13. Magnetic Effects of Electric Current

Magnetic Field and Field Lines

Every magnet has a region around it in which it can influence any other magnet or magnetic substance. This region is known as its magnetic field. In a magnetic field, a magnetic substance will experience attraction or repulsion.

Magnetic field is indicated by imaginary curved lines. These lines are called “magnetic field lines” or “field lines”. These field lines are more intense closer to the magnet and get weaker as the distance from the magnet increases. The closeness of field lines indicates that the magnetic field is stronger in regions close to the magnet. Inside the magnet, these field lines are parallel to each other. Outside the magnet, they are closed curves that originate from the north pole and terminate at the south pole. Also, these field lines are closer to each other near the poles. At any place in a magnetic field, the field line will have only one direction. Thus, two magnetic field lines would never intersect each other.

If a magnetic compass is placed in a magnetic field of another magnet, it orients itself in the direction of the magnetic field as shown below:

Magnetic Field in a Current Carrying Conductor

When electric current is passed through a straight conductor, a magnetic field gets generated around it. Just like the magnetic field around a magnet, magnetic field around a conductor in stronger in regions close to it than regions farther away.

The direction of magnetic field in a current carrying conductor is given by the right-hand thumb rule, which states that “if the conductor is held with the right hand in a manner that the thumb is stretched out in the direction of the current, then the curl of the fingers indicates the direction of the magnetic field”.

If the direction of current is reversed in the conductor, the direction of magnetic field also gets reversed.

Magnetic Field in a Circular Loop

When current is passed through a circular loop, each point of the loop behaves like a straight conductor. Thus the right-hand thumb rule can be applied at any point in the circular loop.

If you want to know the direction of magnetic field at any point in a circular loop, stretch out your thumb and hold the loop at that point with your right hand in a manner that the thumb points in the direction of current. The curl of the fingers will point towards the direction of magnetic field at that part of the loop. At any point in the loop, the field lines are concentric circles.

The magnetic field is perpendicular to the place of the plane of the loop.

Just as in the case of a straight conductor, the direction of magnetic field reverses if the direction of current is reversed.

At the centre of the loop, the magnetic field is uniform and the field lines are straight and parallel. The strength of the magnetic field increases as we move towards the centre of the loop.

Magnetic Field due to a Current in a Solenoid

A solenoid is a long coiled wire used to produce a uniform magnetic field. When current is passed through a solenoid, each of its parts behaves like a straight conductor. Thus, the right-hand thumb rule can be applied at each part to know the direction of magnetic field.

The magnetic field of a solenoid looks similar to that of a bar magnet. Inside the solenoid, magnetic field is constant and field lines are parallel to each other. Outside the solenoid, the field lines are closed curves that start from the end attached to the negative terminal and terminate at the end attached to the positive terminal. Field lines are closer at the last loops indicating that the magnetic field is stronger there.

If the number of loops of a solenoid is increased, its magnetic field also increases. A solenoid is used to magnetize magnetic materials like iron, cobalt and nickel. When these substances are placed inside a solenoid and current is passed through it, they get magnetized.

Force on a Current-Carrying Conductor

When a magnetic compass is placed close to a current-carrying conductor, the direction of the compass needle changes. This is because the current applies a force on the magnet. Similarly, a magnet also applies a force on a current-carrying conductor.

As shown in the image, a rod AB is connected to a battery through wires. A horse-shoe magnet is placed perpendicular to the rod. When current passes through the rod, the magnetic field pushes it to one direction. The direction of movement is given by Fleming’s left-hand rule.

According to this rule, if you stretch out your thumb, index finger and middle finger in a manner that they all are perpendicular to each other, if the middle finger points in the direction of current and index finger in the direction of magnetic field, then the thumb points in the direction of force applied on the current-carrying conductor by the magnetic field.

Electric Motor

An electric motor is a machine that converts electrical energy to mechanical energy. The working of an electric motor is based on Fleming’s left-hand rule.

A rectangular coil is placed between the poles of a magnet in a manner that the plane of the coil is in the direction of the magnetic field. Current is passed through this coil. Applying the left-hand rule to side AB and CD of this coil, if the index finger points towards the magnetic field and middle finger in the direction of current, then force is applied on the coil in the direction of the thumb. This force is downwards on side AB, and upwards on the side CD. The coil thus turns in an anti-clockwise direction.

After a half rotation, the direction of current in the coil gets reversed. On side AB, the direction of force is downwards while on side CD, the direction of force is upwards. The coil thus turns in a clockwise direction. This leads to a flip-flop movement of the coil, continuing as long as the current passes through the coil.

To avoid this flip-flop movement, each side of the coil is attached to one part of a split-ring commutator. Parts of the split ring commutator are attached to two terminals of a source of electricity through two carbon brushes. Due to the split ring commutator, the direction of current in any arm of the coil gets reversed after every half rotation. This leads to reversing of force on the coil and hence its movement becomes continuous.

