Electricity: Magnetic and Heating Effects – Class 8 Science notes, important questions and MCQs

Chapter overview

An electric current heats the wire it flows through and also creates a magnetic field around it. These two effects are used in many household devices.

Heating effect

• Current through a resistance produces heat (heater, iron, kettle, bulb filament).
• A fuse has a thin wire that melts when too much current flows, breaking the circuit.
• ISI-marked wires and MCBs make wiring safe.

Magnetic effect

• Oersted found that a current-carrying wire deflects a compass needle.
• A coil of wire around an iron nail becomes an electromagnet; it stops being a magnet when current stops.
• Uses: electric bell, cranes that lift scrap iron, loudspeakers, motors.

Key terms

Electromagnet

a magnet made by passing current through a coil around soft iron

Fuse

safety device that melts and breaks the circuit on overload

Element

coil in a heater that gets hot when current flows

Formulas

• More current or more turns → stronger electromagnet

Important questions with answers

Quick revision

• Current produces heat and magnetism.
• Oersted showed the magnetic effect.
• Fuse protects against overload.

Exam tips

• Electromagnets can be switched on/off and their strength changed — unlike permanent magnets.
• Never touch switches with wet hands.

Practice MCQs

1. An electric iron works on the

   1. magnetic effect
   2. lighting only
   3. chemical effect
   4. heating effect
   Show answer

   Answer: (d) heating effect. Heating effect.

2. Who first showed that current affects a compass needle?

   1. Oersted
   2. Newton
   3. Faraday
   4. Edison
   Show answer

   Answer: (a) Oersted. Hans Christian Oersted.

3. The strength of an electromagnet increases by

   1. reducing turns
   2. removing the iron core
   3. increasing current or turns
   4. using a plastic core
   Show answer

   Answer: (c) increasing current or turns. More current or turns.

4. A fuse wire should have a

   1. low melting point
   2. high melting point
   3. very thick size
   4. iron coating
   Show answer

   Answer: (a) low melting point. It must melt quickly.

5. An electric heater works on the

   1. magnetic effect
   2. lighting effect only
   3. chemical effect
   4. heating effect
   Show answer

   Answer: (d) heating effect. Element heats up.

6. An electric bell rings again and again because the electromagnet

   1. always stays on
   2. is switched on and off repeatedly by the circuit
   3. uses a permanent magnet
   4. heats up
   Show answer

   Answer: (b) is switched on and off repeatedly by the circuit. Contact breaks and remakes.

7. Why do cranes use electromagnets, not permanent magnets, to lift scrap iron?

   1. they are cheaper
   2. they never fail
   3. they are lighter
   4. the iron can be dropped by switching off the current
   Show answer

   Answer: (d) the iron can be dropped by switching off the current. Magnetism can be turned off.

8. The strength of an electromagnet can be increased by

   1. increasing the number of turns
   2. using a plastic core
   3. reducing current
   4. removing the core
   Show answer

   Answer: (a) increasing the number of turns. More turns, more current, iron core.