6.3: Electric Energy and Power

Energy - the ability to do work

Work

• force (F) multiplied by distance (d) the force is applied

• measured in joules (J)

• Work = force x distance

Force 

• energy exerted; measured in newtons (N)

• 1 N of force is applied for one metre of distance (1 N⋅m)

• 1 N⋅m = 1 joule

Power 

• the rate at which energy is transferred. 

• Measured in watts (W)

• Power = energy / time OR 1 W = 1 joule per second

• Example a 60 W lightbulb uses 60 joules / second. 

If a generator produces 7 J of energy every second, how many watts of power does it produce?

Power companies are the same as an electric utility company. When you are sent a bill, you will find that you are paying for how much electric energy your house consumed during a period of time. The values are measured in watt hour (Wh). One way to reduce the cost per month is by purchasing newer appliances

When you purchase appliances, you may notice an energy star sticker on the appliance. If the appliance is considered energy efficient, the sticker will indicate the energy efficiency rating. 

Calculating Watt Hours

How much does a 15 W light bulb use if it is left on for 2 hours?

1. What units are used to measure the power of electric appliances?

2. How much energy, in kilowatt hours, is used by a 1000 watt hair dryer when it runs for one hour?

3. Why do you think electric power companies measure electric energy in kilowatt hours instead of watt hours?

4. Compare the use of a new air fryer versus a new oven in energy efficiency? Is it always cheaper to use the one with the lowest amount of energy? When might it not be cheaper to use the lowest kW appliance?