NCERT Solutions for Class 8 Science Chapter 4 Electricity Magnetic and Heating Effects (PDF) – 2027

Yes. Place a magnetic compass close to the wire of the circuit — just below it is best — and watch the needle as you close the switch.

• If the needle swings away from its usual north–south direction, current is flowing.
• If the needle stands still, no current is flowing.
• Open the switch again: the needle should return to its original direction.

This works because a wire carrying current produces a magnetic field around it, and this field turns the compass needle.

Yes. A temporary magnet is made by winding a coil of insulated wire around an iron nail and passing current through it — this is an electromagnet.

1. Take about 50 cm of flexible insulated wire and an iron nail.
2. Wind the wire tightly around the nail, turn after turn, all in the same direction.
3. Connect the two free ends of the wire to an electric cell for a few seconds only.
4. The nail now picks up iron paper clips. Disconnect the cell and the clips fall off.

In an electrical appliance nothing burns. The heat comes from the heating effect of electric current.

When current flows through a conductor, the conductor offers some opposition to it, called resistance. Because of this resistance, part of the electrical energy is changed into heat, and the conductor gets hot. Heaters, irons, kettles and stoves have a heating element — a rod or coil of wire, usually of nichrome — that becomes hot in this way.

A cell is dead when the chemicals inside it have been used up, so it can no longer supply electricity. You can tell in these ways:

• A lamp or LED connected to it does not glow, or glows very dimly.
• An electromagnet made with it will not lift paper clips.
• A compass placed near the circuit wire shows little or no deflection.
• A device that used to run for hours now stops after a few minutes.

No — not all cells and batteries can be recharged.

— questions I would like to explore, and where the chapter answers them:

The moment the switch is closed, the compass needle is deflected — it turns away from the north–south direction in which it was resting and settles at an angle to the wire.

This shows that a wire carrying current produces a magnetic field around it, and this field acts on the compass needle, which is a tiny magnet.

The needle returns to its original direction — it swings back and once again settles along the north–south line.

This is because the current stops, so the magnetic field around the wire disappears.

The needle behaves the same way every time: it kicks out of the north–south direction at each ON, and comes back to it at each OFF.

So the needle is deflected only while current flows through the wire.

Yes. A current carrying coil behaves exactly like a magnet, and such a coil is called an electromagnet.

When current flows through a coil of insulated wire, the coil deflects a compass needle and attracts iron clips; an iron nail placed inside the coil makes it much stronger. It is a magnet only while the current flows.

Yes. With the coil connected to the cell, the clips cling to the nail and hang from its two ends — you can lift a small chain of them clear of the table.

The current in the coil turns the iron nail into a magnet (an electromagnet), and iron clips are attracted to a magnet.

Yes — the clips fall off at once.

When the current stops, the magnetic field of the coil disappears and the nail loses its magnetism. An electromagnet is a temporary magnet — it is a magnet only while current flows.

Yes. Both compass needles are deflected as soon as the coil is connected — even though the coil is only wound on a paper cylinder and there is no iron in it at all.

The coil carrying current produces a magnetic field, so it behaves like a magnet and turns the compass needles.

Yes. Both needles swing back and rest once more along the north–south direction.

With no current, the coil produces no magnetic field, so only the Earth’s magnetism acts on the needles and they point north–south again.

Yes — the deflection is much larger with the iron nail inside than with the empty paper cylinder, for exactly the same cell and the same coil.

Iron is a magnetic material. Inside the coil the nail becomes a magnet itself and adds to the coil’s magnetism, so the electromagnet becomes much stronger.

Yes. The clips are pulled towards the two ends of the nail and hang there while the current flows (Fig. 4.3e). Stop the current and they drop.

The coil with its iron core behaves as a magnet, and a magnet attracts most strongly at its ends (poles).

Yes. Like a bar magnet, an electromagnet has two poles — North and South, one at each end of the coil.

This is what Activity 4.4 shows. Hold a compass near end A: one particular pole of the compass needle is attracted to it. Since unlike poles attract, that tells you what end A is. Repeat at end B and you find the opposite pole — so the two ends of an electromagnet are always of opposite kind, exactly like the two ends of a bar magnet.

In the set-up shown in Fig. 4.4a it is the north pole of the compass needle (the red tip) that swings towards end A.

Yes. At end B the other pole of the compass needle is attracted, so end B is the North pole while end A is the South pole.

Both changes make the electromagnet stronger: the compass needle deflects more and the coil holds more clips.

Conclusion: the strength of an electromagnet increases when the current is increased (more cells) and when the number of turns is increased.

Reverse the connections to the cell and the deflection of the compass needle reverses too: the pole of the needle that used to be attracted to end A is now pushed away, and the other pole is attracted instead. In other words, the poles of the electromagnet have swapped over — what was South is now North.

Putting the whole box together:

• The strength of an electromagnet is changed by changing the current, the number of turns, or both.
• The polarity of an electromagnet is reversed by reversing the direction of the current.

Because of moving electric currents deep inside the Earth. In the core, liquid iron is constantly in motion, and this movement creates electric currents. Those currents, exactly like the current in your coil, generate a magnetic field — and that field spreads far out beyond the surface of the planet.

Two consequences of the Earth’s magnetic field:

• Many migratory birds, fish and animals use it to navigate across continents and oceans.
• It acts as a shield, blocking harmful particles coming from space, and so helps protect life on Earth.

Yes — these are called lifting electromagnets. They are very strong electromagnets that may be hung from cranes in place of the usual hook.

They are widely used in factories and scrap yards, to move, lift and sort heavy metal items efficiently.

Yes — and that warmth is the heating effect of electric current, the second effect this chapter is about.