The force on the coil is maximum when the coil and the magnetic field are in the same plane. It is minimum when the planes of the coil and the magnetic field are perpendicular to each other. To reduce the fluctuation in speed of rotation, several coils are added to form a complete circle. Thus the total force on the coils becomes constant and we have a machine which rotates due to electrical energy.

This principle of electric motor is used in case of a car fan, electric car, electric scooter or a battery rickshaw.

Electromagnetic Induction

When a conductor moves in a magnetic field or the magnetic field is varied around a static conductor, electricity is induced in the conductor. This phenomenon is known as electromagnetic induction.

The direction of current induced is given by Fleming’s right-hand rule. Stretch the thumb, the index finger and the middle finger of your right hand in a manner that they all are perpendicular to each other. If your thumb points in the direction of movement of the conductor or the force and the index finger points in the direction of magnetic field, then the current will be induced in the direction of the middle finger.

AC Generator

An AC generator is a machine that converts mechanical energy to Alternating Current. It works on the basis of Fleming’s right-hand rule.

An armature coil is perpendicular to the magnetic field of a horse shoe magnet in a manner that the plane of the coil is parallel to the direction of the magnetic field. The coil is rotated mechanically in a clockwise direction. Applying the right-hand rule to side AB and CD of this coil, the thumb points in the direction of force and index finger in the direction of magnetic field. Then, the middle finger will point in the direction of induced current. On side AB, the direction of force is upwards while on side CD, it is downwards. The direction of current will, thus, be BACD.

As the coil continues to rotate in the clockwise direction, after half a rotation, the direction of force in the coil gets reversed. On side AB, the direction of force is upwards while on side CD, it is downwards. Thus, the direction of current in the coil gets reversed after every half cycle.

To avoid this, each arm of the armature coil is attached to slip rings which are insulated from each other. These slip-rings are connected to an external circuit through carbon brushes and an axle. The axle is rotated mechanically to rotate the armature coil. When AB is moving from north pole to south pole, the galvanometer needle will point towards the negative direction. When AB is moving from south pole to north pole, the galvanometer needle will point towards the positive direction. The direction of induced current gets reversed after every half cycle, thus it’s called Alternating Current (AC).

DC Generator

A DC generator is a machine to convert mechanical energy to Direct Current.

An armature coil is perpendicular to the magnetic field of a horse shoe magnet in a manner that the plane of the coil is parallel to the direction of the magnetic field. The coil is rotated mechanically in a clockwise direction. Applying the right-hand rule to sides AB and CD of this coil, if the thumb points in the direction of force and the index finger in the direction of magnetic field, then the middle finger will point in the direction of induced current. On side AB, the direction of force is upwards, while on side CD it is downwards. The direction of current will, thus, be BACD.

As the coil continues to rotate in the clockwise direction, after half a rotation, the direction of force in the coil gets reversed. On side AB, the direction of force is upwards, while on side CD it is downwards. Thus, the direction of current in the coil gets reversed after every half cycle.

To nullify the change in direction of the induced current, each arm of the armature coil is attached to one half of a split-ring commutator in a DC generator. The two halves of the commutator are insulated from each other. The split-ring commutator is connected to an external circuit with carbon brushes and an axle which mechanically rotates the coil. After half a rotation, the direction of current in one arm gets reversed. But, thanks to the commutator, the current produced is unidirectional.

Domestic Circuits

Any household receives electric supply through overhead or underground cables. What comes to our house is a pair of wires. One of the wires is live (or positive) and is insulated in a red casing while the other is neutral (or negative) and is insulated in black. The pair of wires first reaches the meter through a main fuse. The main fuse prevents the house from fire-like situations due to overheating of electric circuits. The earth wire with a green insulation connects each circuit inside the house to a metallic plate dug deep into the ground. Each appliance in the house is connected through these three wires by forming parallel circuits. Heavy equipment like geysers, room heaters, microwave oven, air conditioners etc require 15A plug sockets while smaller equipment like mobile chargers, table fans, mixer-grinder, TVs etc. require 5A plug sockets.

Fuse

A fuse is a simple device that is connected in series in any circuit to protect electrical equipment from heavy current flows. A fuse has a wire which gets heated and breaks at a certain temperature. This temperature is generally lower than a temperature which would damage electrical equipment. When excess currents flows in a circuit, the fuse wire heats up and will break before the excess current can damage any appliance. Thus, the appliance is protected from excess current flow by the fuse.

Earthing

Earthing is a system to protect a person from shock in case of excess current flow. A metal rod is dug into the ground outside a house while a small part of it stays above the ground. The part above the ground is connected to wire covered in green insulation. The other end of the green wire is connected to the electrical circuits in a house, forming one of the three wires of the domestic wiring. This green wire provides an alternative path for the flow of excess current from an internal domestic circuit during leakages or lightning, thus reducing the chance of someone getting an electric shock by touching the appliances.

Questions

Q1	What are the applications of electric motors?

Q2	What is a commutator?

Q3	What is a magnetic field line?

Q4	What is the use of a galvanometer?

Q5	What is electromagnetic induction?

Q6	What is a generator?

Q7	What is an electromagnet?

Q8	Illustrate and explain the right-hand thumb rule.

