LAKHMIR SINGH AND MANJIT KAUR Solutions for Class 10 Physics Chapter 2 - Magnetic Effects of Electric Current
Chapter 2 - Magnetic Effects of Electric Current Exercise 73
A compass needle gets deflected when brought near a bar magnet because the bar magnet exerts a magnetic force on the compass needle, which is itself a tiny pivoted magnet.
Because the strength of magnetic field produced by the cable is quite weak.
Activity to draw a magnetic field line outside a bar magnet from one pole to another pole:
- Take a small compass and a bar magnet.
- Place the magnet on a sheet of white paper fixed on a drawing board, using some adhesive material.
- Mark the boundary of the magnet.
- Place the compass near the north pole of the magnet. The south pole of the needle points towards the north pole of the magnet. The north pole of the compass is directed away from the north pole of the magnet.
- Mark the position of two ends of the needle.
- Now move the needle to a new position such that its south pole occupies the position previously occupied by its north pole.
- In this way, proceed step by step till you reach the south pole of the magnet .
- Join the points marked on the paper by a smooth curve. This curve represents a magnetic field line.
(b) A freely suspended magnet points in the north-south direction because earth behaves as a magnet with its south pole in the geographical north and the north pole in the geographical south.
(i) The magnetic field lines originate from the north pole of a magnet and end at its south pole.
(ii) The strength of magnetic field is indicated by the degree of closeness of the field lines. Where the field lines are closest together, the magnetic field is the strongest there.
(ii) By using compass
The axis of earth's imaginary magnet is inclined at an angle of 15o with the geographical axis.
Manufacturers use a magnetic strip in the refrigerator's door to keep it closed properly.
(b) bar; south
Magnetic field lines around a bar magnet
The space surrounding a magnet in which magnetic force is exerted, is called a magnetic field. The direction of magnetic field lines at a place can be determined by using a compass needle. A compass needle placed near a magnet gets deflected due to the magnetic force exerted by the magnet. The north end of the needle of the compass indicates the direction of magnetic field at the point where it is placed.
Two magnetic field lines do not intersect each other due to the fact that the resultant force on a north pole at any point can be only in one direction. But if the two magnetic lines get intersect one another, this means that resultant force on a north pole placed at the point of interection will be along two directions, which is not possible.
Chapter 2 - Magnetic Effects of Electric Current Exercise 74
As the north pole of the magnetic needle is pointing in the opposite direction,so the nearer end of the magnet will be north pole.
(b) X, as it repels the north pole (tip) of magnetic needle.
A=N; B=N; C=S; D=S; E=N; F=S
(ii) Magnet 2 is weaker.
Chapter 2 - Magnetic Effects of Electric Current Exercise 81
We conclude that a current carrying wire produces a magnetic field around it.
Chapter 2 - Magnetic Effects of Electric Current Exercise 82
Magnetic effect of current was discovered by Oersted.
Magnetic field becomes very strong.
Maxwell's corkscrew rule.
(i) increasing the number of turns in the coil
(ii) increasing the current flowing in the coil
(iii) reducing the length of air gap between the poles
(a) magnetic field; concentric circles
(b) bar magnet
(c) turns; current; iron
A current-carrying wire concealed in a wall can be located due to the magnetic effect of current by using a plotting compass. If a plotting compass is moved on a wall, its needle will show deflection at the place where current-carrying wire is concealed.
We take a thick insulated copper wire and fix it in such a way that the portion AB of the wire is in the north-south direction as shown in fig. A plotting compass M is placed under the wire AB. The two ends of the wire are connected to a battery through a switch. When no current is flowing in the wire AB, the compass needle is parallel to the wire AB and points in the usual north-south direction. When current is passed through wire AB by closing the switch, we find that the compass needle is deflected from its north-south position. On opening the switch, the compass needle returns to its original position.
Thus, the deflection of compass needle by the current carrying wire shows that magnetic field is associated with an electric current.
(b) Maxwells right-hand thumb rule: According to this rule, imagine that you are grasping the current-carrying wire in your right hand so that your thumb points in the direction of current, then the direction in which your fingers encircle the wire will give the direction of magnetic field lines around the wire.
(b) According to Maxwell's right hand thumb rule: Imagine that you are grasping the current-carrying wire in your right hand so that your thumb points in the direction of current, then the direction in which your fingers encircle the wire will give the direction of magnetic field lines around the wire.
