Magnetic Effects of Electric Current — CBSE Class 10 Science Important Questions

Answer: (a) Concentric circles centred on the wire - lying in a plane perpendicular to the conductor; their direction is given by the right-hand thumb rule.

Answer: (a) Fleming's left-hand rule - forefinger points along the field, middle finger along the current and the thumb gives the direction of the force.

Answer: (a) Uniform and directed along the axis - the field lines inside a solenoid are parallel straight lines, showing that the field is the same at all points inside, like that of a bar magnet.

Answer: (b) $100$100 - the current reverses twice in every cycle, so at $50$50 cycles a second it changes direction $100$100 times, once every $\tfrac{1}{100}$1/100 s.

Answer: (a) Both A and R are true and R is the correct explanation of A - since a current-carrying solenoid acts like a bar magnet with a north and a south pole, when suspended freely it aligns itself along the Earth's magnetic north-south direction.

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 the current, then the direction in which the fingers curl round the conductor gives the direction of the magnetic field lines. It helps us determine the direction of the magnetic field produced around a current-carrying conductor.

(a) The force is maximum when the conductor is placed at right angles (perpendicular) to the magnetic field, and it is zero when the conductor is placed parallel to the field.
(b) The magnitude of the force depends on (i) the magnitude of the current, (ii) the strength of the magnetic field, and (iii) the length of the conductor lying in the field.

Applying the right-hand thumb rule to each part of the loop: inside the loop the field is perpendicular to the table, pointing into it (downwards); outside the loop it is perpendicular to the table, pointing out of it (upwards).

(a) See the diagram: outside the magnet the field lines emerge from the north pole and enter the south pole, forming closed loops (inside the magnet they run from south to north).
(b) Properties: (i) Two magnetic field lines never intersect each other - if they did, there would be two directions of the field at that point, which is impossible. (ii) The field lines are closer together where the field is stronger (near the poles) and farther apart where it is weaker; they form continuous closed loops.

(a) An electromagnet is a temporary magnet made by winding an insulated coil around a soft-iron core; it behaves as a magnet only as long as current flows through the coil.
(b) An electromagnet is a temporary magnet whose strength can be varied (by changing the current or number of turns) and whose polarity can be reversed; a permanent magnet (made of steel) retains its magnetism and its strength/polarity cannot be easily changed.
(c) Uses: (i) in electric bells, cranes for lifting heavy iron loads, and loudspeakers; (ii) in electric motors, generators, relays and telephones. (any two)

Applying Fleming's left-hand rule (forefinger = field, pointing downwards; middle finger = current, pointing north), the thumb - and hence the force - points horizontally towards the west.
If the current is reversed (flowing towards the south), the force also reverses and acts towards the east.

(a) The field lines are like those of a bar magnet: one end of the solenoid acts as a north pole and the other as a south pole.

(b) Inside the solenoid the field lines are parallel straight lines, which shows that the field is the same at all points inside - it is uniform.

(c) (i) Increase the current. (ii) Increase the number of turns. (iii) Place a soft-iron core inside. (any two)

(d) The current in every turn flows in the same direction, so the fields produced by the individual turns add up. With $n$n turns the field is $n$n times that of one turn.

(a) (i) DC flows in one direction only; AC reverses its direction periodically (in India, $50$50 cycles a second). (ii) DC is obtained from sources such as cells and batteries; AC is what power stations supply to our homes.

(b) AC can be transmitted over long distances without much loss of energy.

(c) A fuse is a safety device - a wire of a metal or alloy with a low melting point, connected in series with the circuit. If the current exceeds the safe limit (because of short-circuiting or overloading), the wire heats up and melts, breaking the circuit. It is placed in the live wire so that when it melts the appliance is cut off from the high potential and is safe to touch.

(d) Current drawn:

$I=\frac{P}{V}=\frac{2000}{220}\approx 9.1\text{ A}$I=P/V=2000/220≈ 9.1 A

This is more than the $5$5 A rating, so the circuit is overloaded - the fuse will melt and cut off the supply.

(i) The potential difference between the live and neutral wires is about 220 V.
(ii) The earth wire provides a low-resistance path to the ground; if the live wire accidentally touches the metal body of an appliance, the current flows safely to the earth instead of through the user, preventing a fatal electric shock.
(iii) Short-circuiting occurs when the live and neutral wires come into direct contact (very low resistance), causing a sudden, very large current to flow.
(iv) Overloading occurs when too many appliances are connected to a single socket/circuit (or the supply voltage rises), so the current exceeds the safe value and the wires overheat.