Chapter 13: Magnetic Effects of Electric current    

Internal Questions Answers :

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

Answer:  A compass needle gets deflected when brought near a bar magnet due to the magnetic field produced by the magnet. The compass needle aligns itself with the magnetic field lines, indicating the presence and direction of the magnetic field.

Example 13.1: A current through a horizontal power line flows in east to west direction. What is the direction of magnetic field at a point directly below it and at a point directly above it?

Answer : At a point directly below the horizontal power line, the direction of the magnetic field is vertically upward.

At a point directly above the horizontal power line, the direction of the magnetic field is vertically downward.

Internal Questions Answers :

1. Draw magnetic field lines around a bar magnet.

Answer: The diagram of magnetic field lines around a bar magnet :

      

2. List the properties of magnetic lines of force.

Answer:  Properties of magnetic lines of force:

(i) Magnetic field lines emerge from the north pole and enter the south pole outside the magnet, while inside the magnet they go from south pole to north pole. Thus, they form closed continuous curves.

(ii) No two magnetic field lines intersect each other. If they intersect, it would mean that the magnetic field has two directions at the same point, which is not possible.

(iii) The magnetic field lines are closer near the poles where the magnetic field is strong, and farther apart where the field is weak.

(iv) The direction of the magnetic field at any point is given by the direction tangent to the magnetic field line at that point.

3. Why don’t two magnetic lines of force intersect each other?

Answer: Two magnetic lines of force do not intersect each other because if they were to intersect, it would imply that a given point in space has two different directions of the magnetic field, which is not possible.

Internal Questions Answers :

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:  Inside the loop: If you use your right hand and curl your fingers in the direction of the clockwise current flow, your thumb will point towards the center of the loop. So, the direction of the magnetic field inside the loop is towards the center.

Outside the loop: Again, if you use your right hand and curl your fingers in the direction of the clockwise current flow, your thumb will now point away from the center of the loop. So, the direction of the magnetic field outside the loop is away from the center.

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

Answer: A uniform magnetic field is represented by parallel and equally spaced magnetic field lines pointing in the same direction.

        

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: (d) is the same at all points.

[ Inside a long straight solenoid carrying current, the magnetic field is uniform, and its strength is the same at all points along the axis of the solenoid. This is one of the unique characteristics of a well-designed solenoid.]

Example 13.2 : An electron enters a magnetic field at right angles to it, as shown in Fig. 13.14. The direction of force acting on the electron will be

   

(a) to the right.       (b) to the left.      (c) out of the page.       (d) into the page.

Answer : (d) into the page .

[ The direction of force is perpendicular to the direction of magnetic field and current as given by Fleming’s left hand rule. Again, the direction of current is taken opposite to the direction of motion of electrons. The force is therefore directed into the page .]

Internal Questions Answers :

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.)

(a) mass    (b) speed     (c) velocity    (d) momentum

Answer :  (b) speed

(c) velocity

(d) momentum

[ When a proton moves freely in a magnetic field, its speed, velocity, and momentum can change due to the influence of the magnetic force on the charged particle. The magnetic force can alter the direction of the proton's motion, which will affect its velocity and momentum. However, the mass of the proton remains constant.]

2. In Activity 13.7, 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 : The displacement of rod AB will increase in all the three cases:

(i) If the current in rod AB is increased, the force acting on the rod increases. Therefore, the displacement of the rod increases.

(ii) If a stronger horse-shoe magnet is used, the magnetic field becomes stronger. Hence, the force on the rod increases and the displacement also increases.

(iii) If the length of rod AB is increased, a larger part of the rod remains in the magnetic field. This increases the force acting on it, so the displacement increases.

3. 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

Answer: (c) downward

[ Using Fleming’s left-hand rule : velocity is west, force is north. For a positive charge, magnetic field must be vertically downward to produce this deflection.]

Internal Questions and Answers

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

Answer: (i) Use of fuses or circuit breakers: Fuses and circuit breakers are safety devices used in electric circuits to protect against excessive current .

