CBSE Class 10 Science Magnetic effect of current Notes

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CHAPTER – 13

This Class 10 Science notes page on Magnetic Effect of Current covers magnets and magnetic field lines, the magnetic field due to a straight conductor, a circular loop and a solenoid, electromagnets and permanent magnets, the force on a current-carrying conductor with Fleming's left and right hand rules, the electric motor, electromagnetic induction, the electric generator, domestic electric circuits and magnetism in medicine, along with all intext and exercise questions with verified answers.

Topics Covered

1. Magnetic Effect of Electric Current
2. Magnets and Magnetic Field Lines
3. Magnetic Field due to a Current through a Straight Conductor
4. Magnetic Field due to a Current through a Circular Loop
5. Magnetic Field due to a Current in a Solenoid
6. Electromagnets and Permanent Magnets
7. Force on a Current-Carrying Conductor in a Magnetic Field
8. Electric Motor
9. Electromagnetic Induction
10. Electric Generator
11. Domestic Electric Circuits
12. Magnetism in Medicine
13. Exercise Questions

Magnetic Effect of Electric Current

The magnetic effect of electric current means that an electric current flowing in a wire produces a magnetic field around it. A current flowing in a wire always gives rise to a magnetic field around it. The magnetic effect of current is also called electromagnetism, which means electricity produces magnetism. In the experiment, the deflection of a compass needle placed near a current-carrying wire shows that an electric current produces a magnetic field around it.

Magnets and Magnetic Field

A magnet is an object which attracts pieces of iron, steel, nickel and cobalt. It has two poles at its ends, the North Pole and the South Pole. Like magnetic poles repel each other, and unlike magnetic poles attract each other.

The space surrounding a magnet in which the force of attraction and repulsion is exerted is called a magnetic field.

Magnetic Field Lines

The magnetic field lines are the lines drawn in a magnetic field along which a north magnetic pole would move. They are also known as magnetic lines of force.

Properties of Magnetic Field Lines

1. Magnetic field lines originate from the north pole and end at the south pole.
2. A magnetic field line is a closed and continuous curve.
3. The magnetic field lines are closer near the poles of a magnet, where the magnetic field is strong, and farther apart where the magnetic field is weak.
4. Magnetic field lines never intersect each other.
5. A uniform magnetic field is represented by parallel and equidistant field lines.

Magnetic Field due to a Current through a Straight Conductor

The magnetic field lines around a straight conductor carrying current are concentric circles whose centres lie on the wire.

The magnitude of the magnetic field produced by a straight current-carrying wire at a point is:
(i) directly proportional to the current passing through the wire.
(ii) inversely proportional to the distance of that point from the wire.

Right-Hand Thumb Rule

Hold the current-carrying straight conductor in the right hand such that the thumb points in the direction of the current. The fingers then wrap around the conductor and give the direction of the magnetic field lines. This is known as the right-hand thumb rule.

If the thumb points in the direction of the current, the direction in which the fingers encircle the wire gives the direction of the magnetic field around the wire.

Magnetic Field due to a Current through a Circular Loop

The magnetic field lines are circular near a current-carrying loop. As we move away from the loop, the concentric circles become bigger and bigger. At the centre of the loop, the field lines are straight. All the field lines at the centre point in the same direction, so the strength of the magnetic field increases.

The magnetic field produced by a current-carrying circular loop at its centre is:
(i) directly proportional to the current passing through the loop.
(ii) inversely proportional to the radius of the circular loop.

The strength of the magnetic field produced by a circular coil carrying current is directly proportional to both the number of turns (n) and the current (I), but inversely proportional to its radius (r).

Magnetic Field due to a Current in a Solenoid

An insulated copper wire wound on a cylindrical tube, such that its length is greater than its diameter, is called a solenoid. The word solenoid comes from the Greek word for channel. A solenoid is a long coil containing a large number of close turns of insulated copper wire.

The magnetic field produced by a current-carrying solenoid is similar to the magnetic field produced by a bar magnet. The current in each turn of the solenoid flows in the same direction, so the magnetic field produced by each turn adds up, giving a strong magnetic field inside the solenoid.