Q9	Draw magnetic field lines of forces around a current-carrying circular wire.

Q10	Explain the magnetic field of a magnet.

Q11	Illustrate and describe Fleming’s right-hand rule.

Q12	Describe the magnetic field due to a current in a solenoid.

Q13	Illustrate and explain Fleming’s left-hand rule.

Q14	List few devices that use current-carrying conductors and magnetic fields.

Q15	Explain the application of magnetism in medicine.

Q16	Difference between an AC and DC.

Q17	Illustrate the common domestic circuits with a schematic diagram. Explain the role of an earth, neutral and live wire.

Q18	Explain the importance of an electric fuse in domestic circuit.

Q19	Show an electric motor with a diagram. How does a split-ring help in making the direction of rotation constant instead of flip-flop?

Q20	Differentiate between an AC generator and a DC generator.

Questions

Q1	What are the applications of electric motors?

A1	An electric motor is a rotating device that converts electrical energy to mechanical energy. It is an important component in electric fans, refrigerators, mixers, washing machines, computers, MP3 players, etc.

Q2	What is a commutator?

A2	A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors and DC generators, the split-ring acts as a commutator.

Q3	What is a magnetic field line?

A3	A field line is the path along which a hypothetical free north pole would tend to move. The direction of the magnetic field at a point is given by the direction that a north pole placed at that point would take.

Q4	What is the use of a galvanometer?

A4	A galvanometer is an instrument that can detect the presence of a current in a circuit.

Q5	What is electromagnetic induction?

A5	The production of electromotive force in a current-carrying conductor when placed under the influence of a magnetic field that varies with respect to time is called electromagnetic induction.

Q6	What is a generator?

A6	A generator is an electric device that converts mechanical energy into electrical energy. It works on the basis of electromagnetic induction.

Q7	What is an electromagnet?

A7	An electromagnet is temporary magnet that works on the basis of magnetic effect of electric current. It consists of a core of soft iron wrapped around with a coil of insulated copper wire.

Q8	Illustrate and explain the right-hand thumb rule.

Q9	Draw magnetic field lines of forces around a current-carrying circular wire.

A9

Q10	Explain the magnetic field of a magnet.

A10

Magnetic field is the region around a magnet in which any magnetic substance experiences a force. The direction of force on a magnetic substance is demonstrated by the magnetic field lines. The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it. Therefore, it is taken by convention that the field lines emerge from north pole and merge at the south pole. Inside the magnet, the direction of field lines is from its south pole to its north pole. Thus, the magnetic field lines are closed curves.

Q11	Illustrate and describe Fleming’s right-hand rule.

A11	Flemings Right hand rule: It states that if you stretch the thumb, forefinger and middle finger of your right hand in such a way that they are perpendicular to each other and if the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of the conductor, then the middle finger will show the direction of the induced current.

Q12	Describe the magnetic field due to a current in a solenoid.

A12

An insulated copper wire coiled closely in the shape of a cylinder is called a solenoid. The pattern of the magnetic field lines around a current-carrying solenoid is shown in the figure. One end of the solenoid behaves as a magnetic north pole, while the other behaves as the magnetic south pole. The field lines inside the solenoid are in the form of parallel straight lines. This indicates that the magnetic field is the same at all points inside the solenoid. That is, the field is uniform inside the solenoid.

Q13	Illustrate and explain Fleming’s left-hand rule.

A13	Fleming’s left-hand rule: According to this rule, if you stretch the thumb, forefinger and middle finger of your left hand in such a way that they are mutually perpendicular and if the forefinger points in the direction of the magnetic field and the middle finger in the direction of the current, then the thumb will point in the direction of motion or the force acting on the conductor.

Q14	List few devices that use current-carrying conductors and magnetic fields.

A14	Devices that use current-carrying conductors and magnetic fields include __________. · Electric motors · Electric generators · Loudspeakers · Microphones · Measuring instruments

Q15	Explain the application of magnetism in medicine.

A15	In the human body, the magnetic field is significantly produced in the heart and the brain. The magnetic field inside the body forms the basis of obtaining the images of the different body parts. This is done by using the technique called Magnetic Resonance Imaging (MRI). Through this technique, doctors diagnose the medical conditions of the body. An image showing MRI of the head.

Q16	Difference between an AC and DC.

A16

The direct current (DC) always flows in one direction, whereas the alternating current(AC) reverses its direction periodically.

A DC is produced by a DC generator whereas an AC is produced by an AC generator.

In India, the AC changes direction after every 1/100 second, that is, the frequency of AC is 50 Hz whereas, the frequency of DC current is 0 Hz.

AC current can be transmitted over long distances without much loss of energy whereas there is a significant power loss when DC current is transferred over long distances.

Q17	Illustrate the common domestic circuits with a schematic diagram. Explain the role of an earth, neutral and live wire.

A17

The diagram shows three types of wires: earth wire, live wire and neutral wire.

The live wire (red in colour) and the neutral wire (black in colour) are connected to the electric box from the poles outside our house or though the underground cables. Electricity, which is produced at power stations, reaches our homes through these cables or wires fixed over the poles.

