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NCERT Solutions Curiosity Chapter 4 Chapter opener — Probe and ponder

Book page 46 Updated on2026-09-05

Q1.
If we don't have an electric lamp while making an electric circuit with an electric cell, is there any other way through which we can find out if current is flowing in the circuit?
Answer

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.
Why it happens: a current carrying wire produces a magnetic field around it. The compass needle is itself a tiny magnet, so it is turned by that field. No current means no field, and the needle simply points north–south as usual. This is exactly the test you carry out in Activity 4.1.
Tip: a second, rougher test is the heating effect — a thin nichrome wire in the circuit becomes warm when current flows. The compass test is better because it is instant, needs no touching, and works even for a very small current.
Q2.
Is it possible to make temporary magnets? How can these be made?
Answer

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.
Why it is temporary: the magnetism comes from the current, not from the nail. Each turn of the coil makes its own magnetic field, and because every turn is wound the same way, these fields point the same way inside the coil and add up. The iron nail sitting in that field becomes a magnet itself and makes the whole thing much stronger. The moment the current stops, the coil's field vanishes and the iron loses almost all its magnetism — so the clips drop.
Did you know? This is exactly how a lifting electromagnet on a crane works. The operator does not have to prise the load off the magnet — he simply switches the current OFF.
Q3.
We can generate heat by burning fossil fuels and wood; but how is heat generated in various electrical appliances?
Answer

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

Why it happens: when current flows through a conductor it faces some opposition, called resistance. In pushing past that resistance, part of the electrical energy is converted into heat energy, and the conductor gets hot. The greater the resistance, the more heat is produced for the same current.

Every heating appliance therefore contains a rod or coil of wire called the heating element, usually made of nichrome, which offers far more resistance than copper of the same size and length. In an electric room heater or a stove you can see this element glowing red hot.

Burning wood or fossil fuelElectrical appliance
Chemical energy of the fuel is released as heat by burningElectrical energy is converted into heat by resistance
Gives smoke, soot and ashGives no smoke, soot or ash
Hard to switch off or control quicklyHeat starts and stops with the switch
Q4.
How do we know if a cell or a battery is dead? Can all cells and batteries be recharged?
Answer

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.

TypeCan it be recharged?What to do when it is finished
Dry cell (torch, clock, remote)No — it is a single use cellDispose of it at an e-waste facility
Rechargeable battery (phone, laptop, inverter, vehicle)Yes — many times overAfter many charge cycles it slowly wears out; then send it for recycling
Why a rechargeable battery still does not last forever: each charge and discharge slightly changes the materials inside it. After a year or two of daily charging those changes add up, the battery holds less charge, and the phone needs charging more often.
Check it yourself: a battery that has stopped working is not really 'dead' — it still holds acids and metals such as lead, cadmium, nickel or lithium. Thrown into ordinary garbage these can cause fires or harm the environment, and valuable material is wasted. Always hand used batteries to an e-waste recycling point; ask your teacher if you are not sure where one is.
Q5.
Share your questions
Answer

Frame questions that begin with why, how or what if, and that you could actually test with a cell, a coil and a compass. Here are some to start you off — each one is answered somewhere in this chapter, so keep them beside you as you read.

Your questionWhere the chapter answers it
Does the compass needle deflect the same amount however far it is from the wire?Section 4.1, Activity 4.1
What if I wind 100 turns instead of 50 — will the coil lift more clips?Think like a scientist, page 51; Discover, design, and debate, page 61
Why does the coil pick up iron clips but not a plastic button or an aluminium foil?Section 4.1.1 — only magnetic materials are attracted
Why is the heating coil of a heater made of nichrome and not of copper?Section 4.2, page 53
Can a potato or a tomato light an LED, the way a lemon does?Activity 4.6; Discover, design, and debate, page 61
Try This: write your questions in your notebook before you read the chapter, and tick them off as you find the answers. The ones still left unticked at the end are the best ones to take to your teacher.
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