4. Electricity: Magnetic and Heating Effects
Complete solutions to Keep the Curiosity Alive. CBSE / NCERT Class 8 Science (Curiosity).
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Keep the Curiosity Alive
(i) The solution used in a Voltaic cell is called electrolyte.
(ii) A current carrying coil behaves like a magnet.
(i) Dry cells are less portable compared to Voltaic cells. — False
Dry cells are more portable. They have no liquid that can spill.
(ii) A coil becomes an electromagnet only when electric current flows through it. — True
(iii) An electromagnet using a single cell attracts more iron paper clips than the same electromagnet with a battery of 2 cells. — False
A battery of 2 cells gives more current, so the electromagnet is stronger and attracts more clips.
Answer: (c) Both (i) and (ii) are correct
A current shows both effects at the same time. The wire gets warm due to the heating effect. The compass deflects due to the magnetic effect.
| Column A | Column B |
|---|---|
| (i) Voltaic cell | (d) Generates electricity by chemical reactions |
| (ii) Electric iron | (c) Works on heating effect of electric current |
| (iii) Nichrome wire | (a) Best suited for electric heater |
| (iv) Electromagnet | (b) Works on magnetic effect of electric current |
Answer: (ii) generates more heat for a given current.
Nichrome offers high resistance to the flow of current. So it gets very hot. It also does not melt easily.
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- They produce no smoke. So indoor air stays clean and there are fewer breathing problems.
- They do not need firewood. This saves trees and forests.
- They save time and effort spent in collecting firewood.
- They heat quickly, and the heat can be easily controlled.
- They are clean. There is no ash or soot to clean up.
(i) Draw the arrow starting from the positive (+) terminal of the cell. It goes through the connecting wire, around the coil, and back to the negative (−) terminal.
(ii) When current flows through the coil, it produces a magnetic field around it. The coil behaves like a magnet (electromagnet). This magnet attracts one pole of the compass needle. So the needle moves.
(iii) The current will flow in the opposite direction. The poles of the electromagnet will get reversed. So the compass needle will deflect in the opposite direction.
- The cell was used continuously for a long time. It got discharged (used up). So very little current flows, and the electromagnet becomes too weak to lift the clips.
- The wire got heated due to the heating effect of current. Part of the energy of the cell was used up as heat.
- A connection may have become loose, reducing the current.
The wire is still warm because it got heated while the current was flowing, and it takes some time to cool down.
Answer: Case (a)
Lemon juice is acidic. It acts as an electrolyte. The two different metals and the lemon juice make a simple Voltaic cell. It produces current, so the LED glows.
Pure water is a very poor conductor of electricity. So no cell is formed in case (b), and the LED does not glow.
Yes, the coil will still deflect the compass. A current carrying coil behaves like a magnet even without the nail.
But the deflection will be less. The iron core makes the electromagnet much stronger. Without it, the coil is a weaker magnet.
Answer: (iv) In all four circuits
Iron, copper, aluminium and nichrome are all conductors of electricity. Current flows through every coil. Any current carrying coil produces a magnetic effect. So the compass needle deflects in all four cases.
Discover, Design, and Debate
The “Discover, design, and debate” projects are hands-on tasks. Do them with your classmates under the guidance of your teacher, and write your own observations.