Probe and Ponder — The Opening Questions — "Electricity: Magnetic and Heating Effects"

1. 13 marksCuriosity Grade 8, Chapter 4, page 46

   If we don't have an electric lamp while making an electric circuit with an electric cell, is there any other way through which we can find out if current is flowing in the circuit?

   Hint. A lamp is only one kind of detector. What else changes when current flows?

   Yes — put a magnetic compass next to the wire. This is exactly what the chapter goes on to do in Activity 4.1.

   A lamp tells you current is flowing because the current heats its filament until it glows. But a current does something else as well: it produces a magnetic field around the wire carrying it. A compass needle is a tiny magnet, so it swings away from its usual north–south rest position whenever it is placed in that field.

   So the test is:

   1. Lay the compass right under (or right over) a straight part of the connecting wire.
   2. Let the needle settle.
   3. Close the switch.

   If the needle deflects, current is flowing. Open the switch and the needle swings back, because the magnetic effect disappears the moment the current stops.

   ✦ The point of the question. A detector does not have to be a lamp — it only has to respond to some effect of the current. The chapter is about two such effects, the magnetic effect and the heating effect, and either one can be used to reveal a current you cannot see.

   A second answer, from later in the chapter: touch the wire. If a thin nichrome wire warms up, current is flowing through it. The compass is the safer and more sensitive of the two.

2. 23 marksCuriosity Grade 8, Chapter 4, page 46

   Is it possible to make temporary magnets? How can these be made?

   Hint. Sumana's exhibition model in the chapter opening is exactly this.

   Yes. A coil of wire carrying a current is a magnet, and it stops being one the instant you switch the current off. Such a magnet is called an electromagnet.

   How to make one (this is Activity 4.2 of the chapter):

   1. Take about 50 cm of flexible insulated wire and an iron nail.
   2. Wind the wire tightly around the nail as a coil and hold it with adhesive tape.
   3. Connect the two ends of the wire to an electric cell.

   Bring the nail near iron paper clips and they cling to its ends. Disconnect the cell and the clips fall off, because there is no longer any current, and therefore no longer any magnetic field.

   ✦ Why this is such a useful thing. A permanent bar magnet is always a magnet — you cannot ask it to let go. An electromagnet can be turned on and off by a switch, which is why a scrap-yard crane can pick up a load of steel at one end of the yard and drop it at the other.

   And it is adjustable, not just switchable. The chapter shows that the strength can be increased by using more cells or more turns of the coil, and that the north and south ends swap over if the current is sent the other way round.

3. 33 marksCuriosity Grade 8, Chapter 4, page 46

   We can generate heat by burning fossil fuels and wood; but how is heat generated in various electrical appliances?

   Hint. Nothing burns inside an electric iron. Something else converts the energy.

   By the heating effect of electric current — no burning is involved at all.

   When current flows through any conductor it meets some opposition, called resistance. Because of that resistance a part of the electrical energy is converted into heat energy, and the conductor gets hot. That is the whole mechanism.

   Inside the appliance. Every electric heating appliance contains a rod or a coil of wire called a heating element. It is made of a material such as nichrome, which offers a much higher resistance than a copper wire of the same length and thickness, so it turns far more of the electrical energy into heat. In an electric stove or a room heater you can often see the element glowing red hot.

   ✦ The contrast with a fire. Burning wood is a chemical reaction — the fuel is consumed and turned into ash, smoke and gases. An electric heating element is not consumed; it simply resists the current passing through it, and can be switched on and off instantly.

   The chapter lists room heaters, electric stoves, kettles, irons, immersion rods and hair dryers as everyday examples, and reminds you that the incandescent lamp glows for the very same reason — its filament is heated by the current until it emits light.

4. 43 marksCuriosity Grade 8, Chapter 4, page 46

   How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?

   Hint. A cell runs on a chemical reaction. What happens when the chemicals are used up?

   How you know it is dead. A cell produces electricity from a chemical reaction between its electrodes and its electrolyte. Over time those chemicals get used up, and then the cell can no longer supply electricity — it is said to be dead. In practice you notice this because the device stops working, or works feebly: the lamp does not glow or glows dimly, the electromagnet lifts fewer clips than it used to, the toy slows down.

   No, not all of them can be recharged.

   Type	Can it be recharged?	What you do with it
   Dry cell	No — it is a single use cell	Once used up it has to be disposed of
   Rechargeable battery	Yes, many times over	Recharge and reuse; it saves waste and money

   ✦ Even rechargeable batteries do not last forever. After being charged and used many times they slowly wear out — which is why, as the chapter's character notices, a phone battery needs charging more often after a year or two.

   A caution the chapter adds at the end. A 'dead' battery is not empty. It can still contain acids and metals such as lead, cadmium, nickel or lithium, which may cause fires or harm the environment in ordinary garbage — and many of those materials are valuable and recyclable. Used batteries belong in an e-waste collection point, not the dustbin.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity, Textbook of Science for Grade 8, Chapter 4 'Electricity: Magnetic and Heating Effects', book pages 46-61 (hecu104.pdf, 16 pages, Reprint 2026-27), downloaded from ncert.nic.in and read page by page. Every activity number, figure number, quantity and quoted sentence below was checked against that PDF. Figures read at high resolution: Fig. 4.4a (compass, coil ends A and B, and cell polarity - the cell's - terminal is on the left and + on the right, and the red north tip of the needle has swung towards end A, so end A is the south pole for the connections shown); Fig. 4.12 (a: iron nail and copper strip in lemon juice, b: same electrodes in pure water); Fig. 4.13 (four coils of iron, copper, aluminium and nichrome, each with a cell and a switch). Three deliberate restraints on what is claimed. (1) The chapter gives the heating effect entirely qualitatively - 'the heat generated depends on the magnitude of the electric current' and on 'the material, thickness, length of the wire, and the duration' - and gives no formula. No formula is supplied here either; Joule's law, Ohm's law, power and kWh belong to a later grade and appear nowhere in this book. (2) The two wire-heating projects are answered as experiments to be performed, not as results to be reproduced: the thickness comparison is explained, and the length comparison is explicitly left open because two effects work against each other and the chapter offers no prediction. (3) Solid-state batteries are reported in the chapter's own tense - under development, advantages expected - and the lithium and cobalt supply question is described without naming countries or quantities, since the chapter names none.. Questions are referenced from the NCERT textbook for identification.