1. Power and Efficiency

2. Power Power is defined as energy per unit time Electrical power describes the amount of electrical energy that is converted into a usable form (heat, light, sound, movement) every second The symbol for power is P and the unit for power is the Watt Formula P = E energy - J t -time (seconds) I J/s = 1 watt

3. Problems How much energy does a 1200W dishwashe use when it runs for 30 minutes? Givens:E = ? P = 1200W t = 30 min = 1800s

4. Problem 2 A dishwasher consumes 1200J of energy in 3 minutes. What is its power in Watts? Given: E = 1200J t = 3 min = 180s P = ? T P E T

5. Efficiency Electrical appliances are not 100% efficient. For example: An electric kettle is designed to heat water. Some energy escapes to the outside and does not heat the water. %efficiency of electrical device=Useful energy output x 100 total energy input

6. Problems A lightbulb uses 30000J of electrical energy and emits 900J. What is the percent efficiency of the light bulb? Given E input = 30000 J E output = 900 J % Efficiency = ?

7. Problem 2 An electric kettle has a power rating of 1000W. It takes the kettle 4.0 min to heat 600 ml of water from 22⁰ to 100⁰C. If it takes 19600 J of energy to heat the water, what is the efficiency of the kettle? Given: P= 1000 W T = 4.0 min = 240s E output = 196000 J % efficiency = ? E input = Pt