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NCERT Solutions Curiosity Chapter 4 Science, Society and Interdisciplinary Projects — Discover, design, and debate

Book page 61 Updated on2026-09-05

Q1.
Make coils of turns 25, 50, 75, and 100. Connect them to the same cell one by one. Note the deflection in a magnetic compass placed in the same position in all the cases. Report your observations. Draw conclusion of the effect of number of turns of the coil on the strength of the electromagnet.
Answer

How to do it fairly. Only one thing must change from trial to trial — the number of turns.

  • Use the same cell, the same insulated wire and the same paper cylinder each time.
  • Mark the compass position on the table with chalk and never move it; keep the same distance from the end of the coil.
  • Note the resting direction of the needle first, then the deflected direction, and record the angle turned through.
  • Connect for only a few seconds each time, so the cell does not weaken as you go along.
Number of turnsDeflection of compass needleNumber of clips held
25SmallestFewest
50LargerMore
75Larger stillMore still
100LargestMost

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

Why it happens: each turn carries the same current and makes its own magnetic field. Because every turn is wound in the same direction, all these fields point the same way inside the coil and add together. More turns means more fields adding up in the same region, so the total field is stronger.
Tip: for a fair test the coil must stay the same length as you add turns — wind the extra turns in neat layers, not by stretching the coil out along the cylinder.
Q2.
Take two thin nichrome wires of equal length and different thickness (approximately one of these wire thickness to be double of the other, say 0.3 mm and 0.6 mm). Connect them one by one in a circuit which has a switch and a cell, and allow the current to flow for 30 s in each case. Momentarily touch these wires. Which wire heats up more? Now repeat the same activity with two nichrome wires of same diameter but of different lengths. Prepare a brief report of your activity.
Answer

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.

Part 1 — same length, different thickness. The thinner (0.3 mm) wire heats up more.

Part 2 — same thickness, different length. The longer wire heats up more when the two are compared over the same 30 seconds.

TrialWhat was changedWhich felt hotter
10.3 mm and 0.6 mm, same lengthThe 0.3 mm (thinner) wire
2Short and long, same thicknessThe longer wire
Why: the chapter tells us that the heat generated in a wire depends on the material, thickness, length of the wire and the duration for which the current flows. A thinner wire offers more resistance to the current than a thick one of the same material and length, so more of the electrical energy is turned into heat in it. A longer wire likewise offers more resistance than a short one of the same thickness. This is why the heating element of a heater is made of a long, thin nichrome coil rather than a short fat rod.

Writing the report. State the aim; list the materials with their exact sizes; describe what you did; give a table of your observations; then write the conclusion in one sentence for each part and add the precaution you followed.

Safety first: keep the current flowing for no more than the 30 s stated, and do not touch the wire for an extended period, to avoid any injuries.
Q3.
Try to make an electric cell using various fruits and vegetables. Also try with electrodes of different metals. Prepare a brief report.
Answer

Method. Build the lemon cell of Activity 4.6 again, but change one thing at a time.

  • Changing the fruit or vegetable: keep the same pair of electrodes (copper wire and iron nail) and try lemon, orange, tomato, potato, raw mango, and a cup of salt solution.
  • Changing the metals: keep the same lemon and try the pairs the chapter names — zinc and copper, zinc and silver, aluminium and copper, iron and copper, magnesium and copper, lead and copper.
  • Judge each by how brightly the LED glows. Use the same LED, the same number of cells in the chain and the same spacing of the electrodes each time, otherwise the comparison means nothing.
What you tryWhat to expectReason
Juicy, sour fruits — lemon, raw mango, orangeLED glows wellTheir juice is a fairly strong weak acid, so it is a good electrolyte
Potato, tomatoLED glows, usually more faintlyThe juice is a weaker electrolyte, so the push produced is smaller
Two electrodes of the same metalLED does not glowA cell needs two different metals; identical rods react identically and no push is produced
Zinc with copper, magnesium with copperUsually the brightestCopper acts as the positive electrode while zinc acts as the negative electrode, because of their chemical properties
The idea being tested: every one of these is the same Voltaic cell — two different electrodes and an electrolyte. By changing only the electrolyte, or only the metals, you find out which part decides how much electricity the cell can produce.

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."

Tip: if the LED refuses to glow, first reverse the LED, then add one more fruit to the chain. Do not eat any fruit that has had metal electrodes stuck into it.
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