Keep the Curiosity Alive — Chapter Exercises — "Electricity: Magnetic and Heating Effects"

1. 12 marksCuriosity Grade 8, Chapter 4, page 58, exercise 1

   Fill in the blanks:

   (i) The solution used in a Voltaic cell is called ________.

   (ii) A current carrying coil behaves like a ________.

   Hint. Both answers are single words from the Snapshots.

   (i) The solution used in a Voltaic cell is called an electrolyte.

   From section 4.3.1: It contains two metal rods made of different materials and a liquid called an electrolyte, placed in a glass or plastic container. The electrolyte is usually a weak acid or salt solution — lemon juice in Activity 4.6, or a salt solution.

   (ii) A current carrying coil behaves like a magnet.

   From section 4.1.1: when current is passed through the cylindrical coil, it behaves like a magnet and deflects the needle of a magnetic compass. Such a coil is called an electromagnet, so 'electromagnet' is also an acceptable answer — but the blank follows the book's sentence, and 'magnet' is what fills it.

   ✦ A trap in (ii) worth avoiding. Do not write 'behaves like an iron nail' or add that it needs an iron core, because the coil in Activity 4.3 was wound on rolled paper and it still deflected both compasses. The current alone makes it a magnet; the iron core only makes it a stronger one.

   And in (i), 'acid' is not the answer the blank wants. Lemon juice happens to be acidic, but a salt solution works just as well, so the word that covers both is electrolyte — which is the term the chapter defines.

2. 23 marksCuriosity Grade 8, Chapter 4, page 58, exercise 2

   State whether True or False:

   (i) Dry cells are less portable compared to Voltaic cells.

   (ii) A coil becomes an electromagnet only when electric current flows through it.

   (iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells.

   Hint. Each statement is settled by one sentence of the chapter.

   (i) False. It is the other way round. Dry cells are more portable, and that is the whole reason they exist: Voltaic cells were an important discovery, but they are not convenient for everyday use. A Voltaic cell holds a liquid electrolyte in an open glass or plastic container, which cannot be tipped, shaken or carried in a pocket. A dry cell uses a thick moist paste instead, so it can be sealed and used in any position.

   (ii) True. A current carrying coil that behaves as a magnet is called an electromagnet. Activity 4.2 showed both halves of this: the clips cling while the cell is connected, and fall the moment it is disconnected. No current means no magnetic field, so the word only in the statement is correct.

   (iii) False. Reversed again. A battery with more cells gives a larger current as compared to that with a single cell. This creates a stronger magnetic field, so the deflection of the compass needle is more and the coil can attract more clips. Two cells lift more clips than one, not fewer.

   ✦ Notice the pattern in this question. Two of the three statements are simply the correct fact turned around. Reading them quickly, they sound plausible; reading them against the chapter, the direction is wrong. When a True/False statement compares two things, check which way round the comparison is pointing before answering.

3. 32 marksCuriosity Grade 8, Chapter 4, page 59, exercise 3

   An electric current flows through a nichrome wire for a short time.

   (i) The wire becomes warm.

   (ii) A magnetic compass placed below the wire is deflected.

   Choose the correct option: (a) Only (i) is correct (b) Only (ii) is correct (c) Both (i) and (ii) are correct (d) Both (i) and (ii) are not correct

   Hint. This chapter has two effects in its title. Which of them applies here?

   Answer: (c) Both (i) and (ii) are correct.

   Statement	Verdict	Which effect
   (i) The wire becomes warm	Correct	Heating effect — this is exactly Activity 4.5, which used a nichrome wire and found it warm after 30 s
   (ii) A compass below the wire is deflected	Correct	Magnetic effect — this is exactly Activity 4.1, in which the compass was placed beneath a current-carrying wire

   Why both happen at once. The two effects are not alternatives that you choose between. When electric current flows through a conductor (like a wire), it produces a magnetic field around it — every conductor, always. And when an electric current passes through a conductor, it gets heated — again, every conductor, always. A single current in a single wire does both things simultaneously.

   ✦ The reason the question specifies nichrome. Nichrome has a higher resistance than copper, so the warming in (i) is easy to feel. But the magnetic effect in (ii) does not depend on the wire being nichrome at all — Activity 4.1 used an ordinary connecting wire. The material choice makes one effect noticeable; it does not switch the other one off.

   Exercise 8 is built on this same point: Sumana's electromagnet stopped lifting clips and the wire was still warm. Both effects, in one circuit.

4. 44 marksCuriosity Grade 8, Chapter 4, page 59, exercise 4

   Match the items in Column A with those in Column B.

   Column A: (i) Voltaic cell, (ii) Electric iron, (iii) Nichrome wire, (iv) Electromagnet

   Column B: (a) Best suited for electric heater, (b) Works on magnetic effect of electric current, (c) Works on heating effect of electric current, (d) Generates electricity by chemical reactions

   Hint. Two of the four entries in Column B mention an effect by name; the other two do not.