Every conductor offers some resistance to the flow of current. Because of this, part of the electrical energy is changed into heat, so the wire becomes warm.

With the switch still in the OFF position, the nichrome wire feels cool — it is at room temperature, no different from the cardboard or the nails around it.

Now the wire feels distinctly warm. Repeat the ON–OFF cycle and you get the same result each time, which confirms that the warmth is caused by the current.

The nichrome wire offers resistance to the current, so some of the electrical energy is converted into heat. This warming is called the heating effect of electric current.

The wire heats up more with two cells. For the same 30 seconds, the amount of heat generated in the experiment with 2 cells is greater.

This is because two cells drive a larger current through the wire, and the heat produced depends on the magnitude of the current.

Inside every cell there are two electrodes of different materials dipped in an electrolyte, and it is the chemical reaction between them that produces the electricity.

When the electrodes are joined by a wire through a lamp, the chemical reaction makes current flow from the positive terminal, through the circuit, to the negative terminal. When the chemicals are used up, the cell stops working and is called dead.

Yes — a lemon, a copper wire and an iron nail are enough. That is exactly what you build in Activity 4.6.

• Electrodes: the copper wire and the iron nail — two different metals, as a Voltaic cell requires.
• Electrolyte: the lemon juice, which is a weak acid and helps conduct electricity. A salt solution works just as well.
• Container: the lemon itself.

If the LED is the right way round, it glows — faintly, but clearly visible in a shaded corner of the room. A glowing LED indicates that your cell is working.

Each lemon, with a copper wire and an iron nail in its juice, is a small Voltaic cell. Joined together, the lemons make a battery that drives a current through the LED.

Yes. Turn the LED round in the circuit and it lights up.

An LED lets current pass only one way. It glows only when its longer (positive) wire is joined to the positive side of the battery — the copper wire of the first lemon — and its shorter (negative) wire to the negative side, the iron nail of the last lemon.

The liquid (a weak acid or salt solution) in which the electrodes dip is the electrolyte; a coil carrying current behaves like a magnet and is called an electromagnet.

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

The same current produces both effects at once: the nichrome wire gets warm because of the heating effect, and the magnetic field it produces around itself deflects the compass (magnetic effect).

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

Nichrome offers much higher resistance than a copper wire of the same size and length, so for the same current it converts more electrical energy into heat.

Electric heating devices are more convenient, and better for society, because:

1. No smoke indoors — nothing is burnt, so there is no smoke, soot or ash; women and children near a chulha no longer breathe smoke that harms the lungs.
2. Saves time and effort — no firewood to collect and carry (usually the work of women and children); that time can go to school or work.
3. Instant, controllable heat — it starts and stops with a switch and can be turned up or down.
4. Safer — no open flame, sparks or live coal, so fewer burns and house fires.
5. Saves trees, cleaner air — less firewood and charcoal are burnt, so fewer trees are cut and less smoke goes into the air.
6. Cleaner homes — no ash to clear and no blackened pots to scrub.

(i) The current leaves the positive (+) terminal of the cell, goes up the right-hand wire to end B, flows through the coil from B to A, and comes down the left-hand wire to the negative (−) terminal. Arrows are drawn along this path.

(ii) Current in the coil produces a magnetic field, so the coil behaves like a magnet with North and South poles. The compass needle, itself a small magnet, turns in this field — this is the deflection.

(iii) The current will flow through the coil the opposite way, so the poles of the electromagnet are reversed (end A becomes North, end B South). The compass needle will therefore deflect in the opposite direction; the amount of deflection stays about the same.

The electromagnet stopped lifting the clips because the current in the coil became too small. The wire is still warm, so some current is still flowing — the circuit is not broken.

Possible reasons:

1. The cell has run down. Left connected for a long time, the chemicals inside the cell got used up, so the cell became weak and now gives much less current. A small current makes only a weak magnetic field, too weak to lift the clips. (If left on even longer, the cell becomes completely dead.)
2. A connection may have become loose, letting less current through the coil.

The wire is still warm because current has been flowing through it for a long time (heating effect), and even the smaller current still heats it.

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

In (a), the two different metals and lemon juice (an electrolyte) together make a Voltaic cell, which drives a current through the LED. Pure water is not an electrolyte, so in (b) no current is produced.

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

The magnetic field is produced by the current in the coil, and the current is unchanged, so the needle still deflects. But the iron nail — which had become a magnet and added to the coil’s magnetism — is gone, so the deflection is smaller.

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

Iron, copper, aluminium and nichrome are all conductors of electricity, so current flows in every coil — and any coil carrying current produces a magnetic field around it.

Sample report. Coils of 25, 50, 75 and 100 turns were connected one by one to the same cell, with the compass kept at the same place each time.

Conclusion. With the current kept the same, the strength of an electromagnet increases as the number of turns of the coil increases.

Do this only under your teacher’s supervision. Touch each wire momentarily, never hold it, and switch OFF as soon as the 30 s are over.

Conclusion. The heating of a nichrome wire depends on its thickness and its length. With the same cell, the thinner wire got hotter than the thicker one, and the shorter wire got hotter than the longer one.

Reason. The heat produced in the thin wire is shared by much less metal, so its temperature rises more. The longer wire has more resistance, so the same cell sends a smaller current through it, and a smaller current produces less heat.

Sample report. Lemon cells were made as in Activity 4.6, changing one thing at a time — first the fruit or vegetable, then the pair of metals — and the brightness of the LED was compared.

Sample conclusion for your report: “A cell can be made from any juicy fruit or vegetable provided the two electrodes are of different metals. Sour juices work better than bland ones, and the zinc–copper pair gave the brightest LED. Two electrodes of the same metal never produced any current.”