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Class 8 Science: Electricity: Magnetic and Heating Effects, 8 practice questions with answers
These are 8 practice questions on Chapter 4, Electricity: Magnetic and Heating Effects, of the new NCERT Class 8 Science textbook, Curiosity. Each has a short answer and the textbook page it comes from. They cover what a deflecting compass shows, a fair test of what makes an electromagnet stronger, reversing its poles, a crane's lifting magnet, which wires heat up most, sorting appliances by effect, a home-made cell, and dry cells against rechargeable batteries. For a walk-through of how the book teaches science, see our Class 8 Science Chapter 1 explainer. Try each question before you read the answer.
These questions are our own, drafted with AI help. Every fact was checked against the page it cites by a test script, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.
What the chapter says you need
Facts from the NCERT textbook (Curiosity, Grade 8, Chapter 4, printed pages 46 to 61):
- A current in a wire makes a magnetic field around it, which deflects a compass needle; the field disappears when the current stops. This is the magnetic effect of electric current, found by Oersted in 1820 (pages 47 and 48).
- A coil carrying a current behaves like a magnet, an electromagnet, with a north and a south pole. An iron core, more current or more turns make it stronger, and reversing the current reverses its poles (pages 49 to 51).
- A current heats the wire it flows through, the heating effect of electric current; the heat depends on the current, the material, the thickness and length of the wire, and the time (pages 52 and 53).
- A cell makes electricity from chemical reactions between two different metal electrodes and an electrolyte; dry cells are single use, and rechargeable batteries can be reused but slowly wear out (pages 55 to 58).
The magnetic effect
Question 1. Mira hides a cell, a switch and a bulb inside a closed cardboard box, with one wire of the circuit running just under the lid. She puts a compass on the lid. Each time her friend presses the switch, the bulb lights and the needle swings; each time the switch is released, the needle swings back. (a) What does this tell you about the wire under the compass? (b) The bulb's filament breaks. Will the needle still swing when the switch is pressed? Why? (c) She moves the compass to the far end of the table, well away from the wire. What do you expect now?
Answer: (a) while a current flows in it, the wire has a magnetic field around it; (b) no, because no current can flow; (c) the needle hardly moves, if at all. (a) The compass needle is a tiny magnet, so it turns only when there is a magnetic field. The field appears when the switch lets current flow and disappears when it stops: this is the magnetic effect of electric current. The cardboard lid does not block it, because a magnetic effect acts through non-magnetic materials. (b) No. With the filament broken, the circuit is not complete, so pressing the switch sends no current through the wire, and without a current there is no magnetic field. (c) The magnetic field is felt only in the region around the wire, so far from the wire the needle hardly moves, if at all. (The magnetic effect of current, pages 47 and 48.)
Question 2. A group tests home-made electromagnets by counting the steel clips each one lifts. These are their results:
| Coil | Turns | Cells | Core | Clips lifted |
|---|---|---|---|---|
| A | 20 | 1 | none | 2 |
| B | 40 | 1 | none | 4 |
| C | 40 | 2 | none | 7 |
| D | 40 | 2 | iron nail | 25 |
| E | 20 | 2 | iron nail | 15 |
(a) Which pairs of coils test only the number of turns? Which pair tests only the number of cells, and which only the core? (b) Why is comparing coils A and D not a fair test of anything? (c) In these results, which single change made the biggest difference?
Answer: (a) turns: A and B, and E and D; cells: B and C; core: C and D; (b) three things change at once; (c) adding the iron nail. (a) A fair comparison changes one thing and keeps the rest the same. A and B differ only in turns (20 against 40), and so do E and D. B and C differ only in cells (1 against 2). C and D differ only in the core. In every pair, more turns, more cells or an iron core lifted more clips. (b) From A to D the turns, the cells and the core all change, so the jump from 2 clips to 25 cannot be put down to any one of them. (c) Putting the iron nail in took coil C from 7 clips to 25 in coil D, the biggest jump from a single change. (Connect a coil to the cells only for a few seconds at a time: it runs them down quickly.) (Electromagnets and what makes them stronger, pages 49 to 51.)
Question 3. Two electromagnets lie end to end on a table, with a small gap between them. When both are switched on, they pull towards each other. (a) What can you say about the two ends that face each other? (b) The connections of the cell of one electromagnet are swapped. What happens now, and why? (c) What would happen if, instead, the connections of both cells were swapped?
Answer: (a) they are unlike poles, one north and one south; (b) they push apart; (c) they still pull together. (a) Unlike poles attract, so the facing ends must be one north pole and one south pole. (b) Changing the direction of the current reverses the poles of an electromagnet. The swapped one's facing end changes, say from north to south, so the two facing ends are now like poles, and like poles repel: the electromagnets push apart. (c) If both currents are reversed, both facing ends change, so they are again one north and one south, and the electromagnets still attract. (Poles of an electromagnet, pages 50 and 51.)
Question 4. A scrap yard uses a crane with a large electromagnet instead of a permanent magnet. (a) Why is an electromagnet better here? (b) Which of these would it lift: iron rods, steel cans, aluminium sheets, copper pipes?
