Transferring Electrical Energy

Somewhere in your home there is a small box with a row of digits on it, and those digits have been climbing since the day the building was wired. Nobody watches them. They tick up a little while the kettle boils, faster while the shower runs, imperceptibly while a phone charger sits plugged in and forgotten. Once every three months somebody writes the number down, takes away the number written down last time, multiplies the difference by a price, and posts a bill. That is the whole of this topic in one sentence, and every step of it is examinable.

What you will build here is the bridge between physics and money. You will meet power as a rate rather than an amount, and see why a machine rated at three kilowatts can cost less to run than a lamp rated at a tenth of that. You will learn the one equation that ties power to the current and the potential difference an appliance works at, and then use it for the job the specification names outright: working out the current an appliance actually draws and choosing a fuse that will protect it without cutting it off in normal use. Finally you will find out why electricity is sold in a unit invented for the purpose, the kilowatt-hour, and how to turn two meter readings and a price into a total.