The earth wire which is green in colour, is connected to the electric appliances.

The role of the earth wire, live wire and neutral are given below.

Earth wire: The earth wire, which has insulation of green colour, is usually connected to a metal plate deep in the earth near the house. The other end of the earth wire is connected to the electric appliance having metallic body. This is used as a safety measure, especially for appliances like iron, table fan, etc. Whenever there is a leakage of current through these appliances, it allows the current to pass to the ground. Thus, it prevents the user from getting electric shock due to the leakage of electric current.
> Live wire: The live wire is connected directly to the power source. Electricity is supplied to different appliances through the live wire.
> Neutral wire: The neutral wire returns the electricity (completes the circuit) to the power source after it has passed through an appliance(s).

Q18	Explain the importance of an electric fuse in domestic circuit.

A18

An electric fuse is an important safety device in all domestic circuits. In any circuit, a fuse prevents damage to the appliances and the circuit due to overloading.

A fuse has a short tin-plated copper (fuse wire) having low melting point. It is always connected in series in the electric circuit. It works on the principle of heating effect of current.

When the current, flowing through the circuit, remains within a safe value, the fuse wire acts like a conductor and lets the current flow in the circuit.

When current flowing through the circuit exceeds a safe value, the appliance can get damaged. A large amount of current can also flow through a circuit when too many electrical appliances are connected to a single socket. If a fuse is connected in the circuit and the current flowing through the circuit exceeds a safe value, the fuse wire melts and breaks the circuit. It, thus, stops the flow of very high current and prevents the electric circuit and the appliance from a possible damage.

Q19	Show an electric motor with a diagram. How does a split-ring help in making the direction of rotation constant instead of flip-flop?

A19

In an electric motor, the split-rings act as a commutator. It helps to reverse the direction of current in the circuit.

The role of the split-ring can be understood from the diagram. An electric motor consists of a rectangular coil (ABCD) of insulated copper wire placed between the two poles of a magnetic field in such a way that the arm 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 the split-rings are insulated and attached to an axle.

The external conducting edges (P and Q) touch two conducting stationary brushes say X and Y, respectively.

When a current is allowed to flow through the coil ABCD, due to magnetic forces, an upward force acts on length CD and a downward force acts on length AB simultaneously. As a result, the coil starts rotating anti-clockwise.

Similarly, when the coil completes half rotation, the positions of DC and BA interchange. The half-ring D comes in contact with brush X and half-ring Y gets reversed. Now the direction of current is through the coil DCBA. It reverses after every half rotation. As a result, the coil rotates in the same direction.

Q20	Differentiate between an AC generator and a DC generator.

A20

The difference between an AC generator and a DC generator can be understood from the two diagrams.

If the slip rings of an AC generator are replaced by a commutator, the AC generator becomes a DC generator. The differences between the two are given below.

AC generator	DC generator
The armature is connected to two slip-rings.	The armature is connected to a split-ring commutator.
One arm of the armature is fixed to one of the slip-rings while the second arm is fixed to the other.	One arm of the armature is fixed to one half of the commutator while the second arm is fixed to the other half.
One end of the armature is always attached to the same brush.	Each end of the armature alternatively is attached to each of the brushes.
The direction of current changes after every half circle by the coil.	The direction of current does not change.
The armature of the coil is kept fixed while the magnet moves around it.	The magnetic poles are fixed while the armature coil rotates between them.

Lesson: Magnetic effect of electric current

Question: 1

Why does a compass needle get deflected when brought near a bar magnet?

Solution

A needle of a compass is also a magnet. It is a fact that, like poles of two magnets repels each other and unlike poles attract. Due to this repulsion and attraction between the poles, a magnetic needle deflects when brought near a bar magnet.

Question: 2

Draw magnetic field lines around a bar magnet.

Solution

Question: 3

List the properties of magnetic lines of force.

Solution

The properties of magnetic lines of force are as follows:

· These lines are closed curves emerging from the north pole and merging at the south pole.

· The direction of field lines inside the magnet is from the south pole to the north pole.

· The field lines are dense near a magnet and their density decreases as we move away from the magnet.

· The field lines are most intense near the poles, thereby indicating that the magnetic force is strongest near the poles.

· Any two field lines never intersect each other. This means that no point in the magnetic field will ever have more than one direction.

Question: 4

Why don't two magnetic lines of force intersect each other?

Solution

Magnetic lines of forces represent the direction of the magnetic field. If two magnetic field lines intersect each other, it would mean there are two directions of the magnetic field at a same point, which is impossible. Thus, magnetic field lines never intersect each other.

Question: 5

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 out the direction of the magnetic field inside and outside the loop.

Solution

For the current (I) flowing inside a wire, the direction of the magnetic field (B) is shown the diagram. The direction of the magnetic field when the current passes through a circular loop is shown in the diagram below.

The direction of the magnetic field inside the loop appears as if the magnetic fields pierce through the table. Whereas, the direction of the magnetic field outside the loop appears as if the magnetic fields have emerged out of the table.

