a clockwise magnetic field around the conductor, the current should be passed in the conductor from top towards bottom. It is concluded by applying right-hand thumb rule.
Answer 45:
It is given that a thick wire is hanging form a wooden table. An anticlockwise magnetic field is needed to be produced around the wire by passing current through it using a battery. Then, according to right hand thumb rule, current should pass through the wire from bottom end to top end. To execute this flow, positive terminal of the battery should be connected to the bottom end and negative terminal of battery should be connected to the top end because current flows from positive terminal to negative terminal.
Exercise : Solution of Questions on page Number : 91
Answer 1:
When a current-carrying conductor is placed in a magnetic field, the conductor experiences a force that makes it move.
Answer 2:
The force experienced by a current-carrying conductor placed in a magnetic field is largest when it is perpendicular to the direction of magnetic field.
Answer 3:
In a statement of Fleming’s left-hand rule:
(a) Direction of centre finger represents current.
(b) Direction of forefinger represents magnetic field.
(c) Direction of thumb represents force.
Answer 4:
Electric motor works on the magnetic effect of electric current.
Answer 5:
Electric motor converts electrical energy into mechanical energy.
Answer 6:
A motor converts electrical energy into mechanical energy.
Answer 7:
False.
An electric motor converts electrical energy into mechanical energy.
Answer 8:
In Fleming’s left-hand rule, when thumb, forefinger and central finger are kept mutually perpendicular to each other:
(a) Thumb represents force.
(b) Forefinger represents magnetic field.
(c) Central finger represents current.
Answer 9:
The function of commutator in an electric motor is to reverse the direction of current in the coil of the motor.
Answer 10:
The split ring used in an electric motor is also called commutator.
Answer 11:
The function of a commutator in an electric motor is to reverse the direction of current in the coil of the motor.
Answer 12:
The brushes of an electric motor are made of carbon.
Answer 13:
The core of the coil of an electric motor is made of soft iron.
Answer 14:
The carbon brushes remain fixed to the base of the motor, whereas the commutator (split rings) rotates.
Answer 15:
The role of split ring in an electric motor is to reverse the direction of current in the coil of the motor.
Answer 16:
Fill in the following blanks with suitable words:
(a) Fleming’s Rule for the motor effect uses the left hand.
(b) A motor contains a kind of switch called a commutator which reverses the current every half rotation.
Answer 17:
(a) When a current-carrying conductor is placed perpendicular to a magnetic field, Fleming’s left-hand rule can be used to find the direction of force acting on the conductor.
(b) The force on a current-carrying conductor in a magnetic field can be increased by increasing the flow of current in the conductor and also by increasing externally applied magnetic field.
(c) Electric motor is a device whose working depends on the force exerted on a current-carrying coil placed in a magnetic field.
Exercise : Solution of Questions on page Number : 92
Answer 18:
According to Fleming’s left-hand rule, when thumb, forefinger and central finger are kept mutually perpendicular:
(a) Thumb represents force.
(b) Forefinger represents magnetic field.
(c) Central finger represents current.
Answer 19:
According to the principle of an electric motor, when a rectangular current carrying conductor is placed in a magnetic field, a couple of forces act on it and try to rotate the conductor in the magnetic field. Electric motors are used in fans, washing machines, grinders, etc.
Answer 20:
(a) In a D.C. electric motor, the reversing of current in the coil is repeated after every half rotation. This causes the coil to continue its rotation as long as the current from the battery is passed through it. The rotating coil can drive a machine connected to it.
(b) A commutator (split rings) reverses the current.
Answer 21:
(a)
(i) The direction of rotation of the motor will get reversed on reversing the direction of flow of current in the coil.
(ii) The direction of rotation of the motor will get reversed on reversing the direction of magnetic field.
(iii) The direction of rotation of the motor will not change if both current and magnetic field are reversed simultaneously.
(b) We can make a motor more powerful by following ways:
(i) If the coil is wound on a soft iron core, the strength of magnetic field increases.
(ii) By increasing the number of turns of insulated copper wire of the coil
(iii) By using a powerful electromagnet in place of permanent magnet
Answer 22:
(a) An electric motor is a device that converts electrical energy into mechanical energy.
Diagram: Electric motor
Working of an electric motor:
An electric motor works on the principle of magnetic effect of electric current. In an electric motor, a rectangular coil ABCD in placed between two magnets in poles N and S. Now, current is passed through it continuously. When current is passed into the coil, it produces a magnetic field around it. The two magnetic fields interact and cause the coil to rotate. When the coil rotates, the shaft attached to it also rotates. In this way, electrical energy supplied to the motor is converted into mechanical energy.
