In-text Question (Page 195)
Question: Why does a compass needle get deflected when brought near a bar magnet?
Answer: A compass needle is actually a tiny bar magnet. When it is brought near another magnet (like a bar magnet), the magnetic field around the bar magnet acts on the compass needle. This magnetic force causes the compass needle to turn and point in a certain direction. That is why it gets deflected.
In-text Questions (Page 201)
Question 1: Draw magnetic field lines around a bar magnet.
Answer: To draw the magnetic field lines around a bar magnet, follow these steps:
• Place the bar magnet on a sheet of paper.
• Using a compass, mark the direction in which the compass needle points around the magnet.
• These directions, when joined, form curved lines that start from the magnet’s north pole and end at its south pole.
• The field lines are closer near the poles and spread out as you move away.
(Students should draw the usual pattern of field lines emerging from the north pole, curving around, and entering the south pole.)
Question 2: List the properties of magnetic field lines.
Answer: The properties of magnetic field lines are:
• They start from the north pole of a magnet and end at the south pole.
• They never intersect or cross each other.
• They are closer together where the magnetic field is strong (near the poles) and spread out where the field is weak.
• Inside the magnet, the field lines run from the south pole to the north pole.
Question 3: Why don’t two magnetic field lines intersect each other?
Answer: Two magnetic field lines do not intersect because if they did, it would mean there are two different directions of the magnetic field at that point. This cannot happen. Each point in space can have only one direction of the magnetic field.
In-text Questions (Page 201, regarding the circular loop)
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: If the current in the loop is clockwise (as seen from above), then by using the right-hand thumb rule (curl your fingers in the direction of current, and your thumb shows the direction of the field), the magnetic field inside the loop will be directed downwards (towards the table), and outside the loop it will be directed upwards above the plane of the loop.
Question 2: The magnetic field in a given region is uniform. Draw a diagram to represent it.
Answer: A uniform magnetic field can be represented by a set of parallel and equally spaced straight lines pointing in the same direction. (Draw parallel lines with arrows in one direction.)
In-text Questions (Page 203)
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.)
(a) mass (b) speed (c) velocity (d) momentum
Answer:
• The mass of the proton will not change.
• The speed may remain unchanged if the field is perpendicular, but its direction of motion can change, so velocity changes.
• When velocity changes, momentum also changes.
So, the correct answers are (c) velocity and (d) momentum.
Question 2 (Related to Activity 12.7): 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) If the current in the rod AB is increased, the force on it increases, so the rod will move more.
(ii) If a stronger magnet is used, the magnetic field is stronger, so the force is larger, and the rod will move more.
(iii) If the rod AB is made longer, it experiences more force because a longer length in the magnetic field means more pushing force, so it will move more.
Question 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: The correct answer is (d) upward.
(Reason: Using Fleming’s left-hand rule, if the particle moves west and the force is towards north, then the magnetic field must be directed upward.)
In-text Questions (Page 205)
Question 1: Name two safety measures commonly used in electric circuits and appliances.
Answer:
- Electric fuse
- Earthing (grounding) wire
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 a 2 kW oven at 220 V is given by Current = Power/Voltage = 2000 W / 220 V ≈ 9.09 A.
This is much higher than the 5 A rating of the circuit. The fuse will blow or the circuit breaker will trip because the oven needs more current than the circuit can safely provide. This prevents damage or fire.
Question 3: What precaution should be taken to avoid the overloading of domestic electric circuits?
Answer:
To avoid overloading, we should:
• Not connect too many appliances to a single socket.
• Use proper fuses and circuit breakers.
• Use appliances that match the current rating of the household wiring.
Exercise Questions (End of Chapter)
- 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 centered on the wire.
- 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.
- 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) True. The field inside a long circular coil is uniform and represented by parallel lines.
(b) False. The green insulated wire is the earth wire, not the live wire. The live wire is usually red or brown.
- List two methods of producing magnetic fields.
Answer:
- By using a permanent magnet.
- By passing electric current through a conductor (like a straight wire or a coil).
- 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 in the conductor is at right angles (90°) to the direction of the magnetic field.
- 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 direction of the magnetic field is downward.
(If the electron beam moves forward and is deflected to the right, then the magnetic field must be directed downward to produce such a force.)
- State the rule to determine the direction of:
(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) Use the right-hand thumb rule: If you hold the conductor with your right hand so that the thumb points in the direction of current, then the curled fingers show the direction of the magnetic field lines.
(ii) Use Fleming’s left-hand rule: If the forefinger is magnetic field, the second finger is current, then the thumb shows the direction of force.
(iii) Use Fleming’s right-hand rule: If the forefinger is magnetic field, the thumb is motion of the conductor, then the second finger gives the direction of induced current.
- When does an electric short circuit occur?
Answer: A short circuit occurs when the live wire and the neutral wire come into direct contact, allowing a large current to flow suddenly. This happens if the insulation is damaged or there is a fault in an appliance.
- 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 electric current if there is any leakage of current through the metallic body of an appliance. Earthing prevents the user from getting an electric shock by sending the leakage current into the ground instead of through a person’s body.