Question: 6

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

Solution

The magnetic field lines inside a current-carrying solenoid are uniform.

Question: 7

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

Solution

Question: 8

Which of the following property 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

Solution

c, d

Question: 9

In activity displayed below, how do we think the displacement of rod AB will be affected if

(i) Current in rod AB is increased;

(ii) A stronger horse-shoe magnet is used; and

(iii) length of the rod AB is increased?

Solution:

(i) If the current in the rod is increased, the magnetic force exerted on the conductor and the deflection of the rod will increase.

(ii) If a stronger horse-shoe magnet is used, the magnetic force exerted on the conductor and the deflection of the rod will increase.

(iii) If the length of the rod AB is increased, there is no effect on the displacement of the rod.

Question: 10

A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is
(a) Towards south
(b) Towards east
(c) Downward
(d) Upward

Solution

Question: 11

State Fleming's left-hand rule.

Solution

Fleming's left hand rule: If we arrange the thumb, the forefinger, and the middle finger of the left hand at right angles to each other, then the thumb points towards the direction of the magnetic force, the middle finger gives the direction of the current, and the forefinger points in the direction of the magnetic field.

Question: 12

What is the principle of an electric motor?

Solution

The principle of an electric motor is based on the magnetic effect of electric current. A motor works on the principle that when a current-carrying rectangular coil is placed in a magnetic field; it experiences a force and rotates. The direction of rotation of the loop is given by the Fleming’s left-hand rule.

Question: 13

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

Solution

The split-ring commutator in the electric motor reverses the direction of current flowing through the coil after every half rotation of the coil. This makes the coil rotate in the same direction continuously.

Question: 14

Explain different ways to induce current in a coil.

Solution

Induced current is produced in a coil which is placed in a region where the magnetic field changes with time. The magnetic field can be changed when either the coil is rotated in the magnetic field or the magnet is rotated around a static coil.

Question: 15

State the principle of an electric generator.

Solution

Electric generator works on the principle that when a straight conductor is moved in a magnetic field, then current is induced in the conductor.

Question: 16

Name some sources of direct current.

Solution

Some sources of direct current are:

· Cell,

· DC generator, etc.

Question: 17

Which sources produce alternating current?

Solution

Some sources producing alternating current are:

· AC generators,

· Power plants, etc.

Question: 18

Choose the correct option.

A rectangular coil of copper wires 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

Solution

Question: 19

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

Solution

· Electric fuse

· Proper earthling of all electric circuits

Question: 20

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

Solution

The current drawn by the electric oven can be obtained by the expression

$I = \frac{P}{V}$

where, the power of the oven, $P = 2 kW = 2000 W$ and the voltage supplied,

$V = 220 V$

Therefore, $I = \frac{2000}{220} = 9.09 A$
Since, the current drawn by the electric oven (9.09 A) exceeds the safe limit of the circuit (5A), the fuse element of the electric fuse will melt and break the circuit.

Question: 21

What precaution should be taken to avoid the overloading of domestic electric circuits?

Solution

Some of the precautions that can be taken to avoid the overloading of domestic circuits are as follows:

· Connecting fuse in the circuit

· Not using faulty appliances in the circuit

· Not connecting too many appliances to a single socket

· Not using too many heavy appliances at the same time

Lesson: Magnetic Effects of Electric Current

Question: 1

Choose the incorrect statement from the following regarding magnetic lines of field:

(a) The direction of magnetic field at a point is taken to be the direction in which the north pole of a magnetic compass needle points

(b) Magnetic field lines are closed curves

(d) Relative strength of magnetic field is shown by the degree of closeness of the field lines

Solution:

Question: 2

If the key in the arrangement in the figure below is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane ABCD, the lines are

(a) Concentric circles

(b) Elliptical in shape

(d) Concentric circles near the point O but of elliptical shapes as we go away from it.

Solution:

Question: 3

A circular loop placed in a plane perpendicular to the plane of paper carries a current when the key is ON. The current as seen from points A and B (in the plane of paper and on the axis of the coil) is anti-clockwise and clockwise respectively. The magnetic field lines point from B to A. The N-pole of the resultant magnet is on the face close to:

(a) A

(b) B

(d) B if the current is small and A if the current is large

Solution:

Question: 4

For a current in a long straight solenoid N- and S-poles are created at the two ends. Among the following statements, the in correct statement is:

(a) The field lines inside the solenoid are in the form of straight lines which indicate that the magnetic field is the same at all points inside the solenoid

(b) The strong magnetic field produced inside the solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the

coil

(c) The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet.

