Activities 4.2 and 4.3 — Building an Electromagnet — "Electricity: Magnetic and Heating Effects"

1. 13 marksCuriosity Grade 8, Chapter 4, page 49, Activity 4.2

   Describe Activity 4.2. Do the paper clips hang from the ends of the nail when the wire is connected to the cell? Do they fall when it is disconnected?

   Hint. Two observations, one when the circuit is complete and one when it is broken.

   What you do. Take about 50 cm of flexible insulated wire, an iron nail, an electric cell and a few iron paper clips. Wind the wire tightly around the nail as a coil (Fig. 4.2) and secure it with adhesive tape. Connect the two ends of the wire to the cell. Bring the nail close to the paper clips and lift.

   The two observations.

   Circuit	What the clips do
   Connected — current flows through the coil	The clips cling to the ends of the nail and are lifted with it
   Disconnected — current stops	The clips fall down

   ✦ Read it against Activity 4.1. There the current's magnetic effect was just strong enough to swing a delicately balanced needle. Here the same effect is strong enough to pick up objects against gravity, because the wire has been wound into many turns around an iron nail instead of running past in a single straight stretch. Same phenomenon, made much stronger by the arrangement.

   And the magnetism is entirely borrowed. The nail is an ordinary iron nail; before the wire was wound on it, it could not pick up a single clip. It is a magnet only while the current flows, so the clips fall the moment you break the circuit.

2. 23 marksCuriosity Grade 8, Chapter 4, page 49, Activity 4.2

   The activity warns: 'Take care to not connect the wires to the cell for more than a few seconds; otherwise, the cell may weaken quickly.' Why does connecting the coil directly across a cell drain it so fast?

   Hint. There is no lamp in this circuit. What is limiting the current?

   Because a coil of copper wire is a very easy path for current, so a large current flows.

   In the circuits you have built before, there was a lamp or some other component in the circuit offering resistance and limiting the current. Here the coil is joined straight across the cell, and the wire itself offers very little resistance. A large current therefore flows.

   What that costs. The current is produced by chemical reactions inside the cell, and those chemicals are used up as current is drawn. A large current uses them up fast — so the cell weakens, and eventually goes dead, much sooner than it would in an ordinary circuit.

   ✦ There is a second reason, which the chapter reaches a few pages later. A large current through a wire also heats it, by the heating effect. That is why the chapter's own character asks, at the start of section 4.2, 'While doing the activity for electromagnet, did you also notice that the wire ends got warm? Why would that happen?' The wasted energy is coming out of the cell as heat.

   So the practical rule is: connect, observe, disconnect. Do not leave it on.

3. 33 marksCuriosity Grade 8, Chapter 4, pages 49-50, Activity 4.3

   Describe how the coil is made in Activity 4.3. Why is it wound on a rolled paper cylinder about as wide as a pencil, and why about 50 turns?

   Hint. Compare the shape here with the shape in Activity 4.2, and think about what each choice makes possible.

   How it is made. Roll a piece of chart paper into a cylinder roughly the diameter of a pencil and tape it. Wind about 50 turns of flexible insulated wire tightly on this cylinder to form a cylindrical coil (Fig. 4.3a), and secure the wire with adhesive tape. You will also need an electric cell, two magnetic compasses, and a few iron or steel paper clips.

   Why a paper cylinder rather than the nail itself. This is the key design change from Activity 4.2. Paper is not a magnetic material, so the coil can first be tested on its own, with nothing magnetic inside it. Then the iron nail can be slid in later (Fig. 4.3d) and the same coil tested again. That lets you separate two questions that Activity 4.2 ran together:

   • Does the coil behave as a magnet by itself?
   • What does the iron core add?

   Winding straight onto the nail would have made that comparison impossible.

   ✦ Why about 50 turns, and why tightly wound. Each turn contributes its own magnetic effect, and the turns add up — the chapter confirms later that increasing the number of turns makes the coil a stronger magnet. Fifty turns is enough to give a clear deflection from a single cell. Winding them tightly and close together keeps their effects lined up and pulling the same way.