(ii) Grounding: Grounding is a safety measure that involves connecting electrical devices and appliances to the Earth using a ground wire.

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 :  Here,  Power P = 2 kW = 2000 W , V = 220 Volt

Current

The oven draws about 9.1 A current, which is greater than the 5 A rating of the domestic circuit. Therefore, the fuse will melt or the circuit breaker will trip, causing the circuit to break and preventing damage due to overheating.

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

Answer: To avoid overloading domestic electric circuits:

(i)  Limit the use of too many appliances simultaneously.

(ii) Don't overload extension cords with high-power devices.

(iii) Unplug unused appliances.

(iv) Distribute heavy power users across different circuits.

(v) Check circuit ratings before connecting devices.        

(vi) Use energy-efficient appliances with lower power consumption.

Exercises Chapter 13: Magnetic Effects of Electric current :

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.

[ When a current flows through a long straight wire, the magnetic field around the wire forms concentric circles. The direction of the magnetic field lines can be determined using the right-hand rule: if you point your thumb in the direction of the current, the curling of your fingers shows the direction of the magnetic field lines around the wire.]

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

(a) reduces substantially.       (b) does not change.     (c) increases heavily.      (d) vary continuously.

Answer : (c) increases heavily.

[ During a short circuit, the low-resistance path causes a rapid surge in current, as there is little impedance to limit the flow. This can be dangerous and damaging.]

3. State whether the following statements are true or false.

(a) The field at the centre 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) The magnetic field lines at the centre of a long circular coil are nearly parallel straight lines, showing that the magnetic field is uniform there.→ True

(b) A wire with green insulation is usually the earth wire, not the live wire.→ False.

4. List three sources of magnetic fields.

Answer: Three sources of magnetic fields are:

(i) A bar magnet (or a natural magnet) : A permanent magnet always has a magnetic field around it.

(ii) A current-carrying straight conductor (wire) : When electric current flows through a straight wire, a magnetic field is produced around it.

(iii) A current-carrying circular loop (or solenoid) : A circular loop or a coil of wire carrying current also produces a magnetic field similar to that of a bar magnet.

5. 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 behaves like a bar magnet because it produces a magnetic field similar to that around a bar magnet. One end of the solenoid acts as the north pole and the other end acts as the south pole.

     

Yes, the north and south poles of a current-carrying solenoid can be determined using a bar magnet. Bring the north pole of a bar magnet near one end of the solenoid:

(i) If the end of the solenoid repels the north pole of the bar magnet, that end is the north pole of the solenoid.

(ii) If it attracts the north pole of the bar magnet, that end is the south pole of the solenoid.

6. 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 (at right angles) to the direction of the magnetic field.

7. 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 electron beam is moving from the back wall towards the front wall. So, the direction of current is opposite to the motion of electrons, that is, from the front wall towards the back wall.

The electron beam is deflected towards the right side. Using Fleming’s Left-hand Rule, the magnetic field must act vertically downward.

Therefore, the direction of the magnetic field is vertically downward.

8. 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.

Answers:  (i) Right-hand thumb rule: If a straight current-carrying conductor is held in the right hand such that the thumb points in the direction of current, then the curled fingers show the direction of the magnetic field around the conductor.

(ii) Fleming’s Left-hand Rule: Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular to each other. If the forefinger points in the direction of magnetic field and the middle finger points in the direction of current, then the thumb gives the direction of force or motion of the conductor.

(iii) Fleming’s Right-hand Rule: Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular to each other. If the forefinger points in the direction of magnetic field and the thumb shows the direction of motion of the conductor, then the middle finger gives the direction of induced current.

9. When does an electric short circuit occur?

Answer: An electric short circuit occurs when the live wire and neutral wire come into direct contact with each other due to damaged insulation or faulty wiring. This causes a sudden increase in current in the circuit.

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

Answer:  The earth wire provides a safe path for excess or leakage current to flow into the ground.

It is necessary to earth metallic appliances so that if the live wire touches the metal body, current goes to earth instead of passing through a person, preventing electric shock.