S4.12.7 For the adult
Power in circuits
Amina's heater is marked 920 W and her lamp is marked 40 W. What does a watt measure in a circuit, and why does the heater need a bigger fuse?
What this chapter teaches
Power is the energy transferred each second. In a circuit P = IV, and with V = IR also P = I²R, so doubling the current quadruples the power. Energy is E = Pt. A fuse is rated just above the normal current I = P ÷ V, so that it melts in a fault but not in normal use.
Three questions to ask
There is no one right wording. Listen for the idea.
- A heater takes 4 A from a 230 V supply. How do you find its power?
- Why does doubling the current make the heating four times as big?
- How do you choose a fuse for an appliance, and why not just use the biggest?
After the chapter, the learner can
- Calculate power using P equals I times V and I squared times R
- Choose the right fuse for an appliance
What they copy into their notebook
P = IV, and P = I²R. Watts are joules each second. E = Pt. Normal current I = P ÷ V. A fuse goes in the live wire, rated just above I. Double I, four times the power.