ENERGY Energy Energy is measured in Joules (J) A joule is a small unit of energy NOT JEWELS like these -> JOULES Energy The amount of energy used over time is called POWER. When describing POWER, we use Watts (W) 1 W = 1 Joule second Energy Watts are convenient to use for small things like light bulbs or laptops. However, energy used by larger devices may be described using kiloWatts (kW) 1 kW = 1000 W = 1000 Joules second Energy For devices that require large amounts of energy over a long period of time, like a house, one may use kiloWatt hours (kW h) 1 kW h = 1000 W h = 1000 Joules x hour second Efficiency An electric kettle is supplied with J of energy and produces J of useful heat energy. What is the efficiency of the electric kettle? Percent Efficiency A man uses 150 J of energy moving a box up a steep slope. The amount of work produced is 37.5J. What is the percent efficiency of the man pushing the box? Watts, kiloWatts and Joules A 55 inch plasma television uses 507 W when displaying High Definition TV. What is the tvs power use in kiloWatts? Watts, kiloWatts and Joules A 55 inch plasma television uses 507 W when displaying High Definition TV. How much energy does the TV use when watching TV for 30 minutes? Cost of Electricity In Toronto, electricity costs $0.074 / kWh. An electric heater uses 2 kW every hour. How much would it cost to leave this heater on for 2 hours and 30 minutes? YOUR TURN