   The matching:

   Column A		Column B	Why
   (i) Voltaic cell	→	(d) Generates electricity by chemical reactions	A chemical reaction between the rods and the electrolyte produces electricity
   (ii) Electric iron	→	(c) Works on heating effect of electric current	It is in the chapter's list of appliances that work on the heating effect (Fig. 4.6d)
   (iii) Nichrome wire	→	(a) Best suited for electric heater	It offers higher resistance compared to a copper wire of the same size and length, so it makes a good heating element
   (iv) Electromagnet	→	(b) Works on magnetic effect of electric current	A current carrying coil that behaves as a magnet is called an electromagnet

   Answer in short: (i)–(d), (ii)–(c), (iii)–(a), (iv)–(b).

   ✦ How to avoid the one likely mix-up. Both (ii) the electric iron and (iii) the nichrome wire belong to the heating half of the chapter, so it is tempting to pair either of them with (c). The way to separate them is to notice what kind of thing each is. An electric iron is a device, so it works on an effect — that is (c). Nichrome is a material, so it is suited for a use — that is (a). Reading Column B's wording carefully settles it.

   Note also that (i) is the only entry that is a source of electricity rather than a user of it, which makes (d) easy to place first and narrows the rest.

5. 53 marksCuriosity Grade 8, Chapter 4, page 59, exercise 5

   Nichrome wire is commonly used in electrical heating devices because it

   (i) is a good conductor of electricity. (ii) generates more heat for a given current. (iii) is cheaper than copper. (iv) is an insulator of electricity.

   Which of these are correct?

   Hint. Check each statement against what the chapter actually says about nichrome.

   Correct: (i) and (ii).

   	Statement	Verdict	Reason
   (i)	Is a good conductor of electricity	Correct	It has to carry the current in the first place. Nichrome is a metal alloy and conducts; the chapter treats it as a conductor throughout Activity 4.5
   (ii)	Generates more heat for a given current	Correct	A nichrome wire offers higher resistance compared to a copper wire of the same size and length, and this resistance causes some of the electrical energy to be converted into heat energy
   (iii)	Is cheaper than copper	Not correct	The chapter says nothing about the cost of nichrome. Cost is not the reason it is chosen, and it is not generally cheaper than copper
   (iv)	Is an insulator of electricity	Incorrect	Flatly contradicts (i) and contradicts the activity. An insulator would carry no current, so there would be no heat at all

   ✦ (i) and (ii) together are the whole answer, and neither alone would do. A material that conducts well but resists little — copper — carries the current without producing much heat, so it makes connecting wires, not heating elements. A material that resists so much that it barely conducts would carry too little current to heat anything. Nichrome is useful because it does both: it conducts, and it resists enough to turn a large part of the energy into heat.

   Why (iii) is the tempting wrong option. It sounds like the sort of practical reason a manufacturer might have, and it is not obviously absurd the way (iv) is. But nothing in the chapter supports it. When an option introduces a fact the text never mentions, that is usually the distractor.

   (i) and (iv) directly contradict each other, so at most one of them can be true — which is a quick way to spot that (iv) must go.

6. 65 marksCuriosity Grade 8, Chapter 4, page 59, exercise 6

   Electric heating devices (like an electric heater or a stove) are often considered more convenient than traditional heating methods (like burning firewood or charcoal). Give reason(s) to support this statement considering societal impact.

   Hint. The question asks for societal impact, so go beyond 'it is easier to use'.

   Reasons, grouped by the kind of impact.

   1. Health. Burning firewood or charcoal indoors produces smoke, and cooking over it means breathing that smoke, often for hours a day. An electric stove or heater produces no smoke at all, because nothing is being burnt — the heat comes from the heating effect of electric current in a heating element. The burden of indoor smoke falls most heavily on whoever does the cooking.

   2. Time and labour. Firewood has to be gathered, carried, chopped and stored, and charcoal has to be bought and fetched. An electric appliance is switched on. The hours saved are hours available for school, work or rest, and that time is very unevenly distributed in most households.

   3. Control and safety. Electric heat starts and stops with a switch and can be regulated. An open fire cannot be turned off, needs watching, leaves hot ash, and can spread. Cleanliness is better too — no soot on vessels, walls or clothes.

   4. Environment. Firewood means cutting trees, and burning wood or charcoal releases smoke into the air outdoors as well as indoors.

   ✦ The honest other side, which a good answer should include. Convenience is not the same as being better in every respect.

   • An electric appliance is useless without a reliable electricity supply, which not every household has all the time.
   • It costs money to buy and to run, which is a real barrier for some families, while firewood may be collected for nothing.
   • Whether it is cleaner overall depends on how the electricity was generated. If it comes from burning coal, the smoke has been moved from the kitchen to the power station rather than eliminated — a large gain for the household's health, but a smaller one for the air in general.