Let AB be the straight wire carrying current in the vertically upward direction from A to B. To find out the direction of the magnetic field lines produced by this current, we imagine that we are grasping the current carrying wire in our right hand as shown in fig. so that our thumb points in the direction of current towards B. Now, the direction in which our fingers are folded gives the direction of magnetic field lines. In this case, the direction of magnetic field lines is in the anticlockwise direction.
According to Maxwell's corkscrew rule: Imagine driving a corkscrew in the direction of current, then the direction in which we turn its handle is the direction of magnetic field.
This rule is used to determine the direction of magnetic field around a straight current carrying conductor.
(b) The strength of magnetic field produced by a current-carrying circular coil can be increased by:
-increasing the number of turns of wire in the coil
-increasing the current flowing through the coil
According to the Clock face rule, we look at one face of a circular wire (or coil) through which a current is passing:
(i) If the current around the face of the circular wire (or coil) flows in the clockwise direction, then that face of the circular wire will be South pole (S-pole).
(ii) If the current around the face of the circular wire (or coil) flows in the anti-clockwise direction, then that face of the circular wire will be North pole (N-pole).
The strength of magnetic field produced by a current-carrying solenoid depends on:
1. The strength of current in the solenoid: Larger the current passed through solenoid, stronger will be the magnetic field produced.
2. The number of turns in the solenoid: Larger the number of turns in the solenoid, greater will be the magnetic field produced.
A coil C of insulated copper wire is wound around a soft iron core NS and the two ends of the copper coil are connected to a battery. Thus, an electromagnet using a soft iron core.
(b) Electromagnetic cranes are used to separate copper from iron in a scrap yard. The current is switched on to energise the electromagnet and pick up the iron pieces from the scrap. Then these iron pieces are moved to another position, the electromagnet in switched off and the iron pieces are released.
(a) An electromagnet produces a magnetic field so long as current flows in its coil i.e., it produces temporary magnetic field.; but a permanent magnet produces a permanent magnetic field.
(b) Electromagnets: Electric bell, electric motors
Permanent magnets: Refrigerator doors, toys
(a) A solenoid is a long coil containing a large number of close turns of insulated copper wire.
(b) The magnetic field produced by a current-carrying solenoid is similar to the magnetic field produced by a bar magnet.
(c) Magnetic field lines inside a current-carrying solenoid are in the form of parallel straight lines. This indicates that the magnetic field inside the solenoid is uniform.
(d) The magnetic field strength of a current-carrying solenoid can be increased by
(i) increasing the number of turns in the solenoid
(ii) increasing the current flowing through the solenoid
(iii) using soft iron as core in the solenoid
(a) An electromagnet is a temporary magnet that works on the magnetic effect of current. It consists of a long coil of insulated copper wire wrapped around a soft iron core that is magnetised ony when electric current is passed through the coil.
To make an electromagnet, we take a rod NS of soft iron and wind a coil C of insulated copper wire around it. When the two ends of the copper coil are connected to a battery, an electromagnet is formed.
(b) An electromagnet is called a temporary magnet because it produces magnetic field so long as current flows in its coil.
(c) Core of an electromagnet should be of soft iron and not of steel because soft iron loses all its magnetism when current in the coil is switched off but steel does not lose its magnetism when the current is stopped.
(d) Strength of electromagnet depends on:
i. The number of turns in the coil - Increasing the number of turns in the coil increases the strength of the electromagnet.
ii. The current flowing in the coil - Increasing the current flowing in the coil increases the strength of the electromagnet.
iii. The length of air gap between its poles: Reducing the length of air gap between the poles of electromagnet increases the strength of the electromagnet.
(e) Electromagnets are used in several electrical devices such as electric bell, electric motor, loudspeaker etc. They are also used by doctors to remove particles of iron or steel from a patient's eye and to remove pieces of iron from wounds.
Chapter 2 - Magnetic Effects of Electric Current Exercise 84
We have used Maxwell's right hand thumb rule here.
End A will be a S-pole because current flows in the clockwise direction at A.
Direction of magnetic field is anticlockwise.
Maxwell's right hand thumb rule. is used to find out the direction of magnetic field.
(a) End A becomes S-pole because current flows in clockwise direction at A.
(b) When A becomes S-pole, the other end becomes N-pole. So the tip of the compass (with also has North polarity) moves away from this end i.e., tip moves towards right.
Magnetic field lines around it will be clockwise (according to Maxwell's right hand thumb rule).
(a) End X is S-pole (because current flows in clockwise direction).
(b) End Y is N-pole (because current flows in anticlockwise direction).
(c) Clock face rule - Looking at the face of a loop, if the current around that face is in anticlockwise direction, the face has north polarity, while if the current at that face is in clockwise direction, the face has south polarity.