(b) Special features of commercial electric motors are:
(i) The coil is wound on a soft iron core; therefore, the strength of magnetic field increases.
(ii) The coil contains large number of turns of insulated copper wire.
(iii) A powerful electromagnet is used in place of permanent magnet.
Answer 23:
(c) half rotation
In an electric motor, the direction of current in the coil changes once in each half rotation.
Answer 24:
(c) into the page
Using Fleming’s left-hand rule, we find that the direction of force acting on the electron beam will be into the page.
Answer 25:
(c) 90°
The force experienced by a current-carrying conductor placed in a magnetic field is largest when the angle between the conductor and the magnetic field is 90°.
Answer 26:
(d) 180°
The force exerted on a current-carrying wire placed in a magnetic field is zero when a current-carrying conductor is parallel to the field.
Answer 27:
(c) vertically downwards
Using Fleming’s left-hand rule, we find that the force on wire should act vertically downwards.
Answer 28:
(d) electrical energy to mechanical energy
Electric motor converts electric energy into mechanical energy.
Answer 29:
(c) a stationary electric charge
Magnetic force is given by Lorentz equation as:
F = q (v × B)
Here, q = amount of electric charge, v = velocity of charge and B = magnetic flux
Thus, when velocity of charge (v) = 0, magnetic force (F) = 0
Exercise : Solution of Questions on page Number : 93
Answer 30:
(d) vertically upwards
Applying Fleming’s left-hand rule, we find that the force on the wire will act vertically upward.
Answer 31:
The current is flowing in clockwise direction around the coil when viewed from above.
Using Fleming’s left-hand rule, the direction of current can be found.
Answer 32:
The wire in the diagram will move outwards (out of the page).
Answer 33:
By applying Fleming’s left-hand rule, we find that the force will act in south direction.
Answer 34:
By applying Fleming’s left-hand rule, we find that the wire tends to move downwards (into the paper).
Answer 35:
The force on a current-carrying wire parallel to the magnetic field is zero because the magnetic force on the wire is due to the force experienced by moving electrons in the conductor. Further, the magnetic force has rotatory effect on electron motion only. Therefore, in parallel condition, this rotating effect is zero and this results in zero force on the conductor.
Answer 36:
The charge is positive in nature. On applying Fleming’s left-hand rule, we find that only positive charge will experience a force pointing vertically outwards due to its interaction with the magnetic field.
Exercise : Solution of Questions on page Number : 102
Answer 1:
An electric generator converts mechanical energy into electric energy.
Answer 2:
(a) A D.C. generator uses a commutator (split rings).
(b) An A.C. generator uses slip rings.
Answer 3:
Electric generator works on the principle that when a straight conductor is moved in a magnetic field, current is induced in the conductor. This phenomenon is called electromagnetic induction.
Answer 4:
Fleming’s right-hand rule gives the direction of induced current.
Answer 5:
The condition necessary for the production of current by electromagnetic induction is that there must be a relative motion between the coil of a wire and magnet.
Exercise : Solution of Questions on page Number : 103
Answer 6:
A.C. generators are used at power stations.
Answer 7:
By replacing the slip rings of an A.C. generator by a commutator, we can convert it into a D.C. generator.
Answer 8:
(a) True. Generator works on the principle of electromagnetic induction.
(b) False. A motor works on the principle that a force is experienced by a current conductor when placed in a magnetic field.
Answer 9:
Brushes maintain the contact between slip rings (commutator) in an A.C. generator (D.C. generator). So, the current produced in the rotating coil can be tapped out through slip rings (commutator) into the brushes.
Answer 10:
The phenomenon is known as electromagnetic induction.
Answer 11:
This phenomenon is known as electromagnetic induction.
Answer 12:
| Simple alternator | Most practical alternator |
| 1. In a simple alternator, small voltage is produced. 2. It is used in cars and bicycles as dynamos. |
1. In a most practical alternator, large voltage is produced 2. It is used in power houses. |
Answer 13:
Thermal power stations are usually located near a river because they require huge and continuous supply of water which is converted into steam by burning coal to turn turbines.
Answer 14:
Three sources of magnetic fields are permanent magnets, electromagnets and current-carrying conductors.
