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Heating Effect of Current and Cells Class 8 Science

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ScienceCBSE Class 8 · Electricity: Magnetic and Heating Effects

Heating Effect of Electric Current and Electric Cells: Class 8 Science

Updated 17 October 2026 · Fact-checked

When current flows through a conductor, some electrical energy changes to heat because the conductor resists the flow; this is the heating effect. A nichrome wire has higher resistance than a copper wire of the same size and length, so it heats more. A cell makes current from chemical reactions; Voltaic cells use two electrodes in an electrolyte, dry cells use a paste and are single-use, and rechargeable batteries can be reused.

Current flows round the circuit, and the resistance of the nichrome wire changes electrical energy into heat energy.

Illustration drawn for QuestionBuddy.

Understand Heating Effect of Current, Voltaic Cells, Dry Cells and Rechargeable Batteries

Heating effect (Activity 4.5). A nichrome wire tied between two nails and joined to a cell and switch feels warm after current flows for about 30 s. (The activity is done under teacher supervision, and the wire is touched only momentarily because it can hurt.) The wire warms because a conductor offers some opposition, resistance, to current; different conductors offer different resistance. A nichrome wire offers more resistance than a copper wire of the same size and length. The resistance converts some electrical energy to heat energy. This is the heating effect of electric current.

What does the heat depend on? The heat is more with 2 cells than with 1, so it depends on the magnitude of the current. It also depends on the material, thickness and length of the wire and on the time for which current flows.

Uses. A rod or coil of wire called a heating element is used in room heaters, stoves, kettles, irons, immersion rods and hair dryers; the element may glow red hot. An incandescent lamp glows because its filament is heated by current. Industrial high-temperature electric furnaces melt and recycle scrap steel.

Problems. Heating causes energy loss in wires during transmission. Wires, plugs and sockets must be rated for the current in the circuit; otherwise they overheat, plastic parts may melt and fires may start. Household circuits have safety devices to reduce such incidents.

How does a battery give electricity?

  • Voltaic (Galvanic) cell: two rods of different metals (electrodes) partly dipped in a liquid electrolyte (weak acid or salt solution). A chemical reaction between rods and electrolyte produces electricity, and current flows from the positive terminal through the circuit to the negative terminal. When the chemicals are used up the cell is 'dead'. The names honour Volta and Galvani: Galvani saw a dead frog's leg kick when touched with copper and iron and thought the electricity came from the frog; Volta showed with saltwater-soaked paper that the metals and liquid together produce current.
  • Lemon cell (Activity 4.6): copper wire and iron nail in juicy lemons joined in a row light an LED. The electrodes are copper and iron; lemon juice is the electrolyte. Current passes through an LED only when its longer (positive) wire joins the positive terminal; reverse it if it does not glow. Common pairs: zinc/copper, zinc/silver, aluminium/copper, iron/copper, magnesium/copper and lead/copper. Copper acts as the positive and zinc as the negative electrode.
  • Dry cell: the electrolyte is a thick moist paste, not a liquid. A zinc container is the negative terminal and a carbon rod with a metal cap at the centre is the positive terminal. It is a single-use cell.
  • Rechargeable batteries: can be recharged and reused many times, preventing waste and saving money, but wear out slowly after many cycles. The lithium-ion battery is the most common type; lithium and cobalt are mined in limited parts of the world, which is why recycling and new technologies such as solid-state batteries (solid electrolyte, safer, faster to charge, longer lasting) are being developed.

The previous topic dealt with the magnetic effect of current and electromagnets.

Heating Effect of Current, Voltaic Cells, Dry Cells and Rechargeable Batteries: formulas and key facts

Heating effect
Current through resistance > electrical energy converted to heat
Nichrome has higher resistance than copper of the same size.
Heat in a wire depends on
  • Magnitude of current
  • Material
  • Thickness
  • Length
  • Duration of current
The chapter reports more heat with 2 cells than 1.
Voltaic cell parts
Two different metal electrodes + electrolyte (weak acid or salt solution)
Chemical reaction produces current.
Dry cell
  • Zinc container (negative)
  • Carbon rod with metal cap (positive)
  • Paste electrolyte
Single use.
Rechargeable battery
  • Can be recharged and reused
  • Wears out slowly
  • Li-ion is most common
Solid-state batteries are future technology.

