ELECTRICAL SYSTEMS 21.3

Chapter Twenty One: Electrical Systems

21.1 Series Circuits

21.2 Parallel Circuits

21.3 Electrical Power

Chapter 21.3 Learning Goals

Define electric power and apply a formula to perform power calculations.

Distinguish direct current and alternating current.

Discuss applications of electricity in daily living.

Investigation 21B

Key Question:How much energy is carried by electricity?

Electrical Energy and Power

21.3 Electrical PowerElectrical 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.

21.3 Important review

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.

21.3 Power

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

21.3 Different forms of the Power Equation

21.3 KilowattMost electrical appliances have a label that lists the power in watts (W) or kilowatts (kW).

The kilowatt is used for large amounts of power.

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.

Solving Problems

1. Looking for: …power of battery

2. Given: …voltage = 12 V; current = 3 amps

3. Relationships: Power: P = I x V

4. Solution: P = 3 A x 12 V = 36 watts

Solving Problems

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.

21.3 Buying ElectricityThere 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.

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.

Solving Problems

1. Looking for: …cost to run stove for 2h

2. Given: … P = 3,000W; T = 2h; price $0.15/kW

3. Relationships: 1000 watts = 1 kW Charge in kWh

4. 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

Solving Problems

= $ 0.90

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.

21.3 AC and DCThe electrical system in your house uses alternating current or AC.

Alternating current constantly switches direction.

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

21.3 Electricity in homesElectrical 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.

21.3 Distributing electricityElectricity 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.

21.3 Distributing electricityMany 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.

Bright IdeasWhat makes one bulb more efficient than another? How much more efficient are the LEDs? What kind of savings does this mean in terms of electricity?