Power and its rate
Show on a big screenHow fast does a circuit use energy?
- One circuit: V = 6 V, I = 2 A, R = 3 Ω.
- P = V × I = 6 × 2 = 12 W.
- P = I² × R = 2² × 3 = 12 W.
- P = V² ÷ R = 6² ÷ 3 = 12 W. All three: 12 W.
Drag the push on a torch bulb. Watch the watts.
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is energy used each second. 1 watt (W) means 1 joule (J) each second.
New word
Power — how fast a circuit uses energy. Unit: the watt (W).
The rule
is voltage times current, in watts. It also equals current squared times resistance. And it equals voltage squared divided by resistance. A longer running time changes the energy, not the power.
P = VI = I²R = V² ÷ R
P is the , in watts. V is the voltage, which is the push. I is the current. R is the resistance. Use the last two forms when R stays fixed.
All three give 12 W for this circuit.
Run that circuit for 10 s. Energy = P × t = 12 W × 10 s = 120 J. Run it for 20 s. Then 12 W × 20 s = 240 J. The energy doubles. The itself stays at 12 W.
A torch bulb is marked 8 V, 16 W. That means 16 W at 8 V. Rearrange P = V² ÷ R to R = V² ÷ P. Then R = 8² ÷ 16 = 64 ÷ 16 = 4 Ω. Treat that resistance as fixed. On a 6 V cell, P = 6² ÷ 4 = 36 ÷ 4 = 9 W. follows the square of the push.
Try the lab to finish this lesson.