Electric circuits: Ohm’s law, resistors and the cost of electricity

Electric power is the energy a device uses each second: P = E ÷ t. Its unit is the watt, 1 W = 1 J/s, so a 9 W bulb uses 9 J every second. The energy used is E = P × t in joules, with t in seconds. Because the joule is very small, electricity bills use the kilowatt-hour: 1 kW·h = 1000 W × 3600 s = 3.6 × 10⁶ J. To get kW·h, multiply the power in kW by the time in hours; the cost is then the kW·h times the price per kW·h. Careful: kW is power, kW·h is energy, and the price here is always an assumption; real prices vary, so we write “assume a price of …”. LED bulbs use less energy than CFLs for the same light, so they cost less to run. Heaters, irons and air conditioners use the most energy. An energy efficiency label shows the monthly consumption and a rating. Safety: do not overload a socket, keep wires insulated, keep hands and tools dry, switch the source off before changing anything, and never use household sources in school experiments. Never open an appliance.