Activity 4.4 — Poles, Strength and Direction — "Electricity: Magnetic and Heating Effects"

1. 13 marksCuriosity Grade 8, Chapter 4, pages 50-51, Activity 4.4

   Describe the procedure of Activity 4.4. How do you find out which pole of the electromagnet end A is?

   Hint. You are not measuring anything — you are reading off which end of the compass needle comes closer.

   The procedure.

   1. Take the electromagnet made in Activity 4.3 (coil with the iron nail inside) and a magnetic compass. Label the two ends of the coil A and B.
   2. Place the magnetic compass near end A (Fig. 4.4a).
   3. Connect the coil to the cell and watch the compass.
   4. Note down which pole of the compass needle is attracted to end A.
   5. Repeat the whole thing with the compass near end B (Fig. 4.4b).

   How the reading is turned into an answer. You already know from Grade 6 that when two magnets are brought close, their unlike poles attract — north pulls south. So whichever pole of the compass needle swings towards end A must be the opposite of whatever end A is.

   ✦ The rule, stated as the chapter states it: if the north pole of the magnetic compass is attracted towards end A of the electromagnet, then end A is south pole.

   In Fig. 4.4a the red (north) tip of the needle has swung round to point towards the coil, so for the connections shown in that figure, end A is the south pole.

   If you build this yourself and get the opposite result, you have not made a mistake — you have simply connected your cell the other way round. Which end is north depends on which way the current goes, as the next questions show.

2. 22 marksCuriosity Grade 8, Chapter 4, page 51

   What is the polarity of end B? Explain how you know without doing a second experiment.

   Hint. A magnet has two poles — and only two.

   End B is the opposite of end A. If end A is the south pole, end B is the north pole. The activity confirms this by experiment — Did you find that the polarity of end B is opposite to the polarity of end A? — and the chapter concludes: Just like a magnet, an electromagnet also has two poles — North and South.

   Why you could have predicted it. You learnt in Grade 6 that a magnet has two poles, one north and one south. An electromagnet behaves as a magnet, so it has one of each. If you have found which one end is, the other end is settled.

   ✦ But doing the second test anyway is the right instinct. The prediction rests on the assumption that the coil behaves like an ordinary bar magnet — and that assumption is exactly what the activity is checking. Confirming it costs one minute and turns an assumption into an observation.

   Practical note. Bring the compass to the coil's end along the axis of the coil, as in the figure, not from the side. Near the middle of the coil the two poles pull the needle in competing directions and the reading is confusing.

3. 33 marksCuriosity Grade 8, Chapter 4, page 51, Think like a scientist

   Repeat Activity 4.3 with 2 cells and then with 4 cells, using the same coil. What do you observe, and why?

   Hint. The coil has not changed at all. Only the source has.

   What you observe. The deflection of the compass needle gets larger, and the coil picks up more clips, as you go from one cell to two cells to four.

   The chain of reasoning the chapter gives:

   1. A single cell provides only a small amount of current, so the magnetic field is weak. The deflection is small and the coil can attract only a few clips.
   2. A battery with more cells gives a larger current than a single cell.
   3. A larger current creates a stronger magnetic field.
   4. So the deflection is greater and more clips are lifted.

   ✦ What makes this a fair test. The coil, its number of turns, the iron nail and the position of the compass are all kept exactly the same. Only the number of cells changes, so the change in deflection can be attributed to the current.

   Two cautions when you try it. Connect for only a few seconds each time — a larger current drains the cells faster and warms the wire. And keep the compass at the same distance and position each time, because the deflection also depends on how far away the compass is, and moving it would spoil the comparison.

4. 43 marksCuriosity Grade 8, Chapter 4, page 51, Think like a scientist

   Repeat Activity 4.3 with 2 cells but with different numbers of turns in the coil. What do you observe?

   Hint. This time the source is fixed and the coil is what changes.

   What you observe. With the same 2 cells, a coil with more turns gives a larger deflection and lifts more clips. The chapter puts it directly: The increase in number of turns of the coil also makes the coil a stronger magnet!

   Why. Every turn of the coil carries the same current, and every turn produces its own magnetic effect. Because the turns are wound side by side in the same sense, their effects add up. Fifty turns therefore give a much stronger field than five, even though the current through the wire is the same.

