Joule's Law
Compute heat Q = I² R t from current, resistance and time; reverse for I, R or t; output in J and kJ.
Inputs
Formula
Q = I² R t
Heat scales with I²; used in heater and overcurrent design.
Fundamentals
Joule's law describes the heating of current through resistance — the basis of electric heating and thermal design.
Joule heat $Q=I^2 R t$, current $I$, resistance $R$, time $t$; heating power $P=I^2R=UI$. The $I^2$ relation makes high current heat sharply.
History
Joule measured the heating effect of current in 1841, giving Q=I²Rt.
Engineering applications
Used for heaters, fuses, cable temperature rise and energy-loss calculation.
Glossary
| Joule heat $Q$ | Heat from resistance (J). |
| Heating power $P$ | Heat per time, $P=I^2R$. |
| Square law | Heat proportional to current squared. |
How to use
- Fill in Current I, Resistance R, Time t in the Inputs section (watch the unit on each field).
- Click Calculate; the tool evaluates the formula shown above.
- Read Heat Q, Heat Q (kJ) in the results area.
Formula notesJoule's law heat Q = I²·R·t (I current, R resistance, t time).
Formula · Worked Example · Knowledge
Formula
Joule heat $Q=I^2Rt$, in J and kJ; heat scales with I².
Worked Example
I=5 A, R=10 Ω, t=60 s → Q=15000 J=15 kJ.
Key Points
- Heaters, fuses and overcurrent protection rely on this.
- Doubling current → 4× heat.
- P=I²R is the heating power per unit time.
Parameters
Inputs
| Parameter | Symbol | Unit | Default |
|---|---|---|---|
| Current I (A) | I | A | 5 |
| Resistance R (Ω) | R | Ω | 10 |
| Time t (s) | t | s | 60 |
Outputs
| Result | Symbol | Unit |
|---|---|---|
| Heat Q | Q | J |
| Heat Q (kJ) | Qk | kJ |
Applications
- Common engineering use cases
FAQ
What formula does this tool use?
This tool computes per ISO / AGMA / ASME standard formulas: Q = I² R t
How accurate are the results?
Results match input precision, based on SI units and common engineering approximations; for critical duty re-check with a safety factor.
Where is it used?
Common engineering use cases