The direction of current will be from east to west.
We have applied MAxwell's right hand thumb rule here.
According to Maxwell's right hand thumb rule: Imagine that you are grasping the current-carrying wire in your right hand so that your thumb points in the direction of current, then the direction in which your fingers encircle the wire will give the direction of magnetic field lines around the wire.
(a) from top towards bottom
(a) Negative terminal
(b) Positive terminal
Because the current should be passed into wire upwards.
Chapter 2 - Magnetic Effects of Electric Current Exercise 85
Chapter 2 - Magnetic Effects of Electric Current Exercise 91
When a current-carrying conductor is placed in a magnetic field, a mechanical force is exerted on the conductor which can make the conductor move.
(b) Magnetic field
(c) Force acting on the conductor
Electrib bell works on the magnetic effect of current. It uses an electromagnet to produce sound.
Electrical energy to mechanical energy.
An electric motor converts electrical energy into mechanical energy.
(b) Magnetic field - direction of fore finger
(c) Force or Motion - direction of thumb
The function of split rings is to reverse the direction of current flowing through the coil every time the coil just passes the vertical position during a revolution.
(b) Commutator; rotation
(a) Fleming's left hand rule
(b) By increasing the current flowing in the conductor; by increasing the strength of magnetic field
(c) Electric motor
Chapter 2 - Magnetic Effects of Electric Current Exercise 92
(i) Direction of rotation would be reversed
(ii) Direction of rotation would be reversed
(iii) Direction of the rotation would remain unchanged
(b) Motor can be made more powerful by winding the coil on a soft iron core of by increasing the number of turns of the coil.
(a) Electric motor is a device used for converting electrical energy into mechanical energy.
Working of an electric motor:
Initially, the coil ABCD is in the horizontal position. On pressing the switch, current enters the coil through carbon brush P and commutator half ring X. The current flows in the direction ABCD and leaves via ring Y and brush Q. The direction of magnetic field is from N pole to S pole of the magnet. According to Fleming's left-hand rule, the force on sides AB and CD is in the downward and upward directions respectively. This makes the coil ABCD move in the anticlockwise direction.
When the coil reaches vertical position, then the brushes P and Q will touch the gap between the two commutator rings and current is cut off. But the coil does not stop rotating as it has already gained momentum. When the coil goes beyond the vertical position, the side CD comes on the left side and side AB comes to the right side, and the two commutator rings change contact from one brush to the other. This reverses the direction of current in the coil, which in turn reverses the direction of forces acting on the sides AB and CD of the coil. The side CD is pushed down and side AB is pushed up. Thus, the coil rotates anticlockwise by another half rotation.
The reversing of current in the coil is repeated after every half rotation due to which the coil (and its shaft) continues to rotate as long as current from the battery is passed through it. The rotating shaft of electric motor can drive a large number of machines which are connected to it.
(b) In commercial electric motors:
i. the coil is wound on a soft iron core. This increases the strength of magnetic field, which makes the motor more powerful.
ii. the coil contains a large number of turns of insulated copper wire.
iii. a powerful electromagnet is used in place of permanent magnet.
Chapter 2 - Magnetic Effects of Electric Current Exercise 93
Clockwise direction (according to Fleming's left hand rule).
According to Fleming's left hand rule, the wire moves in the upward direction (out of the page).
Force on a current-carrying wire that is parallel to magnetic field will be zero.
This is because the magnitude of force depends on the sin of the angle between the direction of current and the direction of magnetic field, so if the current carrying wire is held parallel to the magnetic field, the force will be zero.
Chapter 2 - Magnetic Effects of Electric Current Exercise 102
(a) D.C. Generator
(b) A.C. Generator
Chapter 2 - Magnetic Effects of Electric Current Exercise 103
AC Generator (or Alternator)
If we replace the slip rings of an AC generator by a commutator, then it will become a DC generator.
Function of brushes is to transfer the current from coil to load.
This phenomena is known as electromagnetic induction.
Simple alternator:- Magnet fixed and coil rotates;
Practical alternator:- Coil fixed and magnet rotates.
Electromagnet, permanent magnet, wire carrying current.
(a) Electric generator is based on the principle that when a straight conductor is moved in a magnetic field, then current is induced in the conductor.
(b) Two ways in which the current induced in the coil of a generator could be increased are:
(i) by roating the coil faster
(ii) by using a coil with a larger area
(i) DC current remains same with time in its value and direction.
(ii) AC current changes with time and changes its direction every time after a certain interval of time.
