1. Electric Power, AC, and DC Electricity

2. Electric Power, AC, and DC Electricity
Key Question:
How much does electricity cost and what do you pay for?
*Students read Section AFTER Investigation 20.3

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.

4. Reviewing terms

5. 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 (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.

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

7. 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.
1) You are asked to find the power used by the light bulb.
2) You are given the voltage of the battery and the bulb’s resistance.
3) Use Ohm’s law, I = V/R, to calculate the current; then use the power equation, P=VI, to calculate the power.
4) Solve: I = 1.5V ÷ 1.5Ω = 1A
P = 1.5V × 1A = 1.5 W; the bulb uses 1.5 watts of electric power.

8. 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.

9. Calculate power
1) You are asked to find the cost of using the coffee maker.
2) You are given the power in watts and the time.
3) Use the power formula P = VI and the fact that 1 kWh = 1kW x 1h.
4) Solve: Find the number of kilowatts of power that the coffee maker uses.
1,050 W × 1 kW/1,000 W = 1.05 kW
Find the kilowatt-hours used by the coffee maker each month.
1.05 kW × 1 hr/day x 30 days/month = 31.5 kWh per month.
Find the cost of using the coffee maker.
31.5 kWh/month × $0.14/kWh = $4.41 per month.
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?

10. 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.

11. Alternating and direct current
If 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.

12. 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.

14. Application: Wiring in Homes and Buildings

15. Application: Wiring in Homes and Buildings