Answer 15:
A generator with commutator produces D.C. current.
Answer 16:
Yes, some current will be induced in coil B because with the change in current in coil A, the magnetic field produced by it will change. This change in the value of magnetic field will induce current in coil B.
Answer 17:
(a) An electric generator works on the principle of electromagnetic induction. It states that when a conductor is moved in a magnetic field, current is induced in the conductor.
(b) Two ways in which the current induced in the coil of a generator could be increased:
- By using a coil with more turns
- By using a coil with larger area
Answer 18:
| Alternating current | Direct current |
| 1. In an alternating current, the direction of current changes after equal intervals of time. | 1. In direct current, the direction of current remains the same. |
(a) The major difference between alternating current and direct current is that in an alternating current, the direction of current changes after equal intervals of time, whereas in direct current, the direction of current remains the same.
(b) Device Type of current given
- Dry cell D.C. current
- A power house generator A.C. current
Answer 19:
Fleming’s right-hand rule states that when thumb, forefinger and centre finger of right hand are held mutually perpendicular to one another, thumb, forefinger and centre finger represent directions of motion, field and current induced in the conductor, respectively.
Answer 20:
(a) Fleming’s right-hand rule will be used to find the direction of induced current here. According to this rule, when right hand’s thumb, forefinger and centre finger are held mutually perpendicular to one another, the thumb represents the direction of motion of the coil, the forefinger represents the direction of the magnetic field and the centre finger represents the direction of the induced current. (Fig. 1)
Fig 1
(b) Fleming’s left-rule will be used here to find the direction of force experienced by the conductor. According to this rule, when left hand’s thumb, forefinger and centre finger are held mutually perpendicular to one another, the forefinger represents the direction of the magnetic field, the centre finger represents the direction of the current and the thumb represents the direction of the force experienced by the coil. (Fig. 2)
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Fig 2
Answer 21:
| A.C. generator | D.C. generator |
| In an A.C. generator, two ends of the coil are linked to two full rings of copper called slip rings . | In a D.C. generator, two ends of the coil are linked to a commutator consisting of two half rings of copper. |
(b) The high pressure of steam normally drives the alternators in a thermal power station. Coal, natural gas or oil can be used to heat water in the boiler to form steam of high pressure.
Answer 22:
An electric generator (see figure) consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet. The two ends of this coil are connected to two rings R1 and R2 . The inner side of these rings are insulated. Two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively. The rings R1 and R2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field. Outer ends of the two brushes are connected to a galvanometer to show the flow of current in the given external circuit.
When the axle is attached to the two rings, the axle is rotated so that arm AB moves up (and arm CD moves down) in the magnetic field produced by the permanent magnet. Now, coil ABCD is rotated clockwise in an arrangement as shown in the figure. By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions of AB and CD. Thus, an induced current flows in the direction ABCD. If there are larger numbers of turns in the coil, the current generated in each turn adds up to give a large current through the coil. This means that the current in the external circuit flows from B2 to B1.
After half rotation, arm CD moves up and arm AB moves down. As a result, the directions of induced currents in both the arms change, giving rise to the net induced current in the direction DCBA. The current in the external circuit now flows from B1 to B2. Thus, after every half rotation, the polarity of currents in respective arms changes. Such a current that changes direction after equal intervals of time is called alternating current (A.C.). This device is called an A.C. generator.
Answer 23:
(a) In case of relative motion between a source of magnetic field and conductor, current flows inside the conductor. This phenomenon is called electromagnetic induction. For example, when a bar magnet is moved towards a coil of wire attached to a galvanometer, a deflection in galvanometer needle is observed due to induced current as shown in the figure.
(b) An electric generator works on the principle of electromagnetic induction.
(c) Different ways of inducing current in a wire:
When a magnet is moved towards/away from the coil of wire connected to a galvanometer, a deflection in the galvanometer is observed due to induced current, change in magnetic field and relative motion between magnet and coil. (fig. 1)
Take a coil of wire and connect it to a power supply. Now, place another coil connected to a galvanometer near the first coil and switch on the power supply connected to the first coil. A deflection in the galvanometer is observed due to induced current and change in magnetic field generated by the first coil. (fig 2)
Answer 24:
(a) If current flows in one direction only, it is called direct current, e.g., current generated by battery.
If current reverses its direction after fixed intervals of time, it is called alternating current, e.g., current supplied at homes.
(b) Some sources of direct current are dry cells, battery, car battery and D.C. generators.
