1. Power

2. Definition: The amount of energy converted to heat, light, sound or motion per unit of time Unit: Power is measured in watts (W) Naming: Named after James Watt, a Scottish inventor and engineer

3. Some Math Remember: Power is the amount of energy converted to heat, light, sound or motion every second. QuantitySymbol PowerP EnergyE Timet E t P = E P t P I V P = I x V

4. % Efficiency It is possible to calculate how efficient an electrical device is (such as a kettle at heating up water) No device is 100% efficient Percent efficiency = Useful energy output Total electrical energy input x 100%

5. Efficiency of a kettle problem An electric kettle has a power rating of 1000W. It takes the kettle 4.0 minutes to heat 600mL of water from 22 o C to 100 o C. If it takes 1.96x10 5 J of energy to heat the water, what is the efficiency of the kettle? G: P = 1000 W t= 4.0 minutes (240s) useful energy output = 1.96 x 10 5 J (196 000 J) R: Percent efficiency A: S: Percent efficiency = Useful energy output Total electrical energy input x 100 % E t P = Chemists have shown that it takes 4180J of energy to raise the temperature of 1kg of water by 1 o C

6. Calculating the Cost of Energy Use Step 1 – Find the device’s power rating in “W” (watts), time of use (s, min, h, etc) and price charged (¢ per kW·h) Step 2 – Convert to “kilowatts” (kW)and “hours” (h) Step 3 – Calculate the Energy used by the device using E = P x t, units “kW·h” Step 4 – Calculate the total cost = E x price

7. Household Audit Sample Problem You run your 1200 W dishwasher every night before you go to sleep for 30 min each time. Calculate the total weekly cost if your off-peak price is 7.0¢ per kW·h. P = 1200W t = 30 min Price = 7.0¢ per kW·h 1000 W = 1kW 60 min = 1h 1 wk = 7 d P = 1.2 kW t = 3.5 h (0.5 h x 7 d) E = P x t Total cost = E x Price = 1.2 kW x 3.5 h = 4.7 kW·h x 7.0¢/kW·h = 4.7 kW·h = 32.9 ¢ It will cost about 33¢ to run the dishwasher this week.