Class 10 Science Chapter 12: Magnetic Effects of Electric Current — Important Questions & Sample Paper

They repel each other

Soft iron loses its magnetism quickly when current is switched off

Magnetic field lines

0.09 N

Both A and R are true but R is not the correct explanation of A.

(i) The field strength increases because it is directly proportional to the number of turns per unit length. (ii) It greatly increases because soft iron has high magnetic permeability, concentrating the magnetic field lines inside the solenoid.

A solenoid produces a magnetic field pattern identical to a bar magnet: one end acts as a north pole (field lines emerge) and the other as a south pole (field lines enter). To find polarity without a compass, use the right-hand grip rule: hold the solenoid in the right hand with fingers curling in the direction of current in the coils; the thumb points to the north pole.

If two field lines intersected, there would be two directions of the magnetic field at the point of intersection, which is impossible. Therefore, magnetic field lines never intersect.

The current of 10 A exceeds the 5 A rating of the fuse. According to Joule's law (H = I²Rt), the heat produced in the fuse wire will be high enough to melt it, breaking the circuit. The appliance will not operate, but the circuit is protected from potential overheating.

A fuse is a safety device used to protect an electrical circuit from damage due to excessive current. It consists of a short piece of wire with low melting point and high resistivity. When current exceeds the rated value, the fuse wire melts, breaking the circuit and stopping the flow of current. It is connected in series with the live wire so that when it melts, the appliance is completely disconnected from the high potential (live) and the circuit becomes safe. Characteristics of a good fuse wire: (i) low melting point, (ii) high resistivity, (iii) should not oxidise easily, (iv) made from an alloy like tin–lead or copper–tin. Overloading: When too many appliances are connected in parallel across the same circuit, they draw a large total current due to low equivalent resistance. This current may exceed the safe current rating of the wiring, causing the wires to overheat. The insulation may melt, exposing live and neutral wires, which can come in contact causing a short circuit. Short-circuiting: If the live and neutral wires come into direct contact (due to damaged insulation or fault), a very low resistance path is created, causing an extremely high current to flow. This high current generates excessive heat, which can ignite the insulation or surrounding materials, leading to a fire. The fuse is designed to melt and break the circuit before the current reaches a dangerous level, thus preventing overheating and fire.

Electromagnetic induction is the phenomenon of producing an induced emf (and current) in a circuit when the magnetic flux linked with it changes. Experiment: Take a coil of insulated copper wire and connect its ends to a sensitive galvanometer. Take a bar magnet and rapidly push its north pole into the coil. The galvanometer shows a deflection, indicating an induced current. When the magnet is pulled out, the galvanometer deflects in the opposite direction. Faster movement gives larger deflection. Keeping the magnet stationary yields no deflection. This proves that relative motion between a coil and a magnet induces an emf. Factors affecting induced emf: (i) speed of relative motion between coil and magnet, (ii) number of turns in the coil, (iii) strength of the magnet (magnetic flux density), (iv) area of the coil (related to flux linkage).

a) Works on the principle that a current-carrying conductor in a magnetic field experiences a force. b) Split rings reverse current every half rotation for continuous rotation. c) The rotation direction reverses.