Some sources of alternating current are A.C. generators, power stations.
(c) An important advantage of alternating current over direct current is that alternating current can be transmitted to long distances without much loss of electric energy.
(d) The frequency of A.C. supply in India is 50 Hz.
Answer 25:
(c) half revolution
If a rectangular coil of copper wire is rotated in a magnetic field, the direction of magnetic field to the given side of coil changes after half revolution and the direction of induced current also changes simultaneously.
Answer 26:
(c) producing induced current in a coil due to relative motion between a magnet and the coil
Exercise : Solution of Questions on page Number : 104
Answer 27:
(a) generator
Generator is used to produce both A.C. and D.C. currents.
Answer 28:
(d) A.C. generator has slip rings while D.C. generator has a commutator.
A.C. generator has slip rings, whereas D.C. generator has a commutator.
Answer 29:
(a) return to zero
After some time, the magnetic field gets constant; this means that there is no relative change in the value of magnetic field. Therefore, the circuit will have zero induced current.
Answer 30:
(c) increasing the size of the gap in which the armature turns
The increase in the size of the gap, in which armature turns, has no effect on voltage.
Answer 31:
(d) moving out of the solenoid
Because the relative change in the value of magnetic field is maximum in this condition.
Answer 32:
(d) mechanical energy into electrical energy
An electric generator converts mechanical energy into electric energy.
Answer 33:
(b) electromagnetic induction
A D.C. generator is based on the principle of electromagnetic induction.
Answer 34:
(c) the resistivity of the wire of the coil
Current induced by the process of electromagnetic induction does not depend on the resistivity of wire of the coil used.
Answer 35:
(a) 0 Hz
Because current flows in one direction only.
Answer 36:
(b) 50 Hz
In India, an A.C. source has a frequency of 50 Hz.
Exercise : Solution of Questions on page Number : 105
Answer 37:
(a) When the N-pole of a magnet is removed, deflection will take place in left direction because the induced current will flow in other direction.
(b) When the S-pole of a magnet is inserted, the deflection of galvanometer needle will take place in left direction.
(c) When the magnet is at rest in the coil, no deflection of galvanometer needle will take place.
It is because there is no relative motion between the coil and the magnet. Thus, the current induced is zero.
Answer 38:
(i) The effect is called electromagnetic induction. It causes production of an induced current in the coil on the movement of the magnet.
(ii) The reading shown on the galvanometer will be on the left side when the magnet is moving away from the coil.
(iii) Two changes noticed in the reading on the galvanometer:
The deflection of galvanometer will be more to the right side.
The deflection will occur more quickly.
Answer 39:
A relative motion between the coil of the wire and the magnet is needed to induce current.
Answer 40:
(a) If the wire is moved at a higher speed, the flow of current will increase.
(b) If the wire is moved upwards rather than downwards, the flow of current will reverse.
(c) If we use a stronger magnet, the flow of current will increase.
(d) If the wire is held still in the magnetic field, the flow of current will be zero.
(e) If the wire is moved parallel to magnetic field lines, the flow of current will be zero.
Answer 41:
(i) If current is passed through coil B by plugging key, the galvanometer pointer will move to one side. The movement of galvanometer pointer shows that a current is induced in the coil.
(ii) In this case, galvanometer pointer moves to other side. This shows that the direction of induced current is reversed and then quickly returned to zero.
This phenomenon is based on the principle of electromagnetic induction.
Answer 42:
This is a step-down transformer that reduces voltage.
Exercise : Solution of Questions on page Number : 113
Answer 1:
A safety device called electric fuse automatically cuts off the electricity supply during short-circuiting in household wiring.
Answer 2:
(i) In a lighting circuit, the usual capacity of electric fuse is 5 A.
(ii) In a power circuit, the usual capacity of electric fuse is 15 A.
Answer 3:
In circuit diagrams, an electric fuse is represented by the following symbol:
Answer 4:
(a) False
A wire with green insulation is usually the earth wire.
(b) False
A miniature circuit breaker (MCB) works on the magnetic effect of current.
Answer 5:
Along with live wire and neutral wire, a third wire is also used in domestic electric wiring. This third wire is called earth wire.
Exercise : Solution of Questions on page Number : 114
Answer 6:
The colours of three wires in a cable connected to the plug of an electric iron are:
(a) Live wire – red
(b) Neutral wire – black
(c) Earth wire – green
Answer 7:
The electric potential of neutral wire in a mains supply cable is zero volt.