   A fair conclusion: electric heating is clearly more convenient, much better for the health of the people in the house, and better for the local environment — and its wider environmental benefit grows as more electricity comes from cleaner sources, which is the direction the chapter points to at the end.

7. 75 marksCuriosity Grade 8, Chapter 4, page 59, exercise 7

   Look at Fig. 4.4a. If the compass placed near the coil deflects:

   (i) Draw an arrow on the diagram to show the path of the electric current. (ii) Explain why the compass needle moves when current flows. (iii) Predict what would happen to the deflection if you reverse the battery terminals.

   Hint. For (i), start at the marked terminal of the cell and follow the wire all the way round.

   (i) The path of the current. In Fig. 4.4a the cell lies below the coil, and its + terminal is on the right and its − terminal on the left. Conventional current flows out of the positive terminal, round the external circuit, and back into the negative terminal. So your arrows should trace:

   + terminal (right) → up the right-hand wire → into end B → round and round the coil from B to A → down the left-hand wire → back into the − terminal (left).

   Draw arrowheads at three places — on the right-hand wire pointing up, on the coil showing the direction the turns are followed, and on the left-hand wire pointing down. It is one continuous loop, so every arrow must point the same way round it.

   (ii) Why the needle moves. Because a current-carrying coil produces a magnetic field around it, and the compass needle is itself a tiny magnet. Placed in that field near end A, the needle is attracted and turns away from its usual north–south position. Since unlike poles attract, whichever pole of the needle swings towards end A tells you that end A is the opposite pole — in Fig. 4.4a the red north tip has turned towards the coil, so end A there is the south pole. Stop the current and the field disappears, so the needle returns to north–south.

   (iii) Reversing the battery terminals. The current then flows the other way round the coil, so the poles of the electromagnet swap over — end A becomes north and end B becomes south. The needle therefore deflects in the opposite direction: the tip that had turned towards the coil now turns away, and the other tip comes round.

   ✦ The size of the deflection does not change. You have altered only the direction of the current, not how much of it there is and not the number of turns. The chapter is precise about this: strength is changed by the current and the turns, while its poles can be reversed by changing the direction of the current. A common wrong answer here is 'the deflection becomes smaller' or 'the needle stops deflecting' — neither happens.

8. 85 marksCuriosity Grade 8, Chapter 4, page 60, exercise 8

   Suppose Sumana forgets to move the switch of her lifting electromagnet model to the OFF position (in the introduction story). After some time, the iron nail no longer picks up the iron paper clips, but the wire wrapped around the iron nail is still warm. Why did the lifting electromagnet stop lifting the clips? Give possible reasons.

   Hint. The warm wire is a clue, not just a detail. What does it rule out?

   Start with what the warm wire tells you. It shows that current has been flowing through this coil for a long time, producing heat by the heating effect. That rules out the simplest explanations at once: the circuit has not come apart, a wire has not fallen off the cell, and the switch has not sprung open. Something is still connected.

   The main reason: the cell has run down.

   1. The coil was left connected across the cell, so a large current flowed continuously for a long time.
   2. A cell produces current from chemical reactions inside it, and over time, the chemicals get used up.
   3. The current has therefore fallen a great deal — the cell is weak, or dead.
   4. A smaller current makes a weaker magnetic field, so the electromagnet is no longer strong enough to lift the clips against their weight.

   And that is consistent with a warm wire, because the wire has been heated over a long period and holds that heat for a while, and because a weakened cell may still drive a small current — enough to warm the wire, not enough to lift a clip.

   ✦ Notice what has actually failed. Nothing is broken. The nail is the same nail, the coil has the same turns, the circuit is intact. It is the energy source that has been exhausted — by exactly the mistake Activity 4.2 warns against: Take care to not connect the wires to the cell for more than a few seconds; otherwise, the cell may weaken quickly.

   A second possible reason, worth mentioning as a possibility. Prolonged heating may damage the insulation on the wire where the turns press together. If adjacent turns touch, the current can bypass part of the coil, so fewer turns act — and fewer turns means a weaker magnet. This goes a little beyond what the chapter states, so offer it as a possible contributing reason rather than the answer.

   The lesson Sumana takes away: switch off between demonstrations. Leaving an electromagnet on wastes the cell, warms the wire and eventually stops the model working.

9. 94 marksCuriosity Grade 8, Chapter 4, page 60, exercise 9

   In Fig. 4.12, in which case will the LED glow when the switch is closed? In (a) an iron nail and a copper strip dip into lemon juice; in (b) the same nail and strip dip into pure water. Both are connected through an LED and a switch.

   Hint. Compare each beaker with the definition of a Voltaic cell.