When a piece of soft iron is placed inside a current-carrying solenoid, it becomes strongly magnetised. The magnet thus formed is called an electromagnet. So, a solenoid is used for making electromagnets.

The strength of the magnetic field produced by a current-carrying solenoid depends on:
(i) the number of turns (n).
(ii) the strength of the current (I).
(iii) the nature of the core material. Soft iron as the core produces the strongest magnetism.

Electromagnets and Permanent Magnets

An electromagnet is a temporary strong magnet. It is just a solenoid with its winding on a soft iron core. A permanent magnet is made from steel. Steel has more retentivity than iron, so it does not lose its magnetism easily.

Difference between Electromagnet and Permanent Magnet

Electromagnet:
1. It is a temporary magnet, as it can readily be demagnetised by stopping the current through the solenoid.
2. Its strength can be changed.
3. It produces very strong magnetic forces.
4. Its polarity can be changed by changing the direction of the current.

Permanent magnet:
1. It cannot readily be demagnetised.
2. Its strength cannot be changed.
3. It produces weak forces of attraction.
4. Its polarity is fixed and cannot be changed.

Why soft iron is used for the core of an electromagnet

Soft iron is used because it loses all of its magnetism when the current in the coil is switched off.

Why steel is not used for the core of an electromagnet

Steel is not used because it does not lose all of its magnetism when the current in the coil is switched off.

Force on a Current-Carrying Conductor in a Magnetic Field

A current-carrying conductor placed in a magnetic field experiences a force, except when it is placed parallel to the magnetic field. The force arises from the interaction between the magnetic field produced by the current-carrying conductor and the external magnetic field in which the conductor is placed.

In the activity with a current-carrying aluminium rod AB placed between the poles of a horseshoe magnet, the rod is displaced. The force reverses when the direction of the current is reversed. It also reverses when the poles of the magnet are interchanged. So, the direction of the force depends on the direction of the current and the direction of the magnetic field. When the current and the field are perpendicular to each other, the force is perpendicular to both of them.

Fleming's Left Hand Rule

Fleming's left hand rule (for electric motors) shows the direction of the force on a conductor carrying current in a magnetic field. Hold the left hand with the thumb, first finger and second finger mutually at right angles.
(i) The First finger represents the direction of the magnetic field (north to south).
(ii) The Second finger represents the direction of the current (conventional current, from positive to negative).
(iii) The Thumb represents the direction of the force, or thrust, and the resultant motion.

Fleming's Right Hand Rule

Fleming's right hand rule (for generators) shows the direction of the induced current when a conductor moves in a magnetic field. Hold the right hand with the thumb, first finger and second finger mutually perpendicular.
(i) The Thumb represents the direction of motion of the conductor.
(ii) The First finger represents the direction of the magnetic field (north to south).
(iii) The Second finger represents the direction of the induced current (conventional current, from positive to negative).

Electric Motor

An electric motor is a rotating device that converts electrical energy into mechanical energy. Electric motors are used in electric fans, refrigerators, mixers, washing machines, computers, MP3 players and similar devices.

Principle

When a coil carrying current is placed in a magnetic field, it experiences a torque. As a result of this torque, the coil begins to rotate.

Construction

A rectangular coil ABCD of insulated copper wire is placed between the two poles of a magnet, such that the arms AB and CD are perpendicular to the direction of the magnetic field. The ends of the coil are connected to the two halves P and Q of a split ring. The inner sides of these halves are insulated and attached to an axle. The outer conducting edges of P and Q touch two conducting stationary brushes X and Y.

Working

Current from the battery enters the coil through brush X and flows back to the battery through brush Y. The current in arm AB flows from A to B, while in arm CD it flows from C to D, that is, opposite to arm AB. By Fleming's left hand rule, the force on arm AB pushes it downwards and the force on arm CD pushes it upwards. So the coil and the axle O rotate anticlockwise.