Answer 8:
We will first calculate the current.
Here,
P = 180 W
V = 240 V
Now, power, P = V × I
or, I = P/V = 180/240 = 0.75 A
So, a fuse of 1 A will be most suitable.
Answer 9:
If the plastic insulation of live and neutral wires gets torn, the two wires will touch each other. Due to this direct touching of live and neutral wires, an electric short circuit occurs.
Answer 10:
Switches are introduced in the live wire to switch off an electrical appliance. Switches connect an electrical appliance with the live wire. If it is switched off, there will be no danger of an electric shock.
Answer 11:
In household circuits, a fuse wire is connected in series to prevent any short circuit or to discontinue the flow of current.
Answer 12:
The three colours are red, black and green.
The red coloured wire is live wire, black coloured wire is neutral wire and green coloured wire is earth wire.
Answer 13:
(a) Live wire – red colour
(b) Neutral wire – black colour
(c) Earth wire – green colour
Answer 14:
Earthing means to connect the metal case of an electrical appliance to the earth (at zero potential) by means of a metal wire called earth wire. It is used as a safety measure. The main purpose of earthing an electrical appliance is to avoid the risk of an electric shock.
Answer 15:
Different electrical appliances in a domestic circuit are connected in parallel due to following reasons:
1. In a parallel connection, if one of the appliances is switched off or gets fused, there is no effect on the other appliances and they keep on running.
2. In parallel connection, the same voltage of the mains line is available for all the electrical appliances.
Answer 16:
The electric lamps in a building should be connected in parallel so that the switching on or off in a room has no effect on other lamps in the same building.
Answer 17:
(a) A fuse should always be placed in the live wire of a mains circuit.
(b) The earth wire should be connected to the metal case of an appliance.
Answer 18:
(a) A fuse wire is made of tin-plated copper because it has a low melting point. If the current exceeds a safe value, the wire melts and breaks the circuit.
(b) A pure copper wire cannot be used as a fuse wire because it has a high melting point. Due to this, it will not melt easily in case of a short circuit.
Answer 19:
Cartridge fuse is used in electric appliances like car stereos for their safety.
It consists of a glass tube and a thin fuse wire is sealed inside it. There are two metal caps at the two ends of the glass tube. Further, the two ends of the fuse wire are connected to these metal caps. The metal caps are for connecting the fuse with the circuit in a suitably made bracket.
Answer 20:
| Overloading | Short-circuiting |
| 1. An extremely large amount of current is withdrawn from the circuit when a large number of electric appliances of high power-rating are switched on at the same time or connected in a single socket. This situation is known as overloading . | Short-circuiting occurs when naked live and neutral wires touch each other. In this case, the resistance of the circuit so formed is very small and thus, the current flowing through the wires becomes very large. |
Answer 21:
(a) An electric current is cut off by a fuse when the current exceeds a safe value.
When short circuit or overloading takes place, current becomes large and heats the fuse wire too much. The melting point of fuse wire is much lower than other conducting wires; therefore, the fuse wire melts and breaks the circuit. When the fuse wire breaks, current supply is automatically switched off.
(b) Let the maximum number of bulbs be n.
Power of one bulb = 60 W
So, power of n bulbs, P = 60 × n watts
Potential difference, V = 220 volts
Current, I = 5 A
We know that:
P = V × I
So, 60 × n = 220 × 5
or, n = 18.3
Thus, 18 bulbs can be run in the given case.
Answer 22:
(i) Fuse
A fuse is an important device that disconnects the electricity supply when short circuit or overloading occurs.
It is a short-length wire of low melting point that is used to protect the household electrical system from getting damaged. It also avoids danger of electric fires due to unusual high current. This unusual high current may be due to short circuit, overloading etc.
(ii) Earthing wire
Earthing wire is a safety wire. It connects the metal parts of an appliance to the ground. It is accomplished by thick copper wires. Earthing protects the appliance and human beings in case of insulation failure or accidental short circuit.
Answer 23:
(a) Here,
Power, P = 750 W
Potential difference, V = 230 V
(i) We know that:
P = V × I
Current, I = P / V
I = 750 / 230 = 3.26 A
Thus, the value of maximum current is 3.26 A.
(ii) Given:
Power, P = 750 W = 0.750 kW
Time, t = 30 minutes = 30 / 60 hours = 1 / 2 h
We know that:
Electrical energy, E = P × t
= 0.750 × (1 / 2) = 0.375 kWh or 0.375 units
Thus, the number of units of electricity is 0.375.