   The LED glows in case (a) only — the beaker of lemon juice.

   Why (a) works. Everything a Voltaic cell needs is present:

   Requirement	In beaker (a)
   Two electrodes of different metals	Iron nail and copper strip ✓
   An electrolyte — a weak acid or salt solution	Lemon juice ✓
   A container	The beaker ✓

   This is exactly the cell you built in Activity 4.6, where the electrolyte is the lemon juice, which helps conduct electricity. A chemical reaction between the metals and the juice produces a current, which passes through the LED and lights it.

   Why (b) does not. The two metals are the same, but the liquid is pure water, which is neither a weak acid nor a salt solution. It does not act as an electrolyte, so no appreciable current flows and the LED stays dark.

   ✦ The single variable in this question. The two set-ups are identical in every respect except the liquid — same nail, same copper strip, same LED, same switch. So whatever difference appears must be caused by the liquid, and that is precisely the point being tested: the electrolyte is not optional. Two metals alone do not make a cell.

   One honest qualification. The chapter distinguishes pure water from ordinary water for a reason. Tap water, well water and salty water contain dissolved substances and do conduct to some extent. The figure specifies pure water, and it is pure water — with nothing dissolved in it — that fails to work here.

   If (a) does not glow when you try it, check the LED's direction before concluding anything, exactly as Activity 4.6 instructs.

10. 104 marksCuriosity Grade 8, Chapter 4, page 60, exercise 10

    Neha keeps the coil exactly the same as in Activity 4.4 but slides the iron nail out, leaving only the coiled wire. Will the coil still deflect the compass? If yes, will the deflection be more or less than before?

    Hint. Activity 4.3 tested precisely this arrangement, before the nail was ever inserted.

    Yes, the coil will still deflect the compass — and the deflection will be less than before.

    Why it still deflects. The magnetic field is produced by the current in the coil, not by the nail. Activity 4.3 established this by testing the coil in exactly Neha's condition: wound on a rolled paper cylinder with nothing magnetic inside, and when current is passed through the cylindrical coil, it behaves like a magnet and deflects the needle of a magnetic compass. Removing the nail removes none of that.

    Why the deflection is smaller. The very next step of Activity 4.3 inserted the nail and found that the coil becomes a stronger magnet and the deflection of the magnetic compass needle is much more. Neha has undone that step, so she loses that extra strength and goes back to the weaker deflection of the bare coil.

    ✦ What has not changed. Neha kept the coil exactly the same — same number of turns, same cell, so the same current. Only the core was removed. That is what makes this a fair comparison and lets you attribute the whole difference to the iron.

    A further prediction you can make. If Neha now brings paper clips to the ends of the bare coil, they will most likely not be picked up. In Activity 4.3 the clips clung only after the nail was inserted; the coil alone was strong enough to move a delicately balanced needle but not to lift objects.

    This is the second half of the answer to exercise 11 and to the friend's misconception in section 4.1: iron strengthens an electromagnet, it does not cause it.

11. 114 marksCuriosity Grade 8, Chapter 4, pages 60-61, exercise 11

    Four coils of similar shape and size are made from iron, copper, aluminium and nichrome (Fig. 4.13), each connected to a cell and a switch. When current is passed, compass needles placed near the coils will show deflection:

    (i) Only in circuit (a) (ii) Only in circuits (a) and (b) (iii) Only in circuits (a), (b), and (c) (iv) In all four circuits

    Hint. Ask one question of each coil: can a current flow through it?

    Answer: (iv) In all four circuits.

    The reasoning. When electric current flows through a conductor (like a wire), it produces a magnetic field around it. The rule says a conductor — it does not say a magnetic conductor, or an iron one. Iron, copper, aluminium and nichrome are all conductors, so current flows in all four coils, so all four produce a magnetic field, so all four deflect a nearby compass needle.

    Coil	Conducts?	Deflection?
    (a) Iron	Yes	Yes
    (b) Copper	Yes	Yes
    (c) Aluminium	Yes	Yes
    (d) Nichrome	Yes	Yes

    ✦ The misconception being tested. Many students pick (i), reasoning that iron is the magnetic material so only the iron coil can be a magnet. That confuses two different things. Being a magnetic material means being attracted by a magnet — which is why an iron nail makes a good core. Producing a magnetic field is what a current does, in any conductor. Copper and aluminium are not magnetic materials at all, yet a current in a copper coil deflects a compass perfectly well; the coil in Activity 4.3 was copper wire on paper.

    Will the four deflections be equal? The question does not ask, and you should not claim they are. Nichrome offers a higher resistance than copper of the same size and length, so with the same cell a smaller current would flow in coil (d), giving a smaller deflection. The iron coil is also itself a magnetic material, which may add to its effect. But every one of the four deflects, and that is what makes (iv) the answer.

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.