After half a rotation, Q touches brush X and P touches brush Y. The current in the coil is reversed and flows along the path DCBA. A device that reverses the direction of current through a circuit is called a commutator. In an electric motor, the split ring acts as the commutator.

The reversal of current reverses the force on the two arms, so the coil and the axle keep rotating half a turn at a time in the same direction. The reversal is repeated at each half rotation, giving continuous rotation of the coil and the axle.

Commercial Motors

Commercial motors use (i) an electromagnet in place of a permanent magnet, (ii) a large number of turns of the conducting wire in the coil, and (iii) a soft iron core on which the coil is wound. The soft iron core together with the coil is called an armature. This enhances the power of the motor.

Electromagnetic Induction

The production of electricity from magnetism is called electromagnetic induction. When a straight wire is moved up and down rapidly between the poles of a magnet, an electric current is produced in the wire. This is an example of electromagnetic induction. The process has led to the construction of generators for producing electricity at power stations.

The current produced by moving a conductor in a magnetic field is called an induced current. When a bar magnet is moved towards a coil, a current is set up in the coil circuit, shown by the deflection of the galvanometer needle. If the magnet is moved away, or if the other pole is moved towards the coil, the deflection is reversed. When the coil and the magnet are both stationary, there is no deflection. So, the motion of a magnet with respect to the coil produces an induced potential difference, which sets up an induced current in the circuit.

Electric Generator

In an electric generator, mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.

Principle

Whenever the magnetic field lines linked with a closed circuit change, an induced current is produced.

Construction

A rectangular coil ABCD is placed between the two poles of a permanent magnet and is rotated. The two ends of the coil are connected to two rings R1 and R2, whose inner sides are insulated. Two stationary conducting brushes B1 and B2 are pressed on the rings R1 and R2. The rings are attached to an axle, which can be rotated from outside. The outer ends of the brushes are connected to a galvanometer to show the flow of current in the external circuit.

Working

When the axle is rotated so that arm AB moves up and arm CD moves down in the magnetic field, Fleming's right hand rule shows that induced currents are set up along AB and CD, so the current flows in the direction ABCD. With more turns in the coil, the current from each turn adds up. The current in the external circuit then flows from B2 to B1.

After half a rotation, arm CD moves up and arm AB moves down. The induced currents in both arms reverse, so the net current flows in the direction DCBA, and the external current flows from B1 to B2. The polarity therefore changes after every half rotation. A current that changes direction after equal intervals of time is called an alternating current (AC). This device is called an AC generator.

DC Generator

To get direct current (DC), which does not change its direction with time, a split-ring type commutator is used. One brush is always in contact with the arm moving up in the field, and the other with the arm moving down. This produces a unidirectional current, and the generator is called a DC generator. Direct current always flows in one direction, whereas alternating current reverses its direction periodically.

Domestic Electric Circuits

Electric Short Circuit

If the resistance of an electric circuit becomes very low, the current flowing through the circuit becomes very high. This happens when too many appliances are connected to a single socket, or when high power rating appliances are connected to the light circuits. It also happens when the insulation of the live and neutral wires wears out and the two wires touch each other. The current then increases abruptly, and a short circuit occurs.

Earthing

The metallic body of an electric appliance is connected to the earth through an earth wire, so that any leakage of current is transferred to the ground. This prevents electric shock to the user. That is why earthing of metallic appliances is necessary.

Electric Fuse

An electric fuse consists of a piece of wire made of a metal or alloy of appropriate melting point, such as aluminium, copper, iron or lead. If a current larger than the specified value flows through the circuit, the temperature of the fuse wire rises, melts and breaks the circuit. The fuse is the most important safety device used for protecting circuits against short circuiting or overloading. Fuses used for domestic purposes are rated as 1 A, 2 A, 3 A, 5 A, 10 A, etc.

Magnetism in Medicine

An electric current always produces a magnetic field. Even the weak ionic currents that travel along nerve cells in our body produce magnetic fields. When we touch something, our nerves carry an electric impulse to the muscles we need to use, and this impulse produces a temporary magnetic field. These fields are very weak, about one-billionth of the earth's magnetic field. The two main organs in the human body where the magnetic field produced is significant are the heart and the brain.