(b) A fuse rating of 5 A would be suitable for this electric iron because the fuse used by an appliance should be slightly more than the normal current drawn through it. In this case, the normal current drawn is 3.26 A.
Answer 24:
An earth wire is a safety wire. It connects the metal parts of appliances to the earth. The earth wire conducts the excess current to the earth, thereby reducing the risk of accidents.
It is necessary to earth the metallic bodies of electric appliances to avoid the risk of electric shocks. To do this, we connect the earth wire to the metal body of an electrical appliance using a three-pin plug. By doing so, the metal body always remains at zero potential. In case the live wire touches the metal body of the appliance, the current passes directly to the earth through the earth wire. It does not need our body to pass the current; thus, we do not get an electric shock.
Answer 25:
(a) Given:
Power, P = 3 kW = 3 × 1000 = 3000 W
Potential difference, V = 240 V
We know that:
P = V × I
Current, I = P / V
= 3000 / 240 = 12.5 A
Thus, a current of 12.5 A is taken by an electric geyser of 3 kW, working on 240 V mains.
(b) A fuse of 13 A should be used in this geyser circuit because the rating of the fuse used in an appliance should be slightly more than the normal current drawn through it.
Answer 26:
(a) Fuses are included in circuits to stop the flow of excess currents. They are inserted in the fuse box that makes it easy to replace them.
(b) Miniature circuit breakers (MCBs) can be used in place of fuses.
Answer 27:
(a)
b) Hazards associated with the use of electricity:
1. Short circuit due to damaged wiring or overloading of the circuit can cause electrical fire in a building.
2. There is a risk of shock associated with the use of electricity. If a person touches a live electric wire, he gets a severe electric shock. In some cases, shocks can be fatal.
(c) Precautions in the use of electricity:
1. Electrical appliances should be given earth connections to avoid the risk of electric shock.
2. Household wiring should be done by using good quality wires with proper thickness and insulation.
3. Switches, fuses and circuit breakers should be connected in series with the live wire.
4. Switches in ‘on’ position should not be touched with wet hands.
(d) If a person comes in contact with a live wire, the main switch should be turned off at once to cut off the electricity supply. Moreover, the person can also be provided with an insulated support of wood, plastic or rubber.
(e) We should not operate electric switches with wet hands because water conducts electricity. Thus, touching the switches with wet hands can lead to electric shocks.
Answer 28:
(c) increases heavily
At the time of a short circuit, the live wire and the neutral wire touch each other. The resistance of the circuit so formed is very small; therefore, the current flowing through the wires increases heavily.
Answer 29:
(c) 10 A
We know that:
Power, P = V × I
Current drawn, I = P / V
I = 1250 / 220 (1.25kW = 1250 W)
= 5.68 A
Thus, a suitable fuse would have a current rating of 10 A. If the current exceeds 10 A, the fuse wire will melt.
Exercise : Solution of Questions on page Number : 115
Answer 30:
(d) 30
Let n be the number of tube-lights that can be used safely.
It is given that:
Power of one tube-light = 40 W
Therefore, Power of n tube-lights, P = 40 × n watts
Potential difference, V = 240 volts
Current, I = 5 amperes
We know that:
P = V × I
or, 40 × n = 240 × 5
or, n = 30
Thus, the maximum number of tube-lights that can safely be run is 30.
Answer 31:
(b) 5 A
We should use a fuse with current capacity slightly greater than the current drawn by the device. We will choose a fuse of 5 A current rating and the fuse wire will melt if the value of current exceeds 5 A.
Answer 32:
(b) Switches, fuses and circuit breakers should be placed in the neutral wire.
The correct statement is:
Switches, fuses and circuit breakers should be placed in the live wire.
Answer 33:
(a) 5 A
It is given that:
Power, P = 48 W
Potential difference, V = 12 V
We know that:
P = V × I
Therefore, current, I = P / V
= 48 / 12 = 4 A
We should use a fuse with current capacity slightly greater than the maximum current drawn by the device. Thus, the value of correct fuse for the circuit will be 5 A and the fuse wire will melt if the value of current exceeds 5 A.
Answer 34:
(b) A 13 A fuse is the most appropriate value to use.