The direction of induced current produced in a straight conductor moving in a magnetic field is given by Fleming's right hand rule.
According to Fleming's right hand rule : Hold the thumb, the fore finger and the centre finger of your right-hand at right angles to one another. Adjust your hand in such a way that forefinger points in the direction of magnetic field, and thumb points in the direction of motion of conductor, then the direction in which centre finger points, gives the direction of induced current in the conductor.
(a) Fleming's right hand rule:- Hold the thumb, the fore finger and the centre finger of your right-hand at right angles to one another. Adjust your hand in such a way that forefinger points in the direction of magnetic field, and thumb points in the direction of motion of conductor, then the direction in which centre finger points, gives the direction of induced current in the conductor.
(b) Fleming's left hand rule:- Hold the forefinger, the centre finger and the thumb of your left hand at right angle to one another. Adjust your hand in such a way that the forefinger points in the direction of magnetic field and the and centre finger points in the direction of current, then the direction in which thumb points, gives the direction of force acting on the conductor.
(b) Generally, the alternators in a Thermal Power Station are driven by the power of high pressure steam.
To heat water in the boiler, fuels like coal or natural gas can be used.
A simple AC generator consists of a rectangular coil ABCD which can be rotated rapidly between the poles N and S of a strong horseshoe-type permanent magnet M. The coil is made of a large number of turns of insulated copper wire. The two ends A and D of the coil are connected to two circular pieces of copper metal called slip rings R1 and R2. As the slip rings rotate with the coil, the two fixed pieces of carbon called brushes, B1 and B2, keep contact with them. So, the current produced in the rotating coil can be tapped out through slip rings into the carbon brushes. The outer ends of carbon brushes are connected to a galvanometer to show the flow of current in the external circuit.
Suppose the coil ABCD, which is initially in the horizontal position, is rotated in the anticlockwise direction. The side AB of the coil moves down and side CD moves up. Due to this, induced current is produced in both the sides, which flows in the direction BADC (according to Fleming's right hand rule). Thus, in the first half rotation, the current in the external circuit flows from brush B1 to B2. After half revolution, sides AB and CD will interchange their positions. So, side AB starts moving up and side CD starts moving down. As a result, direction of induced current in the coil is reversed and flows in the direction CDAB. The current in the external circuit flows from brush B2 to B1.
(a) The production of electricity from magnetism is known as electromagnetic induction.
Let us move a wire AB upward rapidly between the poles of the horseshoe magnet. When the wire is moved up, there is a deflection in the galvanometer pointer which shows a current is produced in the wire AB momentarily. Thus, as the wire is moved up through the magnetic field, an electric current is produced in it.
(b) Electric generator
(c) Different ways to induce current in a coil of wire are:
(i) by moving the coil relative to a fixed magnet
(ii) by keeping the coil fixed and moving a magnet relative to it.
(b) Source of DC are dry cell, car battery, DC generator etc.
Source of AC are AC generator, bicycle dynamos etc.
(c) An important advantage of AC over DC is that AC can be transmitted over long distances without much loss of electrical energy.
Chapter 2 - Magnetic Effects of Electric Current Exercise 105
(a) The galvanometer deflects to the left.
(b) The galvanometer deflects to the left.
(c) No deflection in galvanometer.
The deflection in the galvanometer can be increased by
i. increasing the number of turns in the coil
i. using a strong magnet
ii. increasing the speed with which magnet is moved in the coil.
(i) Electromagnetic induction.
(ii) The galvanometer gives a reading to the left.
(iii) Large deflection to right occurs more quickly.
(a) Current increased
(b) Current reversed
(c) Current increased
(d) Zero current
(e) Zero current
(i) Galvanometer pointer moves to one side showing that a momentary current is induced in the coil A.
(ii) Galvanometer pointer moves to the other side showing that the direction of momentarily induced current has been reversed.
The phenomenon taking place here is electromagnetic induction. When the current is passed through coil B or is stopped, the magnetic field linked with coil A changes due to which an induced current is produced in the coil.
Chapter 2 - Magnetic Effects of Electric Current Exercise 113
(i) 5A(ii) 15A
Chapter 2 - Magnetic Effects of Electric Current Exercise 114
Red wire - Live wire
Black wire - Neutral wire
Green wire - Earth wire
P = VI
I = P/V = 180/240 = 0.75A
The fuse wire should be such that it is able to withstand only a little more current than 0.75A. So the fuse of 1A is the most suitable.
To avoid the risk of electric shocks.
(ii) In case of parallel connection, all the appliances are operated on same voltage i.e., the mains supply voltage.