The magnetic field inside the body forms the basis of obtaining images of different body parts. This is done using a technique called Magnetic Resonance Imaging (MRI). Analysis of these images helps in medical diagnosis, so magnetism has important uses in medicine.

Intext Questions Page No. 224

1. Why does a compass needle get deflected when it is brought near a bar magnet?

Answer: A compass needle gets deflected because the forces of the magnetic field of the bar magnet act on its poles.

Teacher's Note:
a) The compass needle is itself a small magnet, so it lines up with the field lines of the bar magnet.
b) This is the same idea used to show that a current-carrying wire produces a magnetic field.

Intext Questions Page No. 228

1. Draw magnetic field lines around a bar magnet.

Answer: Curved lines should be drawn leaving the north pole, spreading out around the magnet, and entering the south pole. The lines are closer together near the poles.

Teacher's Note:
a) Arrows should point from the north pole to the south pole outside the magnet.
b) Lines must never cross each other.

2. List the properties of magnetic lines of force.

Answer: The properties are: (i) field lines start at the north pole and end at the south pole; (ii) they form closed and continuous curves; (iii) they are closer where the field is strong and farther apart where it is weak; (iv) they never intersect each other; and (v) a uniform magnetic field is represented by parallel and equidistant field lines.

Teacher's Note:
a) The source answer only says "Refer in page no. 1", so the five properties are listed here in full.
b) Property (iii) is the one most often used to compare field strength.

3. Why do two magnetic lines of force never intersect each other?

Answer: If two field lines intersected, there would be two different directions of the magnetic field at the point of intersection, which is not possible. That is why magnetic field lines never intersect.

Teacher's Note:
a) A field has only one direction at each point, so two lines cannot cross at a point.
b) Using the same reasoning, the field lines are closed loops, not lines that end in mid-air.

Intext Questions Page No. 229 and 230

1. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find the direction of the magnetic field inside and outside the loop.

Answer: For downward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merge into the table inside the loop. Similarly, for upward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop. In both cases the field inside the loop is perpendicular to the plane of the loop.

Teacher's Note:
a) Curl the fingers of the right hand along the current and the thumb gives the direction of the field at the centre.
b)

2. The magnetic field in a given region is uniform. Draw a diagram to represent it.

Answer: A uniform field is shown by a set of parallel straight lines, all with the same direction and equal spacing.

Teacher's Note:
a) Equal spacing shows that the field strength is the same everywhere in the region.
b)

3. Choose the correct option: The magnetic field inside a long straight solenoid carrying current (a) is zero, (b) decreases as we move towards its end, (c) increases as we move towards its end, (d) is the same at all points.

Answer: (b) decreases as we move towards its end. The magnetic field for a point inside a long straight solenoid carrying current is double that for a point situated at one of its ends. Thus, the correct option is (b).

Teacher's Note:
a) The source answer says the field at the centre is "double" that at the end, which is the correct relation, so the option is (b).
b) Option (d) would hold only for an ideal, infinitely long solenoid, which is not the case here.

Intext Questions Page No. 231 and 232

1. Which of the following properties of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.) (a) mass (b) speed (c) velocity (d) momentum

Answer: (c) velocity and (d) momentum.

Teacher's Note:
a) The mass of the proton does not change.
b) The magnetic force changes the direction of the velocity but not its speed, so the velocity and momentum change while the speed remains the same.

2. In Activity 13.7, how will the displacement of rod AB be affected if (i) the current in rod AB is increased, (ii) a stronger horseshoe magnet is used, and (iii) the length of rod AB is increased?

Answer: (i) The force increases, so the displacement increases. (ii) The force increases because the magnetic field is stronger, so the displacement increases. (iii) The force increases, so the displacement increases.

Teacher's Note:
a) The force on a conductor is directly proportional to the current, the field strength and the length of the conductor in the field.
b) The source answer for (iii) reads "force also increased", which is corrected here to "force increases".