How to solve Heating Effect of Current, Voltaic Cells, Dry Cells and Rechargeable Batteries questions

Use this method for heating-effect and cell questions.

  1. 1Identify the effect: heating (appliances, warm wire) or chemical (cell making current).
  2. 2For heat, ask what changes: current, material, thickness, length, time.
  3. 3Remember nichrome is chosen for heating elements because of its higher resistance than copper.
  4. 4For cells, name electrodes and electrolyte and the polarity of terminals.
  5. 5For the LED, check the polarity; longer wire to the positive terminal.
  6. 6Mention safety: ratings of wires and plugs, touching hot wires only momentarily.

Heating Effect of Current, Voltaic Cells, Dry Cells and Rechargeable Batteries in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Why a heating element, not a copper wire

Suppose a maker wants to build a small heater from a 10 cm wire connected to the same cell. She compares a nichrome wire and a copper wire of the same size.

(a) Which wire will become warmer and why? (b) If she adds a second cell, what happens to the heat in the same time? (c) She leaves the switch ON for 30 s and then 60 s; what happens to the heat? (d) Why should she touch the wire only momentarily?

Try it first, then show the solutionSolution
  1. Nichrome, because it offers higher resistance than copper of the same size and length.
  2. More cells give a larger current, so more heat is generated.
  3. A longer time means more heat since the heat depends on the duration of current.
  4. The wire can become hot enough to cause injury, as the safety note says.

Answer: (a) Nichrome (higher resistance); (b) more heat; (c) more heat in 60 s; (d) to avoid burns.

Memory hook: Higher resistance, hotter wire.

Lemon cell with an LED

Imagine a group connects 5 lemon cells in series with an LED. The LED does not glow at first, but glows after they swap its two connections. Suppose each lemon cell provides about 0.9 V (an invented value).

(a) Why did the LED start glowing after swapping? (b) Name electrodes and electrolyte. (c) What is the total p.d. if the cells add up? (d) What happens when the lemon juice chemicals are used up?

Try it first, then show the solutionSolution
  1. Current passes through an LED only when its positive (longer) wire is connected to the positive terminal of the battery.
  2. Electrodes: copper wire and iron nail; electrolyte: lemon juice.
  3. Total = 5 × 0.9 V = 4.5 V (assuming that cells in series add up).
  4. The cell becomes dead and cannot supply electricity.

Answer: (a) Polarity corrected; (b) copper and iron, lemon juice; (c) 4.5 V; (d) the cell goes dead.

Memory hook: LED is one-way: long leg to plus.

Tricks to remember Heating Effect of Current, Voltaic Cells, Dry Cells and Rechargeable Batteries

Short memory aids. Say each one aloud once, then use it on a question the same day.

  • Resistance makes heat

    Think of rubbing hands: opposition makes warmth. Wire opposition makes heat.

    Use it when: Explaining the heating effect.

  • Dry cell: Zinc outside, Carbon inside

    Zinc container is negative; the carbon rod is the positive centre.

    Use it when: Parts of a dry cell.

  • Voltaic = Volta, Galvanic = Galvani

    Cell names come from the two Italian scientists.

    Use it when: Name origins.

Heating Effect of Current, Voltaic Cells, Dry Cells and Rechargeable Batteries: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Why does a nichrome wire become warm · 3 marks · Short answerBoard level

Explain why a nichrome wire becomes warm when current flows and name two factors on which the heat depends.

Try it first, then show the solutionSolution
  1. A conductor offers resistance to current, and some electrical energy changes into heat. (1)
  2. Nichrome has higher resistance than copper of equal size. (1)
  3. The heat depends on the current, material, thickness, length and time (any two). (1)

Answer: Resistance converts electrical energy to heat; heat depends on current, material, thickness, length and time.

Example 2: Dry cell positive terminal · 1 mark · MCQBoard level

Which row correctly matches the negative terminal and the electrolyte of a dry cell?
(a) Carbon rod; liquid acid
(b) Zinc container; thick moist paste
(c) Zinc container; liquid salt solution
(d) Carbon rod; thick moist paste

Try it first, then show the solutionSolution
  1. The zinc container is the negative terminal (the carbon rod with metal cap is positive), and the electrolyte is a thick moist paste, not a liquid.