(d) The N- and S-poles exchange position when the direction of current through the solenoid is reversed

Solution:

Question: 5

A uniform magnetic field exists in the plane of paper pointing from left to right as shown in figure. In the field an electron and a proton move as shown. The electron and the proton experience:

(a) Forces both pointing into the plane of paper

(b) Forces both pointing out of the plane of paper

(d) Force pointing opposite and along the direction of the uniform magnetic field respectively

Solution:

Question: 6

Commercial electric motors do not use:

(a) An electromagnet to rotate the armature

(b) Effectively large number of turns of conducting wire in the current-carrying coil

(d) A soft iron core on which the coil is wound

Solution:

Question: 7

In the arrangement shown in the figure, there are two coils wound on a non-conducting cylindrical rod. Initially the key is not inserted. Then the key is inserted and later removed. Then

(a) The deflection in the galvanometer remains zero throughout

(b) There is a momentary deflection in the galvanometer but it dies out shortly and there is no effect when the key is removed

(d) There are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions

Solution:

Question: 8

Choose the incorrect statement

(a) Fleming’s right-hand rule is a simple rule to know the direction of induced current

(b) The right-hand thumb rule is used to find the direction of magnetic fields due to current-carrying conductors

(c) The difference between the direct and alternating currents is that the direct current always flows in one direction; whereas the alternating current reverses its direction periodically

(d) In India, the AC changes direction after every $\frac{1}{50}$ second

Solution:

Question: 9

A constant current flows in a horizontal wire in the plane of the paper from east to west as shown in figure. The direction of magnetic field at a point will be North to South:

(a) Directly above the wire

(b) Directly below the wire

(d) At a point located in the plane of the paper, on the south side of the wire

Solution:

Question: 10

The strength of magnetic field inside a long current carrying straight solenoid is

(a) More at the ends than at the centre

(b) Minimum in the middle

(d) Found to increase from one end to the other

Solution:

Question: 11

To convert an AC generator into DC generator:

(a) Split-ring type commutator must be used

(b) Slip rings and brushes must be used

(d) A rectangular wire loop has to be used

Solution:

Question: 12

The most important safety method used for protecting home appliances from short circuiting or overloading is:

(a) Earthing

(b) Use of fuse

(d) Use of electric meter

Solution:

Question: 13

A magnetic compass needle is placed in the plane of paper near point A as shown in figure. In which plane should a straight current-carrying conductor be placed so that it passes through A and there is no change in the deflection of the compass? Under what condition is the deflection maximum and why?

Solution:

When the magnetic field and the current-carrying conductor are in the same plane, the deflection in the conductor will be zero.

When the magnetic field and the direction of current are in mutually perpendicular directions to each other, the deflection in the conductor is maximum.

Question: 14

Under what conditions permanent electromagnet is obtained if a current-carrying solenoid is used? Support your answer with the help of a labelled circuit diagram.

Solution:

Electric current can be used to make temporary magnets called electromagnets. An electromagnet consists of a long coil of insulated copper wrapped inside around a soft iron core.

This iron core is magnetized when current passes through the coil.

Permanent electromagnet is obtained when:

1.The current flowing through the solenoid is direct current.

2. Magnetic material like steel is used instead of the soft iron core.

Question: 15

AB is a current carrying conductor in the plane of the paper as shown in figure. What are the directions of magnetic fields produced by it at points P and Q? Given $r_{1} > r_{2}$ , where will the strength of the magnetic field be larger?

Solution:

The direction of the magnetic force at P and Q are anti-clockwise. At P, it is into the plane of paper and at Q, it is out of it.

The strength of the magnetic field reduces with distance. Since Q is closer than P, the strength of the magnetic field is larger at Q.

Question: 16

A magnetic compass shows a deflection when placed near a current-carrying wire. How will the deflection of the compass get affected if the current in the wire is increased? Support your answer with a reason.

Solution:

The strength of magnetic field around a current-carrying conductor is directly proportional to the magnitude of current passing through it. If the current in the wire is increased, the deflection of the compass too increases.

Question: 17

It is established that an electric current through a metallic conductor produces a magnetic field around it. Is there a similar magnetic field produced around a thin beam of moving (i) alpha particles, (ii) neutrons? Justify your answer.

Solution:

i) Alpha particles: Alpha particles are positively charged. So, a magnetic field would be created around its path.

ii) Neutrons: Neutrons are neutrally charged. So, no magnetic field would be created around its path.

Question: 18

What does the direction of thumb indicate in the right-hand thumb rule? In what way this rule is different from Fleming’s left-hand rule?

Solution:

In right-hand thumb rule, the thumb indicates the direction of current.

The right-hand thumb rule is used to find the direction of a magnetic field when the current passes through a conducting wire.

Right-hand thumb rule: If a current-carrying straight conductor is held with the right hand, such that the thumb points towards the direction of current, then the curl of the fingers points in the direction of the field lines of the magnetic field.

The Fleming’s left-hand rule is used to find the force on a current-carrying conductor in a magnetic field.

Fleming’s left-hand rule: If we stretch our thumb, forefinger and middle finger of our left hand such that they are mutually perpendicular and if the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current, then the thumb points in the direction of motion or the force acting on the conductor.

Question: 19

Meena draws magnetic field lines of field close to the axis of a current-carrying circular loop. As she moves away from the centre of the circular loop, she observes that the lines keep on diverging. How will you explain her observation?

Solution:

The magnetic field lines of a current-carrying circular loop are shown below.

As the distance from the current-carrying conductor increases, the strength of the magnetic field decreases. This is indicated by the reduced density of field lines.