3. A positively charged particle (alpha particle) projected towards the west is deflected towards the north by a magnetic field. The direction of the magnetic field is (a) towards south (b) towards east (c) downward (d) upward.

Answer: (d) upward. The direction of the motion of the positively charged particle is the direction of current (towards west). The direction of force on the particle is towards north. By Fleming's left hand rule, the field must point upward.

Teacher's Note:
a) The source answer refers to a proton, but the question is about an alpha particle; the reasoning is the same because both are positive.
b) For a positive charge the current is in the direction of motion, which is why it is taken as west.

Intext Questions Page No. 231 and 232 (State and Explain)

1. State Fleming's left-hand rule.

Answer: If the thumb, the centre finger and the forefinger of the left hand are held at right angles to each other, then the thumb points in the direction of the magnetic force, the centre finger gives the direction of the current, and the forefinger points in the direction of the magnetic field.

Teacher's Note:
a) Remember the order: First finger for Field, Second finger for Current, Thumb for Thrust.
b) This rule is used for motors and for the force on a current-carrying conductor.

2. What is the principle of an electric motor?

Answer: The working of an electric motor is based on the magnetic effect of current. A current-carrying loop placed in a magnetic field experiences a force and rotates. The direction of rotation is given by Fleming's left-hand rule.

Teacher's Note:
a) The coil experiences a torque, not a single force, because the two arms are pushed in opposite directions.
b) Reversing the current at each half rotation keeps the coil turning in the same direction.

3. What is the role of the split ring in an electric motor?

Answer: The split ring acts as a commutator. It reverses the direction of the current through the coil after every half rotation, so the coil keeps rotating in the same direction.

Teacher's Note:
a) Without the commutator, the torque would reverse after half a turn and the coil would only oscillate.
b) In a DC generator the same split ring is used to produce unidirectional current.

Intext Questions Page No. 236

1. Explain different ways to induce current in a coil.

Answer: (a) Move a coil rapidly between the two poles of a horseshoe magnet, and a current is induced in the coil. (b) Move a magnet relative to a coil, and a current is induced in the coil.

Teacher's Note:
a) In both cases the key requirement is a change in the magnetic field linked with the coil.
b) If the magnet and the coil are both at rest, no current is induced.

Intext Questions Page No. 237

1. State the principle of an electric generator.

Answer: An electric generator works on the principle of electromagnetic induction. It produces electricity by rotating a coil in a magnetic field.

Teacher's Note:
a) The source states the principle as "whenever the magnetic field lines change in a closed circuit, an induced current is produced".
b) The change in the magnetic field lines is produced by the rotation of the coil.

2. Name some sources of direct current.

Answer: Some sources of direct current are cell, DC generator, etc.

Teacher's Note:
a) A cell or battery gives a steady current in one direction.
b) A DC generator uses a split-ring commutator to produce unidirectional current.

3. Which sources produce alternating current?

Answer: AC generators, such as those used in power plants, produce alternating current.

Teacher's Note:
a) Mains electricity supplied to homes is alternating current.
b) An AC generator uses slip rings instead of a commutator.

4. Choose the correct option: A rectangular coil of copper wire is rotated in a magnetic field. The direction of the induced current changes once in each (a) two revolutions (b) one revolution (c) half revolution (d) one-fourth revolution.

Answer: (c) half revolution. The direction of the induced current changes once in each half revolution.

Teacher's Note:
a) In one full revolution the current therefore changes direction twice.
b) Over one full revolution the direction of the current is the same at the start and end of the cycle.

Intext Questions Page No. 238

1. Name two safety measures commonly used in electric circuits and appliances.

Answer: Two safety measures are: (i) each circuit must be connected with an electric fuse, which melts when the current exceeds the safe limit and so stops the flow of current; and (ii) earthing, which transfers any leakage of current to the ground so that users do not get a shock.

Teacher's Note:
a) The fuse element must be chosen to match the current rating of the circuit.
b) Earthing protects the user, while the fuse protects the circuit and the appliance.

