The rate at which a device converts energy. The unit for power is the “watt” (we also use kilowatt and gigawatt depending upon the size) There are two ways to calculate the power used by a device: 1. P = I x V (power = current x voltage) E.g. Mr. P’s hair dryer is plugged into a 120 V outlet. The current flowing through the dryer is 8.33 A. How much power is the dryer using?

Answer: P = I x v P = 8.33A x 120V P = 1000 W

The other way to calculate power is by using energy. P = E/t ( P = power, E = energy [in joules], t = time [in seconds] E.g. A microwave cooks a roast in 30 minutes, using J of energy. What is the power rating for the microwave?

P = E/t P = /(30 min. x 60 s/min) P = /1800s P = 800 W

It doesn’t take long for devices to use large amounts of energy. Therefore, it is often measured in kilowatt hours (e.g. in our homes). Calculation: kWh = power (in kW) x time (in h) When we are charged for the amount of energy used in our homes, the # of kWh used is multiplied by the rate ( cents/kWh for Red Deer area. Currently cents/kWh)

E.g. In a hurry to go skiing for the weekend, you rush out of the house at 5:00 p.m. on Friday and return home at 6:00 p.m. on Sunday. How much would it cost your parents if you left the light on in your room for the entire weekend? Your fixture has three 100W bulbs and the rate for electricity is cents/kWh. During that same period, you also forgot to turn down the furnace which was set at 21 o C so the furnace ran for 4 hours each day. Your furnace consumes 800W of power per hour of usage. How much did this add to your parents’ bill?

On all large appliances sold in Canada. Gives the buyer a rough estimate of how much power will be used per year by the appliance. The bar with the arrow above it allows the buyer to compare where that particular model ranks in power usage compared to other similar models. (This one is pretty good!!)

Energy cannot be created nor destroyed. It can only be transformed from one form into another. On Earth, the amount of energy going into a device (input energy) is larger than the amount of anticipated energy coming out of the device (usable output energy). The “lost or missing” energy is converted into some other form (usually heat)

The ratio of useful output energy to input energy is called efficiency. It is usually calculated as a percent. Percent efficiency = joules of output energy x 100 joules of input energy The units must be the same for the input and output energy (joules, kilojoules, etc.) Remember – efficiency can not be over 100%. Otherwise, the device is creating more energy than went into it!

E.g. A kettle takes in 2000J of energy/second. It transfers 1500J of heat energy into the water in the kettle. Calculate the efficiency of the kettle. Percent efficiency = joules of output energy x 100 joules of input energy PE = (1500/2000) x 100 = 75%

A typical incandescent light bulb is 100 W. If they are only 5% efficient and was left on 30 minutes, how many joules of light energy was produced?