(b) Pure copper wire cannot be used as a fuse wire because it has a high melting point due to which it will not melt easily when a short circuit takes place.
It consists of a glass tube T having a thin fuse wire sealed inside it. The glass tube has two metal caps at its two ends. The two ends of the fuse wire are connected to these metal caps. The metal caps are for connecting the fuse in the circuit in a suitably made bracket.
Live wire coming in contact with the neutral wire is known as short circuit.
When too many electrical appliances of high power rating are switched on at the same time or are connected to a single socket, they draw extremely large current. This is known as overloading.
(a) A fuse cuts off current when the current exceeds a safe value (due to short circuiting or overloading). When the current becomes large, it heats the fuse wire too much. Since the melting point of fuse wire is low, it melts and breaks the circuit. Thus, current in the circuit is cut off.
(b) Let the maximum number of bulbs be y.
Power of y bulbs, P=60y
We know that
P = VI
60y = 220 x 5
60y = 1100
So, number of bulbs required are 18.
(ii) An earthing wire is used to save us from the risk of electric shock in case the live wire touches the metal case of the electric appliance.
(a) V=230V, P=750 W, t=30/60=0.5hr
(i) Let max current be I
We have P=VI
I= 3.26 A
(ii) Electric energy consumed, E = Pxt = 0.75kW x 0.5h = 0.375 kWh
No. of units used in 30 min = 0.375
(b) 5 A fuse rating will be suitable for this electric iron as the maximum current for this iron is 3.26 A.
When the live wire of a faulty appliance comes in direct contact with its metallic case, which has been earthed, the large current passes directly to the earth without passing through the user's body. Thus, it is necessary to earth the metallic bodies of electrical appliances so as to avoid fatal electric shocks.
(a) P = 3kW = 3000W
V = 240V
P = V x I
I = P/V
= 3000/240 = 12.5 A
(b) A 13A fuse should be used in the geyser circuit.
(b) Two hazards associated with the use of electricity are:
i. If a person happens to touch a live electric wire, he gets a severe electric shock.
ii. Short-circuiting due to damanged wiring or overloading of the circuit can cause electrical fire in a building.
(c) Important precautions which should be observed in the use of electricity are:
(i) Use of good quality wires
(ii) Use of fuse and proper earthing.
(iii) Use of appliances in dry condition only.
(d) If a person comes in contact with a live wire, we will switch off the main switch immediately so as to cut off the electricity supply..
(e) Electric switches should not be operated with wet hands because water is a good conductor of electricity, so the user may get electric shock.
Chapter 2 - Magnetic Effects of Electric Current Exercise 115
Given: P=3.2kW=3200W, Fuse current rating=10 A, V=220 V
3200 = 220 x I
As the required current for the air-conditioner is 14.54A and the rated current of the fuse is 10A, so the fuse will blow cutting off the power supply.
P1=60 W, P2=1200W, P3=500W
Fuse rating = 10A
We have, P=VI
The required current is 8A and fuse rating is 10A. So, all the appliances will work normally and the fuse will not blow.
(b) If a 13A fuse is fitted in the circuit, it will not protect the vacuum cleaner against the very high current flowing through it. This may damage the appliance.
Circuit A is not dangerous after fuse blows because fuse is fitted in the live wire; but circuit B is dangerous even if fuse blows because the fuse is in the neutral wire.
Chapter 2 - Magnetic Effects of Electric Current Exercise 116
We know, P=VI
Total current required,
If both the appliances are switched on together, the fuse will get burnt. So, both the appliance cannot be used at same time.
No earth connection is required for the bulb connection as it does not draw heavy current and we hardly touch it. A socket for using an electric iron has an earth connection because electric iron has a metallic body and draws a large current.
(b) Bird's body is not connected to the earth, so no current flows through its body into the earth. So, it is safe for birds to sit on naked power lines fixed atop tall electric poles.
Power of y tube-lights, P=36y
We know that
P = VI
36y = 230 x 5
36y = 1500
So, number of tube-lights required are 31.
Other Chapters for CBSE Class 10 PhysicsChapter 1- Electricity Chapter 3- Sources of Energy Chapter 4- Reflection of Light Chapter 5- Refraction of Light Chapter 6- The Human Eyes And The Colorful World
LAKHMIR SINGH AND MANJIT KAUR Solutions for CBSE Class 10 SubjectsLAKHMIR SINGH AND MANJIT KAUR Solutions for Class 10 Biology LAKHMIR SINGH AND MANJIT KAUR Solutions for Class 10 Chemistry
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