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NCERT Solutions for Class 8th Science Chapter 4 End-of-chapter questions — Keep the curiosity alive

Book page 58–61 Updated on2026-09-19

Q1.
Fill in the blanks: (i) The solution used in a Voltaic cell is called ________. (ii) A current carrying coil behaves like a _______ .
Answer
(i) The solution used in a Voltaic cell is called electrolyte.
(ii) A current carrying coil behaves like a magnet.
Why these are the answers:
  • Electrolyte — in a Voltaic cell the two electrodes are partly dipped in a liquid, usually a weak acid or a salt solution. It is the chemical reaction between the electrodes and this liquid that produces the electricity, and the liquid is given the name electrolyte.
  • Magnet — current in the coil produces a magnetic field. The coil then deflects a compass needle and attracts iron clips, and it has a North and a South pole, exactly as a bar magnet does. Such a coil is called an electromagnet.
Q2.
Choose the correct option: (i) Dry cells are less portable compared to Voltaic cells. (True/False) (ii) A coil becomes an electromagnet only when electric current flows through it. (True/False) (iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (True/False)
Answer
StatementAnswerReason
(i) Dry cells are less portable compared to Voltaic cells.FalseA dry cell has no liquid at all — its electrolyte is a thick moist paste sealed inside a zinc container. A Voltaic cell needs an open glass or plastic container of liquid, which can spill and must be carried upright. The dry cell is more portable, which is exactly why it replaced the Voltaic cell for everyday use.
(ii) A coil becomes an electromagnet only when electric current flows through it.TrueThe magnetism is produced by the current. Stop the current and the field disappears and the clips fall off — an electromagnet is a temporary magnet.
(iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells.FalseIt is the other way round. Two cells drive a larger current, so the magnetic field is stronger and more clips are held. A single cell gives a small current and a weak field.
Q3.
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
Answer

The correct option is (c) Both (i) and (ii) are correct.

Why both happen at once: a single current produces both effects at the same time in the same wire.
  • Heating effect — the nichrome offers resistance to the current, so part of the electrical energy is converted into heat and the wire becomes warm. This is Activity 4.5.
  • Magnetic effect — the current also produces a magnetic field around the wire, so a compass placed below it is deflected. This is Activity 4.1.
Neither effect switches the other off. Nichrome simply makes the heating easier to notice, and it conducts perfectly well, so the compass deflects too.
Q4.
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
Answer
Column AMatchesColumn BWhy
(i) Voltaic cell(d)Generates electricity by chemical reactionsTwo different electrodes in an electrolyte; the reaction between them produces the current
(ii) Electric iron(c)Works on heating effect of electric currentIts heating element gets hot as current passes through it, and the hot plate presses the clothes
(iii) Nichrome wire(a)Best suited for electric heaterIt offers high resistance, so it gives out a lot of heat for a given current and can glow red hot without melting
(iv) Electromagnet(b)Works on magnetic effect of electric currentCurrent in the coil produces a magnetic field, so the coil behaves as a magnet
(i) → (d)   (ii) → (c)   (iii) → (a)   (iv) → (b)
Q5.
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.
Answer

The correct answer is (ii) generates more heat for a given current.

Why (ii) is the reason: a nichrome wire offers higher resistance than a copper wire of the same size and length. The greater the resistance, the more of the electrical energy is converted into heat as the current passes through. So for the same current, nichrome gives far more heat than copper — which is exactly what a heater, a stove or an iron needs.
OptionVerdictWhy
(i) is a good conductor of electricityTrue, but not the reasonCopper is an even better conductor, yet copper wire is not used as a heating element — precisely because it heats too little
(ii) generates more heat for a given currentCorrectThis is the property that makes it a heating element
(iii) is cheaper than copperNot the reasonCost is not why it is chosen; it is chosen for the heat it gives
(iv) is an insulator of electricityWrongAn insulator would not let current pass at all, so no heat could be produced. Nichrome is a conductor
Q6.
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.
Answer

Electric heating converts electrical energy into heat inside a heating element, so nothing has to be burnt in the room. That single difference changes a great deal for the people using it.

  • Clean air indoors. Firewood and charcoal give smoke and soot. Breathing that smoke day after day causes coughing and lung illness, and it is the women and small children who sit closest to the chulha who suffer most. An electric stove produces no smoke, no soot and no ash.
  • Time saved, especially for girls. Collecting and carrying firewood takes hours every week, and that work usually falls on women and children — hours that could go to school, study or paid work. An electric appliance needs only a switch.
  • Instant and controllable heat. The heat begins the moment the switch is ON and stops when it is OFF, and it can be turned up or down. A wood fire has to be lit, tended and then put out.
  • Fewer accidents. There is no open flame, no flying spark and no live coal, so there are fewer burns and fewer house fires — a serious matter in crowded homes.
  • Forests and fuel. Less firewood and charcoal means fewer trees cut, and no smoke released into the outside air either.
  • Cleaner homes and utensils. No ash to clear, no blackened pots to scrub.
The honest other side: the electricity itself has to be generated somewhere. If it comes from burning coal, the pollution has only been moved away from the home to the power station — which is why the chapter ends by pointing to environmentally friendly sources of electrical power. Electric heating also needs a reliable supply, and the wires, plugs and sockets must be rated for the current the appliance draws, otherwise they overheat.
Q7.
Look at the Fig. 4.4a.
−+NESWABcellmagnetic compass(a)
Fig. 4.4(a), page 51 — redrawn sketch: the magnetic compass placed near end A of the electromagnet, which is connected to the cell.
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.
Answer

(i) The path of the current. In Fig. 4.4a the cell lies below the coil, with its negative terminal on the left and its positive terminal on the right. So the current leaves the positive terminal, goes up the right-hand wire to end B, passes through the turns of the coil from B to A, comes down the left-hand wire and enters the negative terminal. Mark the arrows in that order all the way round the loop.