The magnetic field around the wire is stronger and is indicated by more dense concentric circles around the wire than at the centre. The concentric circles representing the magnetic field become bigger as we move away from the wire. By the time we reach the centre of the circular loop, the arcs of these big circles would appear as straight lines.

Thus, when Meena moves away from the centre of the circular loop she observes that the lines keep on diverging from the centre.

Question: 20

What does the divergence of magnetic field lines near the ends of a current-carrying straight solenoid indicate?

Solution:

The divergence of magnetic field lines near the ends of a current-carrying straight solenoid indicates that the strength of the field inside the solenoid is more compared to its strength near the ends of it.

Question: 21

Name four appliances wherein an electric motor, a rotating device that converts electrical energy to mechanical energy, is used as an important component. In what respect motors are different from generators?

Solution:

Four appliances wherein an electric motor is used as an important component are:

· Electric fans

· Mixers

· Washing machines

· Vacuum cleaners

A motor converts electrical energy into mechanical energy. A generator, on the other hand, converts mechanical energy into electrical energy.

Question: 22

What is the role of the two conducting stationary brushes in a simple electric motor?

Solution:

The two conducting stationary brushes in a simple electric motor draw current from the battery and supply it to the armature of the motor.

Question: 23

What is the difference between a direct current and an alternating current? How many times does AC used in India change direction in one second?

Solution:

If the current flows in one direction only, it is called a direct current. If the current reverses its direction periodically, it is called alternating current. An alternating current reverses its direction in every $\frac{1}{100^{t h}}$ second. Therefore, AC changes direction 100 times in one second.

Question: 24

What is the role of fuse, used in series with any electrical appliance? Why should a fuse with defined rating not be replaced by one with a larger rating?

Solution:

A fuse is a safety device that protects a device or a circuit from excessive flow of current through it or current overloading. It blows off when a current, more than the rated value, flows through it.

A fuse is used in series connection with an appliance and is connected before the appliance in the circuit. So, the entire current passes through it before passing through the appliance.

A fuse with defined rating should not be replaced by the one with a larger rating, as a rating of fuse is done based on the capacity of the appliance to handle a load or current supplied to it. A larger rating would let overloading or excessive flow of current in the circuit. This may damage the appliances.

Question: 25

Why does a magnetic compass needle pointing North and South in the absence of a nearby magnet get deflected when a bar magnet or a current carrying loop is brought near it. Describe some salient features of magnetic lines of field concept.

Solution:

A magnetic compass needle points to North and South in the absence of a nearby magnet due to the earth’s magnetic field.

The magnetic compass is deflected when a bar magnet or a current carrying loop is brought near it, as the magnetic field of the bar magnets or the current-carrying loop acts on the compass needle.

Salient features of magnetic field lines are given below.

1. Magnetic field lines emerging from the North Pole converge to the South Pole.

2. Magnetic field lines do not intersect one another.

3. Closer field lines indicate stronger magnetic field and vice-versa.

4. Magnetic field lines are closer near the poles.

5. Inside the magnet, the direction of field lines is from its south pole to its north pole.

6. Magnetic field has a direction and a magnitude at a particular point around a magnet.

Question: 26

With the help of a labelled circuit diagram illustrate the pattern of field lines of the magnetic field around a current-carrying straight long conducting wire. How is the right-hand thumb rule useful to find direction of magnetic field associated with a current-carrying conductor?

Solution:

Right-hand thumb rule is useful to find the direction of the magnetic field around a current-carrying conductor.

If we imagine holding a current-carrying straight conductor in our right hand, such that the thumb points towards the direction of current, then our fingers wrapped around the conductor indicates the direction of the field lines of the magnetic field.

Question: 27

Explain with the help of a labelled diagram the distribution of magnetic field due to a current through a circular loop. Why is it that if a current-carrying coil has n turns, the field produced at any point is n times as large as that produced by a single turn?

Solution:

The magnetic field produced by a current-carrying straight wire depends inversely on the distance of the field line from it.

At every point of a current-carrying circular loop, the concentric circles represent the magnetic field around it. These circles become larger and larger as we move away from the wire and appear as straight lines as we reach the centre of the loop. Thus, every point on the wire carrying current would give rise to the magnetic field appearing as straight lines at the centre of the loop.

Number of turns of coil

The magnetic field at a point is the sum of fields produced by each turn of the coil. Therefore, if there are 'n' turns of coil, the magnitude of the magnetic field will increase ‘n’ times that of the magnetic field produced by a single turn of coil.

Question: 28

Describe the activity that shows that a current-carrying conductor experiences a force perpendicular to its length and the external magnetic field. How does Fleming’s left-hand rule help us to find the direction of the force acting on the current carrying conductor?

Solution:

Let’s take a small aluminium rod (AB), a horse-shoe magnet, battery, plug key, wires and a stand as shown in the given figure.

Let’s insert the plug key to initiate current supply to the rod. It is observed that the aluminium rod deflected towards the left. When the direction of the current was reversed the aluminium rod deflected towards the right.