Answer: (b)

Example 3: Compare the cells · 5 marks · Long answerHarder board level

Compare a Voltaic cell, a dry cell and a rechargeable battery in a table on three points: electrolyte, whether reusable, and an example of use.

Try it first, then show the solutionSolution
  1. Voltaic cell: liquid electrolyte (weak acid or salt solution); cell goes dead when chemicals are used; a classroom model or lemon cell. (2)
  2. Dry cell: paste electrolyte; single use; torches, remote controls. (2)
  3. Rechargeable battery: can be recharged and reused many times but wears out slowly; phones, laptops, inverters, vehicles. (1)

Answer: Voltaic: liquid, dies; dry cell: paste, single use; rechargeable: reusable, used in phones, laptops, inverters, vehicles.

Example 4: An appliance and a safe plug · 4 marks · Case-basedHarder board level

Read the passage and answer.

A family connects a powerful heater to a thin, old extension board. After some time the plastic of the plug becomes soft and smells.
(a) Which effect of current causes this? (1)
(b) Why is the plug unsuitable? (1)
(c) Name two other problems the heating effect creates in wires. (1)
(d) Name two appliances that use heating elements. (1)

Try it first, then show the solutionSolution
  1. (a) The heating effect of electric current. (1)
  2. (b) Wires, plugs and sockets must be rated for the current of the connection; one rated for less current overheats and its plastic can melt. (1)
  3. (c) Energy loss in transmission wires; risk of fire. (1)
  4. (d) Any two: room heater, stove, kettle, iron, immersion rod, hair dryer. (1)

Answer: (a) Heating effect; (b) not rated for the current; (c) energy loss and fire; (d) e.g. kettle and iron.

Example 5: Frog, metals and the first cell · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

Galvani saw a dead frog's leg kick when touched with copper and iron and thought the electricity was from the frog. Volta thought it came from the metals and used paper soaked in salt water instead of the frog and still got a current.
(a) What was Volta's idea? (1)
(b) How did his experiment test it? (1)
(c) What conclusion followed? (1)
(d) In the lemon cell, which part corresponds to the saltwater paper? (1)

Try it first, then show the solutionSolution
  1. (a) The electricity came from the metals, not from the frog. (1)
  2. (b) He removed the frog and used salt-water paper; current still flowed, so the frog was not needed. (1)
  3. (c) The combination of two metals and a liquid produces current, leading to the Voltaic cell. (1)
  4. (d) The lemon juice, the electrolyte. (1)

Answer: (a) Metals are the source; (b) replaced the frog by soaked paper; (c) metals plus liquid give current; (d) lemon juice.

Where students lose marks in Heating Effect of Current, Voltaic Cells, Dry Cells and Rechargeable Batteries

The slips examiners see most often, and what to write instead.

  • Mistake: Saying a dry cell has a liquid electrolyte.

    Confusion with the Voltaic cell.

    Wrong working

    A dry cell contains a liquid electrolyte.

    Right working

    A dry cell has a thick moist paste as electrolyte.

    Do this: Dry means paste.

    Usually costs: 1 mark.

  • Mistake: Naming the zinc container as positive.

    Terminals are mixed up.

    Wrong working

    Zinc case is the positive terminal.

    Right working

    Zinc container is negative; carbon rod with metal cap is positive.

    Do this: Zinc = negative.

    Usually costs: 1 mark.

  • Mistake: Forgetting why nichrome is used.

    Students say 'it is a good conductor'.

    Wrong working

    Nichrome is used because it is the best conductor.

    Right working

    Nichrome has higher resistance, so it generates more heat for a given current.

    Do this: Always mention resistance.

    Usually costs: 1 mark.

  • Mistake: Connecting the LED the wrong way and not saying it.

    LED is one-way.

    Wrong working

    LED did not glow, so the cell is dead.

    Right working

    Reverse the LED connections: longer wire to the positive terminal.

    Do this: Check polarity first.

    Usually costs: 1 mark.

Exam tips

  • Mention resistance when explaining the heating effect.
  • Quote the factors on which heat depends: current, material, thickness, length, time.
  • For cells, always state electrodes, electrolyte and terminal polarity.
  • For lemon-cell questions say copper and iron are electrodes and lemon juice is the electrolyte.
  • For safety, state that wires and plugs must be rated for the current.

