Question Page no: 196
Question: 1. Why does a compass needle get deflected when brought near a bar magnet?
Answer: A compass needle is itself a small magnet with a north pole and a south pole. When it is brought near a bar magnet, the magnetic field of the bar magnet exerts a force on the poles of the compass needle. This force makes the needle experience a torque, and it gets deflected until it aligns with the bar magnet’s field.
Question Page no: 200
Question: 1. Draw magnetic field lines around a bar magnet.
Answer: The field lines emerge from the north pole, curve around outside the magnet, and enter the south pole. Inside the magnet they run from the south pole back to the north pole, forming closed curves.
Question: 2. List the properties of magnetic field lines.
Answer: Field lines emerge from the north pole of a magnet and merge at its south pole; inside the magnet they run from south to north, so they form closed curves.
- The direction of the magnetic field at any point is the direction in which the north pole of a compass needle points at that point.
- The relative strength of the field is shown by the closeness of the field lines — the closer (more crowded) the lines, the stronger the field.
- No two field lines ever cross each other.
Question: 3. Why don’t two magnetic field lines intersect each other?
Answer: The direction of the magnetic field at a point is the direction in which the north pole of a compass needle points at that point. If two field lines intersected, there would be two directions of the field at the point of intersection, meaning the compass needle would have to point in two directions simultaneously — which is not possible. Hence field lines never cross.
Question Page no: 201 – 202
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: Since the current flows clockwise as seen from above, by the right-hand thumb rule the magnetic field at all points inside the loop is directed into the plane of the table (away from the observer), while outside the loop the field lines are directed out of the plane of the table (towards the observer). The field lines are circular near the wire and nearly straight at the centre 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 is represented by equally spaced 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
(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. The field lines inside a solenoid are parallel straight lines, which means the magnetic field is uniform inside the solenoid.
Question Page no: 203 – 204
Question: 1. Which of the following properties 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: Correct options are (c) velocity and (d) momentum.
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: The force on a current-carrying conductor in a magnetic field increases when the current increases, when the magnetic field becomes stronger, and when the length of the conductor in the field increases. Therefore:
- (i) If the current in rod AB is increased, the displacement of the rod increases.
- (ii) If a stronger horse-shoe magnet is used, the displacement increases.
- (iii) If the length of rod AB is increased, the displacement increases.
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: (d) upward. By Fleming’s left-hand rule: point the middle finger (current) towards the west (direction of motion of the positive alpha-particle) and the thumb (force) towards the north; then the forefinger (magnetic field) points upward.
Questions Page no: 205
Question: 1. Name two safety measures commonly used in electric circuits and appliances.
Answer: Two safety measures are:
- Electric fuse – it melts due to Joule heating when the current is too high (due to overloading or short-circuiting) and breaks the circuit.
- Earth wire – it connects the metallic body of an appliance to the earth, providing a low-resistance path for leakage current so the user does not get a severe electric shock.
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 oven is:
I= P/V
I=2000W/220 V
I≈9.09 A
Since the circuit is rated for only 5 A, the oven draws a current much larger than the safe limit. The fuse wire will melt due to Joule heating and break the circuit, protecting the appliance and wiring from damage.
Question: 3. What precaution should be taken to avoid the overloading of domestic electric circuits?
Answer: Too many appliances should not be connected to a single socket, since overloading can also occur due to connecting too many appliances to one socket. Appliances should be connected in parallel with separate switches, and the wiring and fuse should be of proper rating.
Exercises Page no: 207
Question: 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.
Question: 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.
Question: 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.
Answer: (a) True — at the centre of a long circular coil, the field lines are nearly parallel straight lines.
(b) A wire with a green insulation is usually the live wire of an electric supply.
Answer: (b) False — green insulation is for the earth wire, not the live wire.
Question: 4. List two methods of producing magnetic fields.
Answer:
- By passing an electric current through a conductor (for example, a straight wire, a circular loop, or a solenoid).
- By using a permanent magnet, such as a bar magnet, which produces a
Question: 5. 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 is at right angles (90∘) 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 electrons move from the back wall towards the front wall, so the conventional current is in the opposite direction — from front to back (towards you). The force (deflection) is to your right. Applying Fleming’s left-hand rule: middle finger (current) pointing towards you, thumb (force) pointing to your right, the forefinger (magnetic field) points downward, i.e. vertically downwards.
Question: 7. State the rule to determine the direction of a:
(i) magnetic field produced around a straight conductor-carrying current,
Answer: (i) Right-hand thumb rule (Maxwell’s corkscrew rule): Hold the conductor in the right hand with the thumb pointing in the direction of the current; the curled fingers show the direction of the magnetic field around it.
(ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it,
Answer: (ii) Fleming’s left-hand rule: Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular; if the forefinger points along the magnetic field and the middle finger along the current, the thumb gives the direction of force (motion) on the conductor.
(iii) current induced in a coil due to its rotation in a magnetic field.
Answer: (iii) Fleming’s right-hand rule: Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular; if the forefinger points along the magnetic field and the thumb along the motion of the conductor, the middle finger gives the direction of the induced current.
Question: 8. When does an electric short circuit occur?
Answer: A short circuit occurs when the live wire and the neutral wire come into direct contact — for example, when the insulation of wires is damaged or there is a fault in an appliance. In such a situation, the current in the circuit abruptly increases.
Question: 9. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer: The earth wire, which has green insulation, is connected to a metal plate deep in the earth near the house. It provides a low-resistance conducting path for the current. When there is a leakage of current to the metallic body of an appliance, the earth wire keeps the body at earth potential, so the user does not get a severe electric shock. It is therefore necessary to earth appliances with metallic bodies, such as electric press, toaster, table fan and refrigerator.