2. An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.

Answer: Using P = VI, the current drawn by the oven is \( I = \frac{P}{V} = \frac{2000}{220} \) = 9.09 A. This is more than the 5 A safe limit of the circuit, so the fuse element will melt and break the circuit.

Teacher's Note:
a) The power of the oven is 2 kW, which is 2000 W, and the supply voltage is 220 V.
b) The oven should be run on a separate circuit with a suitable fuse rating.

3. What precautions should be taken to avoid the overloading of domestic electric circuits?

Answer: (a) Too many appliances should not be connected to a single socket. (b) Too many appliances should not be used at the same time. (c) Faulty appliances should not be connected to the circuit. (d) A fuse should be connected in the circuit.

Teacher's Note:
a) Overloading causes a high current that heats the wires, which is why these precautions are needed.
b) Regular checks of wiring and appliances help to avoid faults.

Exercise Questions Page No. 240

1. Which of the following correctly describes the magnetic field near a long straight wire? (a) The field consists of straight lines perpendicular to the wire (b) The field consists of straight lines parallel to the wire (c) The field consists of radial lines originating from the wire (d) The field consists of concentric circles centred on the wire.

Answer: (d) The field consists of concentric circles centred on the wire.

Teacher's Note:
a) The circles lie in planes perpendicular to the wire, with their centres on the wire.
b) The field lines are never straight radial lines, so option (c) is wrong.

2. The phenomenon of electromagnetic induction is (a) the process of charging a body (b) the process of generating a magnetic field due to a current passing through a coil (c) producing induced current in a coil due to relative motion between a magnet and the coil (d) the process of rotating a coil of an electric motor.

Answer: (c) producing induced current in a coil due to relative motion between a magnet and the coil.

Teacher's Note:
a) The source answer says "when a straight coil and a magnet are moved", which is corrected here to "a coil and a magnet".
b) Option (b) describes the magnetic effect of current, not induction.

3. The device used for producing electric current is called a (a) generator (b) galvanometer (c) ammeter (d) motor.

Answer: (a) generator. It converts mechanical energy into electrical energy.

Teacher's Note:
a) A galvanometer and an ammeter detect or measure current; they do not produce it.
b) A motor does the reverse conversion, from electrical to mechanical energy.

4. The essential difference between an AC generator and a DC generator is that (a) AC generator has an electromagnet while a DC generator has a permanent magnet (b) DC generator will generate a higher voltage (c) AC generator will generate a higher voltage (d) AC generator has slip rings while the DC generator has a commutator.

Answer: (d) AC generator has slip rings while the DC generator has a commutator.

Teacher's Note:
a) Slip rings are two complete rings, whereas a commutator is a split ring.
b) The magnet type and the voltage level do not decide whether a generator gives AC or DC.

5. At the time of short circuit, the current in the circuit (a) reduces substantially (b) does not change (c) increases heavily (d) varies continuously.

Answer: (c) increases heavily.

Teacher's Note:
a) When two bare wires of the circuit touch, the resistance falls to a very low value.
b) The sudden rise in current can melt the fuse and stop the flow of current.

6. State whether the following statements are true or false. (a) An electric motor converts mechanical energy into electrical energy. (b) An electric generator works on the principle of electromagnetic induction. (c) The field at the centre of a long circular coil carrying current will be parallel straight lines. (d) A wire with green insulation is usually the live wire of an electric supply.

Answer: (a) False. An electric motor converts electrical energy into mechanical energy. (b) True. A generator produces electricity by rotating a coil in a magnetic field, based on electromagnetic induction. (c) True. A long circular coil is a long solenoid, and the field lines inside a solenoid are parallel straight lines. (d) False. In domestic circuits the live wire has red insulation and the earth wire has green insulation.

Teacher's Note:
a) Statement (a) is the reverse of what a motor does.
b) Statement (c) refers to the field inside the coil, which is the part that is parallel and straight.
c) Statement (d) confuses the earth wire with the live wire.

7. List three sources of magnetic fields.

Answer: (a) Current-carrying conductors, (b) permanent magnets, and (c) electromagnets.