Practice questions on Heating Effect of Current, Voltaic Cells, Dry Cells and Rechargeable Batteries

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

  1. Q1 · 2 marks · Very short answerBoard level

    A student joins a copper wire and a nichrome wire of the same size and length, one after the other, to the same cell and claims they will heat equally. Is she right? Give a reason.

    Hint

    Compare the resistance of the two metals.

    Answer

    No (1). Nichrome has a higher resistance than copper of the same size and length, so more electrical energy is changed to heat in the nichrome (1).

  2. Q2 · 1 mark · MCQBoard level

    Which one of these devices does not rely mainly on a heating element?
    (a) Water-heating immersion rod
    (b) Hair dryer
    (c) Electric kettle
    (d) Lifting electromagnet on a crane

    Hint

    One uses the magnetic effect.

    Answer

    (d)

  3. Q3 · 1 mark · Assertion-reasonHarder board level

    Assertion (A): A nichrome wire becomes warmer than a copper wire of the same size and length when the same current flows.
    Reason (R): Nichrome offers more resistance than copper.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

    Hint

    Resistance converts energy to heat.

    Answer

    (a) R explains A.

  4. Q4 · 4 marks · Short answerHarder board level

    A torch uses 3 dry cells in series, each of 1.5 V, which are replaced every 40 days. Suppose each dry cell costs 10 units, and a rechargeable set of 3 costs 6 times the price of 3 dry cells and (assume) lasts the whole year. (a) Find the total p.d. of the 3 cells. (b) How many sets of 3 dry cells would be used in 360 days? (c) Compare the cost of dry cells with the cost of the rechargeable set over 360 days. (d) Name one disadvantage of rechargeable batteries.

    Hint

    Dry cell sets: 360 ÷ 40. Cost of one set of 3 dry cells = 30 units.

    Answer

    (a) 3 × 1.5 = 4.5 V. (b) 360 ÷ 40 = 9 sets. (c) Dry cells: 9 × 30 = 270 units. Rechargeable set: 6 × 30 = 180 units, so it is cheaper by 90 units (if it lasts the 360 days, which the chapter says is likely over many uses but not forever). (d) They wear out slowly after many charges (1 mark each).

  5. Q5 · 4 marks · Case-basedHOTS / competency

    Read the passage and answer.

    A student connects a nichrome wire to 1 cell for 30 s and notes the warmth. She repeats with 2 cells for 30 s, and then with 2 cells for 60 s.
    (a) Arrange the three trials in increasing order of heat produced and give reasons. (b) Name the factor changed in each comparison. (c) Why should she touch the wire only momentarily? (d) Would a copper wire of the same size and length be warmer or cooler than the nichrome wire in the 2-cell, 30 s trial?

    Hint

    Current and time both matter.

    Answer

    (a) 1 cell 30 s < 2 cells 30 s < 2 cells 60 s: more current gives more heat, and a longer time gives more heat. (b) Current (number of cells), then time. (c) The wire may be hot enough to cause burns. (d) Cooler, because copper has lower resistance (1 mark each).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

  1. 1.What is a heating element?Show answer

    A rod or coil of wire in an appliance that heats up when current flows.

  2. 2.Which wire has higher resistance, nichrome or copper?Show answer

    Nichrome.

  3. 3.What is the electrolyte in a dry cell?Show answer

    A thick moist paste.

  4. 4.What does a dead cell mean?Show answer

    Its chemicals have been used up and it can no longer supply electricity.

  5. 5.Which battery type is most common in devices?Show answer

    The lithium-ion (Li-ion) battery.

  6. 6.Name two metals used in a Voltaic pair.Show answer

    Zinc and copper.

Heating Effect of Current, Voltaic Cells, Dry Cells and Rechargeable Batteries: frequently asked questions

Why is the heating effect sometimes a problem?

It wastes energy in wires during transmission and can overheat plugs and sockets, melting plastic or causing fires; hence proper ratings and safety devices.

Can all cells be recharged?

No. Dry cells are single-use. Rechargeable batteries can be reused but also wear out slowly.

What is a solid-state battery?

A future battery that replaces the liquid or paste electrolyte with a solid; the chapter says it would be safer, charge faster and last longer.

Why does the LED not glow in a lemon cell sometimes?

An LED conducts only one way. If it does not glow, reverse its connections so that the longer wire is at the positive terminal.