Activity 4.5 — The Heating Effect of Electric Current — "Electricity: Magnetic and Heating Effects"

1. 13 marksCuriosity Grade 8, Chapter 4, pages 52-53, Activity 4.5

   List the materials for Activity 4.5 and describe how it is set up. What safety instruction does the chapter give?

   Hint. Note the exact thickness and length of wire the chapter specifies.

   Materials. A cardboard piece about 10 cm × 10 cm; two nails; a nichrome wire of thickness about 0.3 mm (26–28 gauge) and length 10 cm; an electric cell; a cell holder; a switch; and connecting wires.

   Setting it up.

   1. Mount the two nails on the cardboard, about 5 cm apart.
   2. Tie the nichrome wire between the nails.
   3. Make the connections as in Fig. 4.5, with the switch in the OFF position.
   4. Touch the nichrome wire and note what you feel.
   5. Move the switch to ON for about 30 s, then back to OFF, and touch the wire momentarily.
   6. Repeat the last two steps to confirm the observation.

   The safety instruction. Do not touch the wire for an extended period to avoid any injuries. The activity also says in the step itself: Do not hold the nichrome wire — touch it and take your hand away.

   ✦ Why the wire is 10 cm long but the nails only 5 cm apart. The extra length lets you tie the wire firmly at both nails while leaving a clear 5 cm stretch in the middle, held above the cardboard, which is the part that heats and the part you touch. It also keeps the hot part away from the cardboard.

   And note step 4 — touching the wire before switching on. That is the control. Without knowing how the wire feels cold, you have nothing to compare the warm reading against.

2. 22 marksCuriosity Grade 8, Chapter 4, page 53, Activity 4.5

   What difference do you feel when you touch the nichrome wire before switching on and after current has flowed through it for about 30 seconds?

   Hint. Two touches, one observation.

   Before switching on: the wire feels cool — no different from the nails or the cardboard around it, at ordinary room temperature.

   After about 30 s of current: the wire feels distinctly warm.

   The chapter records the result as: You may have observed that nichrome wire feels warm when current is passed through it.

   ✦ Why the activity asks you to repeat the two steps. Warmth is judged by hand, and hands are unreliable — a cold hand makes anything feel warm. Doing the pair of touches twice, and getting the same difference both times, is what makes the observation trustworthy. It is the same reasoning as switching the compass on and off repeatedly in Activity 4.1.

   Touch it momentarily and do not hold it. With more cells, or a longer time, the wire can get hot enough to burn — which is why the chapter's follow-up experiment with a 2-cell battery must be done under a teacher's supervision.

3. 34 marksCuriosity Grade 8, Chapter 4, page 53

   Why does the nichrome wire get warm when current passes through it? Define the heating effect of electric current.

   Hint. Something opposes the current, and the energy has to go somewhere.

   The explanation, step by step, as the chapter gives it:

   1. When electric current flows through any conductor, it faces some opposition or resistance to its flow.
   2. Different conductors offer different levels of resistance to the flow of current. A nichrome wire, for example, offers higher resistance than a copper wire of the same size and length.
   3. This resistance causes some of the electrical energy to be converted into heat energy.
   4. So the conductor gets heated.

   Definition. When an electric current passes through a conductor, it gets heated. This warming is known as the heating effect of electric current. The Snapshots put it as: Generation of heat in conductors due to flow of electric current is known as the heating effect of electric current.

   ✦ The effect is universal, but not equally noticeable. Every conductor carrying a current warms up, including the copper connecting wires in your circuit. You notice it in the nichrome because its resistance is much higher, so far more of the electrical energy is turned into heat in that short 5 cm stretch.

   This also explains the character's remark in the chapter: the incandescent torch lamp used to get warm during the Grade 7 activity for the same reason — and the wire ends of the electromagnet got warm in Activity 4.2 for the same reason too.