Teacher's Note:
a) Any moving charge, such as a current in a wire, produces a magnetic field.
b) The Earth itself also behaves as a magnet, though it is not one of the three sources asked for.

8. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current-carrying solenoid with the help of a bar magnet? Explain.

Answer: A current-carrying solenoid produces a magnetic field similar to that of a bar magnet, with field lines running from one end to the other. Its poles can be found with a bar magnet. When the north pole of a bar magnet is brought near the end connected to the negative terminal of the battery, the solenoid repels it. Like poles repel, so that end behaves as a north pole, and the other end behaves as a south pole.

Teacher's Note:
a) The result depends on the direction in which the wire is wound, and the answer holds for the winding shown in the source figure.
b) If the bar magnet is attracted instead of repelled, the end nearer to it is a south pole.

9. When is the force experienced by a current-carrying conductor placed in a magnetic field largest?

Answer: The force is largest when the direction of the current is perpendicular to the direction of the magnetic field.

Teacher's Note:
a) When the conductor is parallel to the field, the force is zero.
b) The source text has the spelling "current-currying", which is corrected here.

10. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from the back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of the magnetic field?

Answer: The magnetic field is directed downward. The electrons move towards the front wall, so the conventional current flows from the front wall towards the back wall. The force is to the right. The direction of magnetic force is rightward. Hence, using Fleming's left hand rule, the direction of the magnetic field inside the chamber is downward.

Teacher's Note:
a) Conventional current is opposite to the motion of the negatively charged electrons.
b) The source answer gives the same result, and it is checked here using the force on a negative charge.

11. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?

Answer: An electric motor converts electrical energy into mechanical energy. Its principle is that a current-carrying coil placed in a magnetic field experiences a torque and rotates. The rectangular coil ABCD is placed between the poles of a magnet with its arms AB and CD perpendicular to the field. By Fleming's left hand rule, the arms are pushed in opposite directions, so the coil rotates anticlockwise. The split ring, which acts as a commutator, reverses the current after every half rotation, so the coil keeps rotating in the same direction.

Construction: The coil ABCD of insulated copper wire is placed between the two poles of a magnet so that arms AB and CD are perpendicular to the field. The ends of the coil are connected to the two halves P and Q of a split ring, whose inner sides are insulated and attached to an axle. The external conducting edges of P and Q touch two stationary brushes X and Y.

Working: Current from the battery enters the coil through brush X and flows back to the battery through brush Y. The current in arm AB flows from A to B and in arm CD from C to D, opposite to that in AB. By Fleming's left hand rule, the force on arm AB pushes it downwards and the force on arm CD pushes it upwards, so the coil and the axle O rotate anticlockwise. At half rotation, Q makes contact with brush X and P with brush Y, so the current in the coil is reversed and flows along the path DCBA. The reversal also reverses the force on the arms, so the arm AB, earlier pushed down, is now pushed up and arm CD is pushed down. The coil and axle rotate half a turn more in the same direction. The reversing of current is repeated at each half rotation, giving continuous rotation.

Teacher's Note:
a) Refer to the labelled diagram of the motor in the notes above, with the coil, split ring P and Q, brushes X and Y, and axle O.
b) Electric fans, refrigerators, mixers and washing machines all use electric motors.

12. Name some devices in which electric motors are used.

Answer: (a) Water pumps, (b) electric fans, (c) electric mixers and (d) washing machines.

Teacher's Note:
a) Any device that converts electrical energy into rotary motion uses a motor.
b) Refrigerators, computers and MP3 players also contain motors.

13. A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?

Answer: (i) When the magnet is pushed in, a current is induced momentarily and the galvanometer needle deflects in one direction. (ii) When the magnet is withdrawn, a current is induced in the opposite direction and the needle deflects the other way. (iii) When the magnet is held stationary, no current is induced and the galvanometer shows no deflection.

Teacher's Note:
a) A current is induced only while the magnet is moving relative to the coil.
b) The direction of the deflection reverses when the motion reverses.

