Joule Heating
For an ohmic resistor with constant resistance, heat is I squared Rt, equivalently V squared t/R or VIt. Choose the form matching what is held fixed.
Why this shows up in the exam
Resistance heaters · Comparing heat in series or parallel branches · Fuse and wiring losses
Learn the idea
A resistor converts electrical energy to heat at a rate set by current, voltage, and resistance. Carrier collisions transfer ordered electrical energy to the lattice. Doubling current at fixed resistance makes heating four times faster.
🧠 Memory hook: Fixed I favors larger R; fixed V favors smaller R.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- H = I² R t — heat for constant current and resistance
- H = V² t/R — heat for constant voltage and resistance
- H = V I t — electrical energy converted over time
How to approach it
- 1State what is common: I, V, or circuit connection
- 2Select the matching heat formula
- 3Use branch values, not source totals
Common slip-ups that cost marks
- •Using the wrong fixed-current or fixed-voltage comparison
- •Forgetting time
- •Assuming resistance stays constant during extreme heating
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A cell of emf 6 V and internal resistance 1 ohm is connected to a 2 ohm resistor. Find the circuit current.
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