Given:
P = 1 kW = 1000 W
V = 250 V
∴ Current, I = P/V = 1000/250 = 4 A
Because the current drawn is 4 A, a fuse of 13 A cannot be considered the most appropriate.
Answer 35:
(d) 2 A
It is given that:
Power, P = 230 W
Potential difference, V = 230 V
Therefore, current, I = P / V = 230 / 230 = 1 A
So, the correct option is 2 A because the value of fuse used by any appliance should be slightly larger than the value of normal current drawn by it.
Answer 36:
(d) MCB
Miniature circuit breakers (MCBs) are used in domestic wiring in place of fuses.
Answer 37:
(c) magnetic effect of current
MCB contains an electromagnet. When current increases heavily, the electromagnet becomes strong enough to separate the pair of contacts and break the circuit.
Answer 38:
We will first calculate the current drawn by this air-conditioner.
Power, P = 3.2 kW = 3.2 × 1000 W = 3200 W
Potential difference, V = 220 V
We know:
Power, P = V × I
or, 3200 = 220 × I
or, current drawn, I = 14.5 A
Now, the current drawn by this air-conditioner is 14.5 A which is very high and the fuse in this circuit is of 10 A capacity.
So, when a very high current of 14.5 A flows through 10 A fuse, the fuse wire will get heated. Further, it will melt and break the circuit, thereby cutting off the power supply.
Therefore, when the given 3.2 kW power rating air-conditioner is switched on, the fuse will cut off the power supply in this circuit.
Answer 39:
Voltage of power supply, V = 220 V
Power rating of the first appliance = 60 W
So, current drawn by the first appliance = 60 / 220 = 0.27 A
Power rating of the second appliance = 500 W
So, current drawn by the second appliance = 500 / 220 = 2.27 A
Power rating of the first appliance = 1200 W
So, current drawn by the first appliance = 1200 / 220 = 5.45 A
Net current drawn by all the appliances = 0.27 + 2.27 + 5.45
= 5.99 A
It is given that the rating of the fuse is 10 A. It will not blow when all the appliances are used together because current capacity of the fuse is more than the value of current drawn by the device.
Answer 40:
(a) The appropriate value of the fuse should be 3 A because the capacity of fuse used by an appliance should be slightly larger than the value of normal current drawn through it.
(b) If a fuse of 13 A is fitted in in the circuit of the vacuum cleaner, it can allow very high current to flow through the vacuum cleaner. This can damage the vacuum cleaner during short-circuits and overloading.
Answer 41:
Circuit (b) will still be dangerous even if the fuse blows off and electric iron stops working during a short-circuit. This is because the live wire that carries A.C. power is still is in contact with the electric iron .
Exercise : Solution of Questions on page Number : 116
Answer 42:
Voltage of the power supply, V = 220 V
Power rating of the kettle, Pk = 1200 W
Maximum current drawn by the kettle, ik = Pk/V = 1200/220 = 5.5 A
Power rating of the toaster, Pt = 1000 W
Maximum current drawn by the toaster, it = Pt/V = 1000/220 = 4.5 A
When both the appliances are switched on together, we get:
Maximum current drawn = ik + it
= 5.5 + 4.5
= 10 A
It is greater than the rating of the fuse; therefore, both the appliances cannot be used at the same time.
Answer 43:
Main difference in the wiring of an electric bulb and a socket for using an electric iron:
An earth connection is given to the socket for an electric iron, whereas no earth connection is given to an electric bulb.
It is because earth connections are given to only those appliances that have metallic body, draw heavy current and can be touched. Therefore, an electric iron is provided with earth connection.
However, earthing is not done with electric bulbs because we hardly touch them when they are on.
Answer 44:
(a)
In an A.C. power supply, the wire that carries alternating current is called live wire. So, it is dangerous to touch live wire. Neutral wire is grounded at the power station; therefore, it is not dangerous to touch neutral wire.
(b)
When a bird sits on a naked power line, it only touches the power line and nothing else. The alternating current in the wire doesn’t have any path to flow through the bird to anywhere else. In the absence of a path for the current to flow, the bird remains safe.
Answer 45:
Main supply voltage, V = 230 V
Maximum current that the fuse can hold, I = 5 A
Maximum power that can be delivered, P = V × I
= 230 × 5 W
= 1150 W
Power of each bulb = 36 W
Let n be the number of bulbs that can be used safely.
Therefore, P = 36 × n
or, n = P/36
or, n = 1150/36
or, n = 31.9
Thus, the required number of bulbs is 31.