20 1 Short circuit A short circuit is

20. 1 Short circuit • A short circuit is a parallel path in a circuit with zero or very low resistance. • Short circuits can be made accidentally by connecting a wire between two other wires at different voltages. • Short circuits are dangerous because they can draw huge amounts of current.

20. 3 Electric Power, AC, and DC Electricity • The watt (W) is a unit of power. • Power is the rate at which energy moves or is used. • Since energy is measured in joules, power is measured in joules per second. • One joule per second is equal to one watt.

20. 3 Reviewing terms

20. 3 Power in electric circuits • One watt is a pretty small amount of power. • In everyday use, larger units are more convenient to use. • A kilowatt (k. W) is equal to 1, 000 watts. • The other common unit of power often seen on electric motors is the horsepower. • One horsepower is 746 watts.

20. 3 Power Voltage (volts) Power (watts) P = VI Current (amps)

20. 3 Calculate power • A light bulb with a resistance of 1. 5Ω is connected to a 1. 5 -volt battery in the circuit shown at right. • Calculate the power used by the light bulb.

20. 3 Paying for electricity • Electric companies charge for the number of kilowatt-hours used during a set period of time, often a month. • One kilowatt-hour (k. Wh) means that a kilowatt of power has been used for one hour. • Since power multiplied by time is energy, a kilowatt-hour is a unit of energy. • One kilowatt-hour is 3. 6 x 106 joules.

20. 3 Calculate power • Your electric company charges 14 cents per kilowatt-hour. Your coffee maker has a power rating of 1, 050 watts. • How much does it cost to use the coffee maker one hour per day for a month?