Chapter 12: Magnetic Effects of Electric Current Exercise & In-Text Questions & Answers
1Why does a compass needle get deflected when brought near a bar magnet?
Answer & Reference:
A compass needle gets deflected when brought near a bar magnet because the compass needle is itself a small magnet. The magnetic field of the bar magnet exerts a force on the compass needle, causing it to deflect. (Page 196)
2Draw magnetic field lines around a bar magnet.
Answer & Reference:
Magnetic field lines around a bar magnet emerge from the North pole and merge at the South pole. They form closed curves and are crowded near the poles where the magnetic field is stronger. (Page 197, Fig. 12.4)
3List the properties of magnetic field lines.
Answer & Reference:
Properties of magnetic field lines:
1. They are closed curves.
2. They emerge from the North pole and merge at the South pole outside the magnet.
3. Inside the magnet, they go from South pole to North pole.
4. The relative strength of the magnetic field is shown by the degree of closeness of the field lines.
5. No two field lines cross each other. (Page 197)
4Why don't two magnetic field lines intersect each other?
Answer & Reference:
Two magnetic field lines do not intersect each other because if they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible. (Page 197)
5Consider 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 & Reference:
Using the right-hand thumb rule: If we curl the fingers of our right hand along the direction of current (clockwise), then the thumb points into the table. Therefore, the magnetic field inside the loop is directed perpendicular to the plane of the table in the downward direction (into the table). Outside the loop, the magnetic field is directed upward (out of the table). (Page 201)
6The magnetic field in a given region is uniform. Draw a diagram to represent it.
Answer & Reference:
A uniform magnetic field is represented by parallel, equidistant straight lines with arrows indicating the direction of the field. (Page 201)
7Choose 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 & Reference:
(d) is the same at all points.
The magnetic field inside a long straight solenoid carrying current is uniform, which means it is the same at all points. (Page 202)
8Which 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 & Reference:
(c) velocity and (d) momentum.
When a charged particle like a proton moves in a magnetic field, the magnetic force acts perpendicular to its direction of motion. This force changes the direction of motion (velocity) but not the speed. Since momentum depends on velocity (p = mv), it also changes. Mass and speed remain constant. (Page 203)
9In 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 & Reference:
(i) If current in rod AB is increased, the displacement will increase because the force acting on the rod is directly proportional to the current.
(ii) If a stronger horse-shoe magnet is used, the displacement will increase because the force is directly proportional to the magnetic field strength.
(iii) If the length of the rod AB is increased, the displacement will increase because the force is directly proportional to the length of the conductor in the magnetic field. (Page 203)
10A 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 & Reference:
(d) upward.
Using Fleming's left-hand rule:
- Direction of current (positive charge moving west) → West
- Direction of force (deflection) → North
- Therefore, direction of magnetic field → Upward (out of the page) (Page 204)
11Name two safety measures commonly used in electric circuits and appliances.
Answer & Reference:
Two safety measures commonly used in electric circuits and appliances are:
1. Electric fuse
2. Earthing (connecting the metallic body of appliances to the earth wire) (Page 205)
12An 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 & Reference:
Power of the oven, P = 2 kW = 2000 W
Voltage, V = 220 V
Current drawn by the oven, I = P/V = 2000/220 ≈ 9.09 A
Since the current drawn by the oven (9.09 A) is more than the circuit's current rating (5 A), the fuse will melt and break the circuit to prevent damage. (Page 205)
13What precaution should be taken to avoid the overloading of domestic electric circuits?
Answer & Reference:
Precautions to avoid overloading:
1. Do not connect too many appliances to a single socket.
2. Use appliances within their power rating.
3. Ensure proper insulation of wires.
4. Use appropriate fuse rating for different circuits. (Page 205)
14Which 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 & Reference:
(d) The field consists of concentric circles centred on the wire.
The magnetic field lines around a straight current-carrying conductor form concentric circles with the wire at the center. (Page 199, Fig. 12.6)
15At the time of short circuit, the current in the circuit:
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.
Answer & Reference:
(c) increases heavily.
During a short circuit, the live and neutral wires come into direct contact, causing the current to increase abruptly to a very high value. (Page 205)
16State 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 & Reference:
(a) True. The magnetic field inside a long solenoid (circular coil) is uniform and consists of parallel straight lines.
(b) False. A wire with green insulation is the earth wire, not the live wire. The live wire has red insulation. (Page 202, 204)
17List two methods of producing magnetic fields.
Answer & Reference:
Two methods of producing magnetic fields:
1. Using a permanent magnet (like a bar magnet).
2. Passing an electric current through a conductor (electromagnet). (Page 196, 202)
18When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
Answer & Reference:
The force experienced by a current-carrying conductor placed in a magnetic field is largest when the direction of current is at right angles (perpendicular) to the direction of the magnetic field. (Page 203)
19Imagine 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 & Reference:
The electron beam is moving from back to front (direction of electron flow). Since electrons are negatively charged, the conventional current direction is from front to back.
Using Fleming's left-hand rule:
- Current direction (conventional): Front to back (away from you)
- Force direction: To your right
- Therefore, magnetic field direction: Vertically downward
(Page 203, 206)
20State 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 & Reference:
(i) Right-Hand Thumb Rule: Imagine holding the current-carrying straight conductor in your right hand such that the thumb points in the direction of current. Then the curled fingers will give the direction of magnetic field lines. (Page 200)
(ii) Fleming's Left-Hand Rule: Stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the forefinger points in the direction of magnetic field and the middle finger in the direction of current, then the thumb will point in the direction of force. (Page 203)
(iii) Fleming's Right-Hand Rule: Stretch the thumb, forefinger and middle finger of your right hand such that they are mutually perpendicular. If the thumb points in the direction of motion of conductor and forefinger in the direction of magnetic field, then the middle finger gives the direction of induced current.
21When does an electric short circuit occur?
Answer & Reference:
An electric short circuit occurs when the live wire and the neutral wire come into direct contact. This can happen due to damaged insulation of wires or a fault in the appliance. During a short circuit, the current in the circuit increases heavily. (Page 205)
22What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer & Reference:
The earth wire provides a low-resistance conducting path for the electric current to flow to the earth in case of any leakage of current to the metallic body of an appliance. It is necessary to earth metallic appliances to prevent the user from getting a severe electric shock. When the metallic body is connected to the earth wire, any leakage current keeps the potential of the appliance to that of the earth. (Page 204)
23Compare the magnetic field of a solenoid with that of a bar magnet.
Answer & Reference:
The magnetic field produced by a current-carrying solenoid is similar to that of a bar magnet. One end of the solenoid behaves as a magnetic north pole, while the other behaves as the south pole. The field lines inside the solenoid are in the form of parallel straight lines, indicating that the magnetic field is uniform inside the solenoid. (Page 201-202)
Property
Solenoid
Bar Magnet
Magnetic Field Pattern
Similar to bar magnet with distinct N and S poles
Distinct N and S poles with field lines emerging from N and entering S
Field Inside
Uniform (parallel straight lines)
From S to N pole (not uniform)
Strength Control
Can be changed by varying current or number of turns