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Heating Effect of Electric Current

When current flows through a resistor, electrical energy turns into heat. Joule's law: H = I²Rt, so heat grows with the square of current, with resistance and with time. Heaters, irons, toasters, bulbs and fuses all use this effect.

🎬 Step-by-step story

  1. Here is a heater coil made of nichrome, joined to copper leads. No current flows, so the coil is cold and the heat produced is zero.
  2. Let 1 A flow through the 20 Ω coil. Electrons bump into the coil's atoms and make them shake: the coil warms up. Power = I²R = 20 W, and heat keeps adding up with time.
  3. Now the current doubles to 2 A. The power jumps from 20 W to 80 W: four times, not two. Heat depends on I squared.
  4. The same current flows through the copper leads and the nichrome coil, because they are in series. Only the high-resistance coil glows; the low-resistance copper stays cool.
  5. A fuse is a thin wire with a low melting point, in series. Watch the current climb past its 5 A rating: the fuse wire melts and breaks the circuit before anything else is damaged.
  6. Your turn: change current and resistance, and watch the glow, the power P = I²R and the heat H = I²Rt add up.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Where does the heat actually come from?

The source does work pushing charges through the coil. Electrons crash into the atoms and hand over energy, so the atoms vibrate faster: that is heat.

Why does doubling current give four times the heat, not double?

Twice as much charge flows each second, and each charge also loses twice as much energy (V = IR doubles). 2 × 2 = 4. Watch P go from 20 W to 80 W.

The same current flows in the copper wire and the coil. Why does only the coil glow?

Heat is I²Rt. With the same I and t, the part with more resistance gets more heat. Nichrome has far more resistance than the short copper lead.

How does a fuse save my appliances?

The fuse wire melts easily. If the current rises above its rating, it heats and breaks the circuit first, so no more current reaches the wiring or appliance.

Why is the bulb filament made of tungsten?

Tungsten has a very high melting point (about 3380 °C), so it can get white-hot and give light without melting. In the 3D, only the high-resistance part glows, just like a filament.

What is the heating effect of current?

A cell keeps pushing charges through a resistor. The source does work on them; in a resistor this work does not make anything move, so all of it becomes heat. Moving electrons keep colliding with the metal's atoms, passing on energy and making them vibrate faster, which we feel as a rise in temperature. This is the heating effect of electric current.

Derivation of Joule's law of heating

Let a current I flow through a resistor R for time t, with p.d. V across it. Charge moved: Q = It. Work done by the source: W = VQ = VIt. This work appears as heat: H = VIt. Using Ohm's law V = IR:

H = I²Rt (also H = VIt = V²t/R)

Joule's law of heating: heat produced in a resistor is (i) directly proportional to the square of the current, (ii) directly proportional to the resistance for a given current, and (iii) directly proportional to the time for which current flows. H is in joules when I is in A, R in Ω and t in s.

Applications of the heating effect

When is heating unwanted?

In wires, motors, computers and chargers the heat is wasted energy and can damage parts. That is why connecting wires are thick copper, electronic devices have fans or vents, and power is sent over long distances at very high voltage (so that the current, and the I²R loss, stays small).

Key formulas and definitions

Worked examples

1. A current of 2 A flows through a 10 Ω resistor for 30 s. Find the heat produced.

H = I²Rt = 2² × 10 × 30 = 1200 J.

2. An iron of resistance 50 Ω draws 4 A. How much heat does it produce in 1 minute?

H = 4² × 50 × 60 = 48 000 J = 48 kJ.

3. A 20 Ω heater is connected to 220 V for 10 s. Find the heat produced.

H = V²t/R = 220² × 10 / 20 = 24 200 J.

4. If the current through a heater is made three times, how many times does the heat produced in the same time become?

H ∝ I², so heat becomes 3² = 9 times.

5. A kettle produces 60 000 J in 2 minutes with a current of 5 A. Find its resistance.

t = 120 s. R = H / (I²t) = 60 000 / (25 × 120) = 20 Ω.

6. A 1100 W heater works on 220 V. Which fuse would you use: 2 A, 5 A or 10 A?

Normal current I = P/V = 1100/220 = 5 A. A fuse must be a little above this, so choose the 10 A fuse; a 5 A fuse would blow in normal use.

Common mistakes

Practice quiz

1. Joule's law of heating is:
2. The filament of a bulb is made of:
3. A fuse wire should have:
4. If current is halved, heat produced in the same time becomes:
5. A fuse is connected:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is Joule's law of heating?

Heat produced in a resistor H = I²Rt: it varies as the square of current, directly as resistance and directly as time.

Give three applications of the heating effect of current.

Electric heater/iron/toaster, the filament bulb, and the electric fuse.

Is this in the CBSE Class 10 syllabus?

Yes. The heating effect, Joule's law derivation and its uses (heater, bulb, fuse) are part of the Electricity chapter in CBSE Class 10 Science.

Where this is taught

RomaniaClasa a X-aProducing and using direct current
CBSE (India)Class 10Effects of Current
China九年级（初三）Ch.18 Electric power
China高一Compulsory 3 Ch.12 Electric energy

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