14. Two circular coils A and B are placed close to each other. If the current in coil A is changed, will some current be induced in coil B? Give reason.

Answer: Yes. When the current in coil A changes, the magnetic field associated with it changes. This changing field passes through coil B and induces an electric current in it. This is electromagnetic induction.

Teacher's Note:
a) The source text says "placed closed to each other", which is corrected here to "placed close to each other".
b) A steady current in coil A produces no induced current in coil B; only a change does.

15. State the rule to determine the direction of a (i) magnetic field produced around a straight current-carrying conductor, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.

Answer: (i) Maxwell's right-hand thumb rule. (ii) Fleming's left-hand rule. (iii) Fleming's right-hand rule.

Teacher's Note:
a) Use the left hand for motors (force) and the right hand for generators (induced current).
b) The right-hand thumb rule gives the direction of the field around a wire, not the force.

16. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

Answer: In an electric generator, mechanical energy rotates a coil in a magnetic field, and electricity is produced by electromagnetic induction. The rectangular coil ABCD is rotated between the poles of a permanent magnet. Its ends are connected to rings R1 and R2, and brushes B1 and B2 press on the rings and carry the current to the external circuit. As the coil rotates, the induced current reverses after every half rotation, which gives alternating current in an AC generator. A split-ring commutator gives direct current in a DC generator. The brushes are stationary conductors that make contact with the rotating rings and take the current out to the external circuit.

Construction: The rotating rectangular coil ABCD is placed between the two poles of a permanent magnet. Its two ends are connected to rings R1 and R2, whose inner sides are insulated. Two stationary conducting brushes B1 and B2 are pressed separately on rings R1 and R2. The rings are internally attached to an axle that can be rotated mechanically from outside, and the outer ends of the brushes are connected to a galvanometer to show the flow of current in the external circuit.

Working: When the axle is rotated so that arm AB moves up and arm CD moves down in the field, with the coil rotated clockwise, Fleming's right-hand rule shows that the induced currents are set up along AB and CD, so the induced current flows in the direction ABCD. With more turns, the current from each turn adds up to give a large current. The current in the external circuit flows from B2 to B1. After half a rotation, arm CD starts moving up and AB moving down, so the induced currents in both arms change and the net current flows in the direction DCBA; the current in the external circuit now flows from B1 to B2. The polarity changes after every half rotation. A current that changes direction after equal intervals of time is called alternating current (AC), and this device is called an AC generator.

To get direct current (DC), which does not change direction with time, a split-ring type commutator must be used. One brush is always in contact with the arm moving up in the field and the other with the arm moving down, as in the electric motor, so a unidirectional current is produced and the generator is called a DC generator. Direct current always flows in one direction, whereas alternating current reverses its direction periodically.

Teacher's Note:
a) Refer to the labelled AC generator diagram in the notes above, with rings R1 and R2 and brushes B1 and B2.
b) The brushes are a sliding contact, so the current can flow out while the coil keeps rotating.

17. When does an electric short circuit occur?

Answer: A short circuit occurs when the resistance of the circuit becomes very low and the current becomes very high. This happens when the live and neutral wires touch each other because their insulation has worn out, or when too many appliances or high power rating appliances are connected to one circuit.

Teacher's Note:
a) A short circuit is different from overloading, though both cause a high current.
b) A fuse in the circuit protects it from the high current.

18. What is the function of an earth wire? Why is it necessary to earth metallic appliances?

Answer: The earth wire connects the metallic body of an appliance to the earth, so that any leakage of current is transferred to the ground. This prevents electric shock to the user, which is why earthing of metallic appliances is necessary.

Teacher's Note:
a) The earth wire normally carries no current, so it is safe to touch an earthed appliance.
b) Earthing is one of the two safety measures for domestic circuits, along with the fuse.

3. Why don’t two magnetic lines of force intersect each other?
Ans. If two magnetic lines of force intersect then there would be two directions of magnetic field at that point, which is absurd. That is why they never intersect.

• strength of current(I)

• nature of core material used in solenoid – use of soft iron as core in a solenoid produces the strongest magnetism.