A B S N − + current up to B current down from A compass
Q7 — the current leaves the + terminal, travels up to end B, passes through the coil from B to A, and returns to the − terminal. The red tip of the needle (its north pole) is attracted to end A, so end A is the South pole.

(ii) Why the compass needle moves. Current going round the turns of the coil produces a magnetic field, and the coil therefore behaves as a magnet with a North and a South pole at its two ends. The compass needle is itself a tiny magnet. Placed in the coil's magnetic field, it is pulled round until it lines up with that field — which is what you see as a deflection. Switch the current off and the field disappears, so the needle goes back to north–south.

(iii) If the battery terminals are reversed. The current then goes round the coil the opposite way, 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 amount of deflection stays about the same, because reversing the cell does not change how much current flows — only which way it flows.

The idea behind it: strength depends on how much current flows (and on the number of turns); polarity depends on which way it flows. Change the amount and the deflection grows or shrinks; change the direction and the deflection flips.
Q8.
Suppose Sumana forgets to move the switch of her lifting electromagnet model to OFF position (in 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.
Answer

The wire is still warm, so some current is still flowing — the circuit has not broken. What has failed is the strength of the magnetic effect, not the circuit.

Possible reasons

  1. The cell has run down — the most likely reason. Left connected for a long time, the chemicals inside the cell get used up. The cell becomes weak, the current falls, and a small current makes only a weak magnetic field — too weak to hold the clips against their own weight. If it is left long enough the cell goes fully dead.
  2. Heat has been building up all this while. Current has been flowing continuously through the coil for a long time, and the heating effect works on every bit of that time. So the coil is warm even though the current is now much smaller than it was at the start. A wire that has been warmed for many minutes stays warm for a while.
  3. A connection may have loosened. The warmth softens tape and loosens twisted joints; a partly loose joint lets only a trickle of current through — enough to keep the wire warm, not enough to lift clips.
The key idea: the two effects of current do not fail together. The magnetic effect needs a reasonably large current to lift anything, while a small current flowing for a long time is quite enough to leave a wire feeling warm. Warm wire plus clips falling off is exactly the signature of a current that has dropped, not of a current that has stopped.
Tip: this is why Activity 4.2 warns you not to keep the coil joined to the cell for more than a few seconds. Switch an electromagnet OFF the moment you are not using it — it saves the cell and keeps the coil cool.
Q9.
In Fig. 4.12, in which case the LED will glow when the switch is closed?
Iron nailCopper stripLemon juice(a)Iron nailCopper stripPure water(b)
Fig. 4.12, page 60 — redrawn: the same circuit of an LED, a switch and two electrodes (an iron nail and a copper strip) dipped in (a) lemon juice and (b) pure water.
Answer

The LED glows in case (a) — the beaker containing lemon juice. In case (b), with pure water, it does not glow.

CaseLiquid in the beakerDoes it act as an electrolyte?LED
(a)Lemon juiceYes — it is a weak acidGlows
(b)Pure waterNoDoes not glow
Why: in each beaker there is an iron nail and a copper strip — two electrodes of different metals. That is only half of a Voltaic cell; the other half is the electrolyte. Lemon juice is a weak acid, so in (a) the three parts of a Voltaic cell are complete: chemical reactions take place at the two metals, a current flows round the circuit, and the LED lights. Pure water is not an electrolyte, so in (b) no such reaction is set up and the water does not let current through either. Closing the switch changes nothing.
Try This: stir a spoon of common salt into the beaker of pure water and close the switch again. A salt solution is an electrolyte, and the LED should now glow — showing that it was the liquid, not the metals, that was at fault.
Q10.
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?
Answer

Yes, the coil will still deflect the compass — but the deflection will be less than it was with the iron nail inside.

Why it still works: the magnetic field is produced by the current going round the turns of the coil, and the current is unchanged. You saw this yourself in Activity 4.3: at the stage shown in Fig. 4.3c the coil was wound on nothing but a paper cylinder, and both compass needles were already deflected — before any nail was put in.
Why it is now weaker: iron is a magnetic material. Sitting inside the coil's field, the nail becomes a magnet itself and adds its own magnetism to the coil's. Slide the nail out and that extra contribution is gone; only the coil's own field is left. So the needle still turns, but through a smaller angle — and the coil may no longer be able to hold paper clips.
Check it yourself: mark the resting position of the needle on a sheet of paper, then note the deflected position with the nail in and with the nail out. The difference between the two angles is the iron core's contribution.
Q11.
We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in Fig. 4.13.
Iron+−(a)Copper+−(b)Aluminium+−(c)Nichrome+−(d)
Fig. 4.13, page 60 — redrawn: four identical circuits, each with a cell, a switch and a coil of the named metal.
When current is passed through the coils, 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
Answer

The correct answer is (iv) In all four circuits.

Why: iron, copper, aluminium and nichrome are all metals, and all four are conductors of electricity. So when each switch is closed, current flows in every one of the four coils — and any current carrying coil produces a magnetic field around it. Every one of the four compasses will therefore be deflected.
CoilConducts current?Produces a magnetic field?Compass deflects?
(a) IronYesYesYes
(b) CopperYesYesYes
(c) AluminiumYesYesYes
(d) NichromeYesYesYes
Did you know? The four deflections need not be equal. Nichrome offers the most resistance, so it lets the least current through and gives the smallest deflection — while it warms up the most. The magnetic effect is present in all four; only its size differs.
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