4. 43 marksCuriosity Grade 8, Chapter 4, page 53

   Why is nichrome wire, rather than copper wire, used for this activity and in heating appliances?

   Hint. Compare the two for equal size and length.

   Because nichrome offers a much higher resistance than copper. The chapter states it directly: A nichrome wire, for example, offers higher resistance compared to a copper wire of the same size and length.

   Since it is the resistance that converts electrical energy into heat, a nichrome wire carrying a given current produces much more heat than a copper wire would. That makes it the right choice for a heating element — the rod or coil of wire that every electric heating appliance contains.

   And it makes copper the right choice for the opposite job. Connecting wires are made of copper precisely because its resistance is low: you want the current to reach the appliance, not to be turned into heat on the way.

   	Copper	Nichrome
   Resistance (same size and length)	Lower	Higher
   Heat produced for a given current	Less	More
   Used as	Connecting wires	Heating elements

   ✦ One claim to be careful about. Exercise question 5 offers 'is cheaper than copper' as a possible reason. The chapter never says that, and it is not the reason nichrome is chosen — resistance is. Do not add cost to this answer.

   Nichrome also has to survive being red hot without melting or burning away, which is a second reason it is used rather than simply any high-resistance wire — though the chapter does not go into that.

5. 54 marksCuriosity Grade 8, Chapter 4, page 53, Think like a scientist

   Repeat Activity 4.5 with a battery of 2 cells. For the same duration, does the wire heat up more with one cell or two? On what does the heat generated depend?

   Hint. One change gives a bigger effect — and the chapter then lists four factors in all.

   This activity must be carried out strictly under the supervision of a teacher — the chapter says so before anything else, because two cells can make the wire hot enough to burn.

   The result. The amount of heat generated is more in the experiment with 2 cells. For the same 30 s, the wire is noticeably hotter.

   Why. A battery of 2 cells drives a larger current through the wire, and the heat generated depends on the magnitude of the electric current.

   The full list of factors the chapter gives: The heat generated in a wire depends on the material, thickness, length of the wire, and the duration for which the current flows — together with the magnitude of the current.

   Factor	Where you meet it
   Magnitude of the current	This experiment — 1 cell vs 2 cells
   Material of the wire	Nichrome vs copper
   Thickness of the wire	The end-of-chapter project — 0.3 mm vs 0.6 mm
   Length of the wire	The same project — two lengths of equal thickness
   Duration of the current	Longer ON time, more heat

   ✦ The chapter gives no formula for this, and you should not supply one. At this level the relationship is entirely qualitative — more current means more heat, not how much more. The quantitative law comes in a later grade.

6. 63 marksCuriosity Grade 8, Chapter 4, pages 53-54

   Name the household appliances the chapter lists as working on the heating effect of electric current. What is a heating element?

   Hint. Fig. 4.6 shows six of them.

   The appliances (Fig. 4.6): electric room heaters, electric stoves, electric kettles, electric irons, water heating immersion rods, and hair dryers. The chapter also counts the incandescent lamp, learnt in Grade 7, which glows because its filament is heated by an electric current.

   Heating element. All these devices contain a rod or a coil of wire, called a heating element. It is the part that carries the current and turns electrical energy into heat. In some appliances where this element is visible, it can be seen glowing red hot — the exposed coil of an electric stove or a room heater, for instance.

   ✦ The same principle, put to six different uses.

   Appliance	What the heat is used for
   Room heater	Warming the air in a room
   Electric stove	Cooking
   Electric kettle	Boiling water
   Electric iron	Heating the metal base to press cloth
   Immersion rod	Heating water directly in a bucket
   Hair dryer	Warming the air the fan blows
   Incandescent lamp	Heating a filament until it glows and gives light

   The lamp is the odd one out and worth noticing. In the other six, heat is the product you want. In the lamp, heat is the route to light — and most of the energy still leaves as heat rather than light, which is why an incandescent bulb is warm to hold and why LED lamps have replaced them.

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.