   ✦ Why this is a separate experiment from the last one. In the previous test the current changed and the coil stayed the same; here the coil changes and the current source stays the same. Changing only one thing at a time is what allows you to say which factor was responsible. If you had added cells and turns together and seen a bigger deflection, you would not know which change did it.

   This is the experiment the chapter asks you to extend as a project at the end — coils of 25, 50, 75 and 100 turns on the same cell, with the compass in the same position every time.

5. 53 marksCuriosity Grade 8, Chapter 4, page 51, Think like a scientist

   Repeat Activity 4.4 after changing the direction of the current. What happens to the poles of the electromagnet?

   Hint. Swap the two connections at the cell and watch which way the needle now turns.

   The poles swap over. The end that was the north pole becomes the south pole, and the end that was the south pole becomes the north pole. The chapter states it as: its poles can be reversed by changing the direction of the current.

   How you see it. Reverse the two connections at the cell so the current runs the other way round the coil, and place the compass near end A again. The other tip of the needle now swings towards end A. Since unlike poles attract, that means end A has changed polarity.

   ✦ The strength does not change — only the direction. You have not altered the current's size or the number of turns, so the deflection is just as large as before; it is simply the other way round. That distinguishes this experiment from the previous two, which changed strength but never polarity.

   Where this is put to use. A permanent magnet's poles are fixed forever. An electromagnet's can be flipped by a switch — which is what lets an electric motor keep turning and a loudspeaker cone push and pull in time with a signal.

   This also answers a puzzle from Activity 4.4: there is no way to predict from the coil's appearance which end is north. It depends on which way you connected the cell.

6. 63 marksCuriosity Grade 8, Chapter 4, page 51

   Summarise the ways in which the strength of an electromagnet can be changed, and the way its polarity can be changed.

   Hint. Three levers in total — two for strength, one for direction.

   The chapter's summary. The strength of an electromagnet can be changed by changing the amount of electric current flowing through the coil or the number of turns of the coil, or both. Also, its poles can be reversed by changing the direction of the current.

   What you change	What happens
   More current (more cells)	Stronger magnet — larger deflection, more clips
   More turns in the coil	Stronger magnet — larger deflection, more clips
   Both together	Stronger still
   Insert an iron core	Stronger — much larger deflection (from Activity 4.3)
   Reverse the current direction	Same strength, but poles swap over

   ✦ Notice the shape of the list. The first four change how strong; only the last changes which way round. Reversing the current does not weaken the magnet, because neither the amount of current nor the number of turns has altered — and adding cells does not flip the poles, since the current still runs the same way round the coil. Keeping the two ideas separate is what most exam questions on this section are really testing.

   And every one of these is something a permanent magnet cannot offer. You cannot make a bar magnet stronger this afternoon and reverse it tomorrow morning.

7. 73 marksCuriosity Grade 8, Chapter 4, page 51, A step further

   Why does the Earth itself behave like a magnet? What does the chapter say the Earth's magnetic field does for living things?

   Hint. The chapter's explanation uses the very effect you have just been studying.

   Why the Earth is a magnet. Deep inside the Earth, the movement of liquid iron in the core creates electric currents, which generate a magnetic field.

   This is the magnetic effect of electric current again, on an enormous scale. The Earth's core contains liquid iron in motion; that moving iron amounts to electric currents; and currents produce magnetic fields. It is the same physics as your coil and cell, which is why the chapter places this box here rather than anywhere else.

   And this is why a freely suspended magnet rests north–south — a fact you met in Exploring Magnets, Curiosity Grade 6. The compass needle in every activity of this chapter is settling into the Earth's own field whenever you switch your circuit off.

   What the field does for life, as the chapter lists it:

   • Many migratory birds, fish, and animals use this field to navigate across continents and oceans.
   • The Earth's magnetic field also acts as a shield, blocking harmful particles from space, and helps protect life on Earth.

   ✦ A note on how much is being claimed. The chapter states these as facts and they are well supported, but exactly how an animal senses the field is still an active research question — different species appear to do it in different ways, and it is not settled. That the navigation happens is not in doubt; the mechanism is.

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.