Chapter 4: Electricity: Magnetic and Heating Effects (Physics)
Answers to all "Keep the curiosity alive" questions of Chapter 4, Electricity: Magnetic and Heating Effects (NCERT Class 8 Science, Curiosity, 2026-27): Voltaic and dry cells, electromagnets and their poles, the heating effect and nichrome, electric versus traditional heating, a lemon cell, and why every current-carrying coil deflects a compass. All 11 questions are answered, with the key answer highlighted.
An electric current has two everyday effects. The magnetic effect: a wire or coil carrying current behaves like a magnet (an electromagnet, made stronger with an iron core, more turns or more current). The heating effect: a wire carrying current gets hot, which is used in heaters, irons and kettles.
True or false? (i) Dry cells are less portable than Voltaic cells. (ii) A coil becomes an electromagnet only when current flows through it. (iii) An electromagnet with one cell attracts more paper clips than the same electromagnet with 2 cells.
Solution
False. Dry cells are sealed, small and contain no liquid, so they are more portable.
True. The magnetism disappears when the current stops.
False. Two cells give a larger current, which makes a stronger electromagnet, so it attracts more clips.
Match: (i) Voltaic cell (ii) Electric iron (iii) Nichrome wire (iv) Electromagnet with (a) best suited for an electric heater (b) works on the magnetic effect of current (c) works on the heating effect of current (d) chemical reactions.
Solution
(i) Voltaic cell → (d) chemical reactions
(ii) Electric iron → (c) heating effect
(iii) Nichrome wire → (a) best suited for a heater
Nichrome is used in heating devices because it (i) is a good conductor (ii) generates more heat for a given current (iii) is cheaper than copper (iv) is an insulator.
Solution
Nichrome has a high resistance, so the same current produces much more heat in it than in copper. It also does not melt or rust easily at high temperatures.
Why are electric heating devices often more convenient than burning firewood or charcoal? Think about the impact on society.
Solution
Cleaner indoor air: there is no smoke, soot or carbon monoxide at home. Smoke from wood and charcoal stoves causes lung and eye diseases, especially in women and children who spend long hours near the stove.
Less cutting of trees for fuel, which protects forests.
Saves time and effort: no collecting or carrying firewood, and no ash to clean up. The time saved can go to school or work.
Easy control and safety: they switch on instantly, the heat can be adjusted, and there is less risk of burns from open flames.
(Electricity still needs to be produced, preferably from clean sources such as solar power, and used carefully.)
They are smoke-free and healthier, save forests and the labour of gathering fuel, and are quick, adjustable and safer to use.
In Fig. 4.4a the compass near the coil deflects. (i) Draw an arrow showing the path of the current. (ii) Why does the needle move? (iii) What happens if the battery terminals are reversed?
Solution
(i) Conventional current leaves the positive (+) terminal of the cell, goes through the wire and around the coil, and returns to the negative (−) terminal.
Current flows from the + terminal of the cell, through the coil, back to the − terminal (arrows). The compass is at end A.
(ii) A coil carrying current behaves like a magnet, with a north and a south pole at its ends. The magnetic field of the coil pushes or pulls the magnetised compass needle, so the needle turns.
(iii) The current flows the other way, so the poles of the coil swap (end A changes from S to N, or N to S). The needle is deflected in the opposite direction.
(i) Current flows from + through the coil to −. (ii) The coil becomes an electromagnet whose field acts on the needle. (iii) The poles reverse, so the needle deflects the opposite way.
Sumana left her lifting electromagnet switched on. After some time it stopped lifting the paper clips, though the wire was still warm. Why?
Solution
The cell has run down. A current kept flowing for a long time (which kept the wire warm), but the current became too weak to make the electromagnet strong enough to lift the clips.
The heating of the wire and nail as time went on also wastes energy and can weaken the magnetism. A loose connection could also reduce the current.
The cell was nearly used up, so the current (and the magnetism) became too weak to lift the clips, even though the wire was still warm from the current that had flowed.
In Fig. 4.12, iron nail and copper strip electrodes are placed in (a) pure water and (b) lemon juice. In which case will the LED glow?
Solution
(b) Lemon juice. It is acidic and contains ions, so it acts as an electrolyte. The two different metals in it make a simple cell.
Pure water hardly conducts electricity, so no useful current flows in (a). (A single lemon cell may give only a faint glow; joining a few in series makes it brighter.)
(b) The lemon juice set-up, because lemon juice is an electrolyte.
Neha slides the iron nail out of the coil in Activity 4.4. Will the coil still deflect the compass? More or less than before?
Solution
Yes. A coil carrying current is a magnet even without a core. But the deflection will be less, because the iron nail made the magnetic field much stronger.
Yes, but the deflection will be smaller without the iron core.
Coils of iron, copper, aluminium and nichrome carry current. In which circuits will the compass deflect? (i) only (a) (ii) only (a) and (b) (iii) only (a), (b) and (c) (iv) all four
Solution
Any wire carrying an electric current produces a magnetic field around it, whatever metal it is made of. Iron only adds extra strength when it is used as a core. So all four coils deflect the compass.
