Page 224
1. Why does a compass needle get deflected when brought near a bar magnet?
Answer:
When a bar magnet is brought near a compass, its magnetic field exerts forces on the north and south poles of the compass needle. These forces are equal and opposite but act in different directions, forming a couple, which causes the compass needle to deflect.
Page 228
1. Draw magnetic field lines around a bar magnet.
Answer:
2. List the properties of magnetic lines of force.
Answer:
Properties of magnetic field lines:
- They originate from the north pole of a magnet and end at its south pole.
- They are closer near the poles of a magnet and widely spaced at other regions.
- Two magnetic field lines never intersect each other.
3. Why don’t two magnetic lines of force intersect each other?
Answer:
At a given point, the magnetic field has a unique direction. If two magnetic field lines were to intersect, then the resultant force on a north pole placed at the intersection would be along two different directions at the same time, which is not possible.
Page 229–230
1. Consider a circular loop of wire lying on 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:
According to the right-hand thumb rule, if the current in the circular loop flows clockwise (as seen from above), then:
- Inside the loop, the magnetic field is directed downwards, i.e., into the plane of the table/paper.
- Outside the loop, the magnetic field is directed upwards, i.e., out of the plane of the table/paper.
Thus, the magnetic field is normal to the plane of the loop—inward inside the loop and outward outside the loop.
2. The magnetic field in a given region is uniform. Draw a diagram to represent it.
Answer:
3. Choose the correct option. The magnetic field inside a long straight solenoid-carrying current
Answer:
(iv) Is the same at all points.
Page 231–232
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.)
Answer:
(iii) Velocity, (iv) Momentum.
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:
(i) When the current in the rod AB is increased, the force exerted on the conductor increases, so the displacement of the rod increases.
(ii) When a stronger horse-shoe magnet is used, the magnitude of the magnetic field increases. This increases the force exerted on the rod and the displacement of the rod.
(iii) When the length of the rod AB is increased, the force exerted on the conductor increases, so the displacement of the rod 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:
Answer:
(iv) Upward.
Page 233
1. State Fleming’s left-hand rule.
Answer:
According to Fleming’s left-hand rule: If the thumb, forefinger, and middle finger of the left hand are stretched out mutually perpendicular to each other—
- Forefinger → direction of the magnetic field
- Middle finger → direction of the current
- Thumb → direction of the force (motion) on the conductor
2. What is the principle of an electric motor?
Answer:
An electric motor works on the principle of the magnetic effect of current. When a rectangular coil carrying current is placed in a magnetic field, a force acts on the coil, causing it to rotate. The shaft attached to the coil also rotates, thereby converting electrical energy into mechanical energy of rotation.
3. What is the role of the split ring in an electric motor?
Answer:
The split ring acts as a commutator. Its role is to:
- Reverse the direction of current in the armature coil after every half rotation.
- This reversal ensures that the direction of force on the coil also reverses, allowing the coil to rotate continuously in the same direction.
Page 236
1. Explain different ways to induce current in a coil.
Answer:
- Moving a magnet towards or away from the coil or vice-versa.
- Changing current in the neighboring coil.
Page 237
1. State the principle of an electric generator.
Answer:
An electric generator works on the principle of electromagnetic induction. When a conductor moves in a magnetic field, the conductor cuts the magnetic field lines, and an induced current is produced in it. In a generator, a rectangular coil rotates rapidly between the poles of a horse-shoe magnet, cutting the magnetic field lines and thus producing current.
2. Name some sources of direct current.
Answer:
Dry cells, button cells, lead accumulators.
3. Which sources produce alternating current?
Answer:
Alternating current is produced by AC generators of nuclear power plants, thermal power plants, hydroelectric power stations, etc.
4. Choose the correct option: A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced current changes once in each:
Answer:
(iii) Half revolution.
Page 238
1. Name two safety measures commonly used in electric circuits and appliances.
Answer:
(i) Earthing
(ii) Electric fuse
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 electric oven draws a current given by:
I = P/V = 2000 W/220 V = 9.09 A
Thus, the electric oven draws a current much higher than the current rating of 5 A. The circuit is overloaded. Due to excessive current, the fuse wire will blow, and the circuit will break.
3. What precautions should be taken to avoid the overloading of domestic electric circuits?
Answer:
To avoid the overloading of domestic electric circuits, the following precautions should be taken:
- The wires used in the circuit must be coated with good insulating materials like PVC, etc.
- The circuit must be divided into different sections, and a safety fuse must be used in each section.
- High-power appliances like air-conditioners, refrigerators, water heaters, etc., should not be used simultaneously.