Explanation:

The displacement of the rod in the above activity suggests:

a) A force is exerted on the current-carrying aluminium rod when it is placed in a magnetic field.

b) The direction of force is also reversed when the direction of current through the conductor is reversed.

Fleming’s left-hand rule: The Fleming’s left-hand rule is used to find the direction of the force on a current carrying conductor in the magnetic field. According to this rule, if we stretch our thumb, forefinger and middle finger of our left hand such that they are mutually perpendicular and if the first finger points in the direction of the magnetic field and the second finger in the direction of the current, then the thumb will point in the direction of motion or the force acting on the conductor.

Question: 29

Draw a labelled circuit diagram of a simple electric motor and explain its working. In what way these simple electric motors are different from commercial motors?

Solution:

Rectangular coil: An electric motor consists of a rectangular coil; let’s say ABCD, of insulated copper wire.

Magnetic poles: The coil is placed between the two poles of a magnetic field such a way that the arm AB and CD are perpendicular to the direction of the magnetic field.

Split-Rings: The ends of the coil are connected to the two halves P and Q of a split-ring.

Axle: The inner sides of the split-rings are insulated and attached to an axle.

External conducting edges: The external conducting edges say P and Q touch two conducting stationary brushes say X and Y, respectively.

How it works:

a) When a current is allowed to flow through the coil ABCD, due to magnetic forces, an upward force acts on length CD and a downward force acts on length AB simultaneously. As a result, the coil starts rotating anti-clockwise.

b) When the coil completes half rotation, the position of DC and BA interchange. The half-ring D comes in contact with brush X and half-ring Y gets reversed.

c) Now the direction of current is through the coil DCBA. It reverses after every half rotation. As a result, the coil rotates in the same direction.

d) The split-rings help to reverse the direction of current in the circuit.

A commercial motor uses:

a) an electromagnet whereas an electric motor uses a permanent magnet.

b) large number of turns of the conducting wire in the current-carrying coil.

c) a soft iron core on which the coil is wound.

Question: 30

Explain the phenomenon of electromagnetic induction. Describe an experiment to show that a current is set up in a closed loop when an external magnetic field passing through the loop increases or decreases.

Solution:

Electromagnetic induction:

The process by which the motion of a magnet with respect to the current-carrying conductor produces an induced potential difference, thereby setting up an induced electric current in the circuit is called electromagnetic induction.

Experimental set up:

Let’s take a coil of wire AB having a large number of turns and connect the ends of the coil to a galvanometer.

Let’s take a strong bar magnet and move its north pole towards the end B of the coil, stop for a moment and then withdraw the north pole of the magnet away from the coil.

Observation: We will observe a momentary deflection of the needle of the galvanometer. This indicates the presence of a current in the coil AB. There is no deflection, the moment the motion of the magnet stops. The galvanometer is again deflected when the North Pole is withdrawn.

Conclusion: A moving magnet induces electric current in a current conducting wire.

Question: 31

Describe the working of an AC generator with the help of a labelled circuit diagram. What changes must be made in the arrangement to convert it to a DC generator?

Solution:

Construction of an AC generator:

a) An electric generator consists of a rotating rectangular coil say ABCD placed between the two poles of a permanent magnet.

b) The ends of the coil are connected to the two rings say R₁ and R₂ insulated from each other.

c) There are two conducting stationary brushes say B₁ and B₂. These are kept pressed separately on the rings $R_{1} and R_{2}$ , respectively.

d) R₁ and R₂ are internally attached to an axle.

e) The axle is mechanically rotated from outside to rotate the coil inside the magnetic field.

f) The outer ends of the two brushes are connected to the galvanometer.

g) The galvanometer shows the flow of current in the given external circuit.

Working of an AC generator:

When the axle, attached to the two rings, is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet, it induces electric current along the directions AB and CD. Thus, an induced current flows in the direction ABCD.

The current in the external circuit flows from $B_{2} to B_{1}$ .

After half a rotation, arm CD starts moving up and AB moving down. These results induced currents in the direction DCBA. The current in the external circuit now flows from B₁ to B₂.

There is a change in direction of the induced current after every half rotation in equal interval of time. The current produced is called AC current.

Converting AC current to DC current:

DC current does not change its direction after a fixed interval of time. To convert AC current to DC current, we can use a split-ring commutator in place of slip-rings. This will ensure no change in the direction of the induced current.

Question: 32

Draw an appropriate schematic diagram showing common domestic circuits and discuss the importance of fuse. Why is it that a burnt-out fuse should be replaced by another fuse of identical rating?

Solution:

A fuse is a safety device that protects a device or a circuit from excessive flow of current through it or overloading. It blows off when a current, more than the rated value, flows through it. A fuse is used in series connection with an appliance so that the entire current passing through the appliance also passes through it.

A fuse with defined rating should be replaced by another with identical rating as the rating of fuse is done based on the capacity of the appliance to handle the load or current supplied to it.

So, a larger rating would let overloading or excessive flow of current in the circuit. This may damage the appliances.

Similarly, a smaller rating would not let the appliance in the circuit work as it will burn before the required amount of current flows through the circuit.

13. Magnetic Effects of Electric Current

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