Magnetic Effects of Electric Current

Intext Questions 1

Question 1

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

Answer

The compass needle is a small magnet. When the compass needle is brought close to a bar magnet, the magnetic field lines of the compass needle interact with the magnetic field lines of bar magnet causing the compass needle to get deflected.

Intext Questions 2

Question 1

Draw magnetic field lines around a bar magnet.

Answer

Below diagram shows magnetic field lines around a bar magnet:

Question 2

List the properties of magnetic field lines.

Answer

The properties of magnetic field lines are:

1. They emerge from the north pole and merge at the south pole.
2. Inside the magnet, the direction of the field lines is from its south pole to its north pole.
3. The relative strength of the magnetic field is shown by the degree of closeness of the field lines.
4. No two field line are found to cross each other.

Question 3

Why don't two magnetic field lines intersect each other?

Answer

No two field lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible. Hence they do not intersect with each other.

Intext Questions 3

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

Answer

Direction of magnetic field:
Inside the loop — Going into the table.
Outside the loop — Emerging out of the table.

Explanation:
On applying the right-hand rule to the downward direction of the current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging out from the table outside the loop and merging into the table inside the loop.
Similarly, for the upward direction of the current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging out from the table outside the loop and merging into the table inside the loop.

Question 2

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

Answer

A uniform magnetic field in a region is shown by drawing parallel straight lines, all pointing in the same direction.

Question 3

Choose the correct option.
The magnetic field inside a long straight solenoid-carrying current

1. is zero.
2. decreases as we move towards its end.
3. increases as we move towards its end.
4. is the same at all points.

Answer

is the same at all points

Reason — The magnetic field inside a long straight current-carrying solenoid is uniform. Therefore, it is the same at all points.

Intext Questions 4

Question 1

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.)

1. Mass
2. Speed
3. Velocity
4. Momentum

Answer

velocity and momentum

Reason — When a proton enters the region of a magnetic field, it experiences a magnetic force. Due to this, the path of the proton becomes circular. Hence, its velocity and the momentum changes.

Question 2

In Activity 12.7, 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?

Answer

(i) The displacement of rod AB increases when current is increased. This is because the magnetic force experienced by the rod increases with increased current leading to larger displacement of the rod.

(ii) The displacement of rod AB increases when a stronger horse-shoe magnet is used. This is because the magnetic force experienced by the rod increases with increase in the strength of magnetic field leading to larger displacement of the rod.

(iii) The displacement of rod AB increases when its length is increased. This is because the magnetic force experienced by the rod increases with increased length leading to larger displacement of the rod.

Question 3

A positively-charged particle (alpha-particle) projected towards the west is deflected towards north by a magnetic field. The direction of magnetic field is

1. towards south
2. towards east
3. downward
4. upward

Answer

upward

Reason — The direction of the magnetic field can be determined using Fleming's Left-hand rule. According to this rule, if we arrange our thumb, forefinger and the middle finger of the left hand right perpendicular to each other, then the thumb points towards the direction of the magnetic force, the middle finger towards the direction of current and the forefinger gives the direction of magnetic field.

As the direction of the positively charged particle is towards the west, the direction of the current will also be towards the west. The direction of the magnetic force is towards the north. Hence the direction of the magnetic field will be upward.

Intext Questions 5

Question 1

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

Answer

Two safety measures commonly used in electric circuits and appliances are:

1. Earthing — Earthing protects the user from electric shocks as it provides a low resistance conducting path for the current. Any leakage of current in an appliance is transferred to the ground by earthing, hence the user may not get a severe electric shock.
2. Fuse — It prevents damage to the appliance due to overloading. When the current in the circuit exceeds the maximum limit of the fuse element, the fuse melts to stop the flow of current protecting the appliance connected to the circuit.

Question 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

The current drawn by the electric oven :

P = V x I

I = $\dfrac{\text{P}}{\text{V}}$

Substituting the values, we get,

I = $\dfrac{2000}{220}$ = 9.09 A

The current drawn by the electric oven is 9.09 A which exceeds the 5A current rating of the circuit, hence, the fuse wire will melt and break the circuit.

Question 3

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

Answer

Precautions to avoid overloading of domestic electric circuits are:

1. We should not connect too many devices to a single socket.
2. We should not use too many appliances at the same time.
3. Faulty appliances should not be connected to the circuit.

Exercises

Question 1

Which of the following correctly describes the magnetic field near a long straight wire?

1. The field consists of straight lines perpendicular to the wire.
2. The field consists of straight lines parallel to the wire.
3. The field consists of radial lines originating from the wire.
4. The field consists of concentric circles centered on the wire.

Answer

The field consists of concentric circles centred on the wire.

Reason — The magnetic field near a long straight wire is concentric circles centred on the wire.

Question 2

At the time of short circuit, the current in the circuit

1. reduces substantially.
2. does not change.
3. increases heavily.
4. vary continuously.

Answer

increases heavily

Reason — During short circuit, the amount of current flowing in the circuit increase heavily.

Question 3

State whether the following statements are true or false.

(a) The field at the center of a long circular coil carrying current will be parallel straight lines.

(b) A wire with a green insulation is usually the live wire of an electric supply.

Answer

(a) True

(b) False
Reason — A wire with a green insulation is the earth wire and wires with red insulation is live wire.

Question 4

List two methods of producing magnetic fields.

Answer

Magnetic fields can be produced by:

1. Using a permanent magnet.
2. Using a current-carrying straight conductor.

Question 5

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

Answer

The force experienced by a current–carrying conductor placed in a magnetic field is largest when the direction of the current is perpendicular to the direction of the magnetic field.

Question 6

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

Answer

The direction of the magnetic field can be determined by applying Fleming's left-hand rule.
The direction of the current is in the opposite direction to the movement of electron beam so it is from the front wall to the back wall. Therefore, the middle finger should point towards the back wall. Also, the direction of force is towards the right, so the thumb points right-side. This gives the direction of fore-finger as downwards Hence, the direction of the magnetic field inside the chamber is downward.

Question 7

State the rule to determine the direction of a

(i) magnetic field produced around a straight conductor carrying current,

(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.

Question 8

When does an electric short circuit occur?

Answer

1. When the insulation of wires in a circuit is damaged or a faulty appliance is used, there is a risk of the live wire and the neutral wire coming in direct contact. This can result in a sudden surge of current in the circuit, leading to a short circuit.
2. When too many appliances are connected to a single socket the current flowing through the circuit increases and short circuit may occur.

Question 9

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

Answer

Earth wire is used as a safety measure to ensure that any leakage of current to the metallic body of appliances does not give any severe shock to the user.
Earthing metallic appliances provides a low-resistance conducting path for the current. Thus, it ensures that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth, and the user may not get a severe electric shock.