2. Chapter Twenty One: Electrical Systems
21.1 Series Circuits
21.2 Parallel Circuits
21.3 Electrical Power

3. 21.3 Electrical Power
Electrical power is measured in watts, just like mechanical power.
Power is the rate at which energy is changed into other forms of energy such as heat, sound, or light.
Anything that “uses” electricity is actually converting electrical energy into some other type of energy.

4. 21.3 Important review

5. 21.3 Electrical Power
The watt is an abbreviation for one joule per second.
A 100-watt light bulb uses 100 joules of energy every second.

6. 21.3 Power
Power is a “rate” and is measured using current and voltage .

7. 21.3 Different forms of the Power Equation

8. 21.3 Kilowatt
Most electrical appliances have a label that lists the power in watts (W) or kilowatts (kW).
The kilowatt is used for large amounts of power.

9. A 12-volt battery is connected in series to two identical light bulbs.
Solving Problems
A 12-volt battery is connected in series to two identical light bulbs.
The current in the circuit is 3 amps.
Calculate the power output of the battery.

10. Looking for: Given: Relationships: Solution: Solving Problems
…power of battery
Given:
…voltage = 12 V; current = 3 amps
Relationships:
Power: P = I x V
Solution:
P = 3 A x 12 V = 36 watts

11. 21.3 Buying Electricity
Utility companies charge customers for the number of kilowatt-hours (kWh) used each month.
A kilowatt-hour is a unit of energy.
The number of kilowatt-hours used equals the number of kilowatts multiplied by the number of hours the appliance was turned on.

12. 21.3 Buying Electricity
There are many simple things you can do to use less electricity.
When added up, these simple things can mean many dollars of savings each month.

13. How much does it cost to run a 3,000 kW electric stove for 2 hours?
Solving Problems
How much does it cost to run a 3,000 kW electric stove for 2 hours?
Use an electricity cost of $0.15 per kilowatt-hour.
Looking for:
…cost to run stove for 2h
Given:
… P = 3,000W; T = 2h; price $0.15/kW

14. Relationships: Solution: = $ 0.90 Solving Problems 1000 watts = 1 kW
Charge in kWh
Solution:
3000 W x 1 kW = 3 kW
1000 W
Charge = 3 kW x 2 h = 6 kWh
Cost = 6 kWh x $ 0.15
1 kWh
= $ 0.90

15. 21.3 AC and DC
Although the letters “DC” stand for “direct current” the abbreviation “DC” is used to describe both voltage and current.
DC current flows in one direction as in a battery.

16. 21.3 AC and DC
The electrical system in your house uses alternating current or AC.
Alternating current constantly switches direction.

17. 21.3 Electricity in homes
Electricity comes into most homes or buildings through a control panel which protect against wires overheating and causing fires.

18. 21.3 Electricity in homes
Electrical outlets in bathrooms, kitchens, or outdoors are now required to have ground fault interrupt (GFI) outlets.
GFI outlets are excellent protection against electric shocks, especially in wet locations.

19. 21.3 Distributing electricity
Electricity is a valuable form of energy because electrical power can be moved easily over large distances.
Alternating current is easier to generate and transmit over long distances.

20. 21.3 Distributing electricity
Many electronic devices, like cell phones or laptop computers, use DC electricity.
An “AC adapter” is a device that changes the AC voltage from the wall outlet into DC voltage for the device.