Electric Power, AC, and DC Electricity

Electric Power, AC, and DC ElectricityKey Question:How much does

electricity cost and what do you pay for?

*Students read Section 20.3 AFTER Investigation 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.

Reviewing terms

Power in electric circuitsOne watt is a pretty small amount of power. In everyday use, larger units are more

convenient to use. A kilowatt (kW) 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.

Power

P = VI Current (amps)Voltage (volts)

Power (watts)

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.

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 (kWh) 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.

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?

Alternating and direct current The current from a

battery is always in the same direction.

One end of the battery is positive and the other end is negative.

The direction of current flows from positive to negative.

This is called direct current, or DC.

Alternating and direct currentIf voltage alternates, so

does current.When the voltage is

positive, the current in the circuit is clockwise.

When the voltage is negative the current is the opposite direction.

This type of current is called alternating current, or AC.

Alternating and direct current AC current is used for almost all high-power

applications because it is easier to generate and to transmit over long distances.

The 120 volt AC (VAC) electricity used in homes and businesses alternates between peak values of +170 V and -170 V at a frequency of 60 Hz.

AC electricity is usually identified by the average voltage, (120 VAC) not the peak voltage.

Application: Wiring in Homes and Buildings

Application: Wiring in Homes and Buildings