Answer: (a) it can be switched off to drop the load; (b) the iron rods and steel cans. (a) When the current is ON, the electromagnet lifts iron and steel; when it is switched OFF, the magnetic field disappears and the objects drop. A permanent magnet cannot let go. (b) Only magnetic materials are attracted: iron and steel. Aluminium and copper are not magnetic, so they are left behind, which also helps to sort scrap. (Lifting electromagnets, page 52.)
The heating effect
Question 5. A nichrome wire and a copper wire of the same length and thickness carry the same current for the same time. (a) Which gets hotter, and why? (b) Name two other things that change how much heat a wire produces.
Answer: (a) the nichrome wire, because it has more resistance; (b) any two of: the current (more cells), the wire's length, its thickness, how long the current flows. (a) Every conductor resists the current a little, and this turns some electrical energy into heat. Nichrome offers more resistance than copper of the same size, so it heats up more, which is why it is used in heating elements. (b) The heat also depends on the current (two cells heat a wire more than one), on the thickness and length of the wire, and on how long the current flows. Never hold a wire that has carried a current: it can burn. (The heating effect, pages 52 and 53.)
Question 6. Sort these by the effect of current they mainly use: electric kettle, electric bell, hair dryer, loudspeaker, water-heating immersion rod, electric motor.
Answer: heating effect: kettle, hair dryer, immersion rod; magnetic effect: bell, loudspeaker, motor. The kettle, hair dryer and immersion rod each have a heating element, a coil or rod that heats up when current flows. The electric bell, loudspeaker and motor use electromagnets, the magnetic effect of current. (Uses of the two effects, pages 49, 53 and 54.)
Cells and batteries
Question 7. At a science fair, Riya has four cups of vinegar, each with two metal strips dipped in it, not touching: copper and copper; zinc and copper; zinc and zinc; iron and copper. (a) Which of the cups can work as a cell, and why not the others? (b) One zinc-and-copper cup on its own gives too little electricity to light an LED. How can Riya use several such cups together to light one, and which way round should the LED be joined? (c) By the last day of the fair, the zinc strip in one cup has worn thin and that cup gives much less current. Why?
Answer: (a) the zinc-and-copper and iron-and-copper cups; (b) join several cups in a row, the copper of one to the zinc of the next, with the LED's longer leg on the copper end; (c) the chemicals that make the electricity are being used up. (a) A cell needs two electrodes made of different metals, dipped in an electrolyte; here vinegar, a weak acid, is the electrolyte. Zinc/copper and iron/copper are both such pairs. Two copper strips, or two zinc strips, are the same metal, so those cups make no current. (b) One cup is too weak to light an LED, whichever way round the LED is joined: a single cell like this gives only about 1 volt, while an LED usually needs about 2 volts (these two figures are from outside the chapter). So Riya should link several cups in a row, the copper strip of one cup to the zinc strip of the next, as the chapter joins five or six lemons. Copper acts as the positive electrode, and current passes through an LED in one direction only, so its longer leg goes to the free copper end and its shorter leg to the free zinc end. If it still stays dark, she can then check the joins and try turning the LED round. (c) The electricity comes from a chemical reaction between the strips and the electrolyte, and the zinc strip is worn away by it. As the chemicals are used up, the current falls, and in the end the cell is "dead". (Voltaic cells and the lemon battery, pages 55 and 56; copper as the positive electrode, page 56.)
Question 8. Compare a dry cell with a rechargeable battery: what is inside a dry cell, and what happens to each when it runs out? How should used batteries be thrown away?
Answer: a dry cell has a zinc case (negative), a central carbon rod with a metal cap (positive) and a moist paste; it is used once and thrown away, while a rechargeable battery is charged again many times until it wears out; both should go to e-waste recycling. A dry cell's electrolyte is a thick paste rather than a liquid, which is why it is called "dry". It is single use. Rechargeable batteries, used in phones, laptops and electric vehicles, can be recharged many times, but they slowly wear out. Used batteries can contain harmful materials and valuable metals, so they should go to an e-waste recycling point, not ordinary garbage. (Dry cells, rechargeable batteries and recycling, pages 57, 58 and 61.)
Practise more, and check your marks
- Free practice, marked instantly in your browser: Class 8 practice, sets on other Class 8 chapters (there is no set on this chapter yet).
- Printable worksheets with answer keys, in English, Hindi and Marathi: all the Class 8 worksheets.
- Magnets first: Class 6 Exploring Magnets.
- The next chapter: Class 8 Force and Pressure.
Where CMDLearn fits
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Sources
All opened and read on 2 November 2026. Page numbers are the printed page numbers of the textbook; printed page = PDF page + 45 for this chapter.
- NCERT, Curiosity: Textbook of Science for Grade 8 (First Edition June 2025, reprinted January 2026), Chapter 4 "Electricity: Magnetic and Heating Effects", printed pages 46-61 (the magnetic effect and Oersted, pages 47-49; electromagnets and lifting electromagnets, pages 49-52; the heating effect, pages 52-54; Voltaic cells, dry cells and rechargeable batteries, pages 55-58; snapshots, exercises and projects, pages 58-61): ncert.nic.in/textbook/pdf/hecu104.pdf
- NCERT, Curiosity, Grade 8, front matter and contents (edition line, chapter list and starting pages): ncert.nic.in/textbook/pdf/hecu1ps.pdf
The facts are from the textbook pages above. The questions, answers and checks are our own. This guide is general information; follow your teacher and the textbook.