Heating effect of current – the H = I²Rt experiment
PhysicsElectricity & CircuitsAges 16–17Virtual lab
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Use it with your class: students open it from a link or a QR code, with no account needed.
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A heater coil in a calorimeter of water: set the current and time, read the thermometer and check H = I²Rt = mcΔT by plotting ΔT against I² and against t. Further screens show why a nichrome element heats while copper leads stay cool, how a fuse melts and an MCB trips on overload, and how a filament lamp, iron and kettle use the effect.
Lesson: Heating effect of electric current, Joule's law of heating
What it shows
When a current flows through a conductor, electrical energy becomes heat: H = I²Rt. In an insulated calorimeter this heat warms the water, H = mcΔT with c = 4186 J/(kg·K), so ΔT is proportional to I² and to t. In a series circuit the heating element and the leads carry the same current, so the part with the larger resistance gives out far more heat. Fuses and circuit breakers use this same effect to protect a circuit. The model ignores the heat capacity of the cup and coil, and the fuse and breaker curves are simplified.
How to use
On Calorimeter, set Current I, Resistance R, Water mass m and Heating time t, then press Switch on. Each run goes into the table. Change only I and do at least three runs for the ΔT against I² graph; choose ΔT against t and change only t for the second graph. On Element and leads, change Element material and I. On Fuse and MCB, choose Rating, set Load current and press Switch on. On Appliances, choose an appliance, mains voltage and time, then press Record data.
Parameters you can change
- Screen Calorimeter, Element and leads, Fuse and MCB, Appliances
- Current I 0.5–4 A
- Resistance R 2–10 Ω
- Water mass m 100–500 g
- Heating time t 60–600 s
- Uninsulated cup
Questions to explore
- When I rises from 2 A to 4 A with R, m and t unchanged, by what factor does ΔT increase?
- Calculate the specific heat capacity c of water from one run. Without insulation, is your value higher or lower?
- What is the smallest load current that melts a 10 A fuse within 60 s, and how long does the 10 A MCB take at that current?