   Why a pencil-width cylinder. It has to be narrow enough for the effect to be concentrated, and just wide enough for an iron nail to slide in afterwards.

4. 43 marksCuriosity Grade 8, Chapter 4, page 50, Activity 4.3

   In Activity 4.3, two compasses are placed near the two ends of the cylindrical coil and the coil is connected to the cell — with no iron nail inside. Do the needles deflect? Do they return when the wire is disconnected? What does this show?

   Hint. There is nothing magnetic inside the coil at this stage — only paper.

   Yes, the needles deflect, at both ends of the coil. Yes, they return to their original positions when the wire is disconnected from the cell.

   What this establishes. The chapter states it plainly: when current is passed through the cylindrical coil, it behaves like a magnet and deflects the needle of a magnetic compass.

   ✦ Why this stage of the activity matters so much. There is no iron in the coil at all — only rolled chart paper, which is not a magnetic material. So the magnetism cannot be coming from the core. It is being produced by the current in the wire, exactly as in Activity 4.1, but now gathered into a coil so that fifty turns act together instead of one straight stretch of wire.

   This is the step that rules out the obvious wrong explanation — that the nail was somehow a magnet all along, or that iron is doing the work. Test the coil empty first, and the nail's role becomes a separate question with its own answer.

   The compasses go at the two ends rather than one, because the next activity asks about the coil's two poles.

5. 53 marksCuriosity Grade 8, Chapter 4, page 50, Activity 4.3

   Now an iron nail is inserted into the paper cylinder and the steps are repeated. Is there any difference in the deflection of the compass needles? Are the paper clips attracted to the ends of the nail?

   Hint. Same coil, same cell, same compasses — only one thing has changed.

   Yes, there is a clear difference. With the iron nail inside, the coil becomes a stronger magnet and the deflection of the magnetic compass needle is much more. And yes — iron paper clips placed near the two ends of the nail are now attracted and cling there (Fig. 4.3e), which the empty coil could not manage.

   Why the comparison is fair. Nothing else was changed. Same coil, same number of turns, same cell, same compass positions. The only difference is the iron nail, so the extra strength must be due to the nail.

   ✦ What the iron core is doing. Iron is a magnetic material. Placed inside the coil, it is magnetised by the coil's magnetic field and adds its own magnetism to it, so the combination is much stronger than the coil alone. This is why the chapter concludes that for practical applications, most electromagnets have an iron core to make them stronger.

   And when the current is stopped, the coil loses its magnetic effect and the clips fall — the iron core does not keep the magnetism going on its own.

6. 63 marksCuriosity Grade 8, Chapter 4, page 50

   What is an electromagnet? Why do most practical electromagnets have an iron core?

   Hint. Give the definition first, then the reason for the core.

   Definition. A current carrying coil that behaves as a magnet is called an electromagnet.

   Note what the definition does not require: it does not mention an iron core. A coil of wire alone, carrying a current, is already an electromagnet — Activity 4.3 showed the empty coil deflecting both compasses.

   Why the core is nevertheless used. For practical applications, most electromagnets have an iron core to make them stronger. The same coil and the same cell give a much larger deflection with the nail inside, and only then are the clips picked up. For anything that has to do real work — lift scrap steel, ring a bell, drive a loudspeaker — the extra strength is worth having.

   ✦ Why iron in particular. Iron is a magnetic material, so it is itself magnetised by the coil's field and reinforces it. It also gives up that magnetism when the current stops, so the electromagnet still switches cleanly off — which is the whole advantage of an electromagnet over a permanent magnet.

   Summary of the properties:

   Property	Electromagnet
   Magnetic when?	Only while current flows
   Strength	Adjustable — by current and by number of turns
   Poles	Two, North and South, and they can be swapped by reversing the current
   Core	Usually iron, to make it stronger

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.