# ELECTRICAL SYSTEMS 21.3 - PowerPoint PPT Presentation

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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.

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ELECTRICAL SYSTEMS 21.3

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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.

Key Question:

How much energy is carried by electricity?

### Investigation 21B

Electrical Energy and Power

### 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.

### 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 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.

### 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.

### Solving Problems

• Looking for:

• …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

• 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.

• 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.

### 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

### Solving Problems

• Relationships:

• 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

### 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 DC

• The electrical system in your house uses alternating current or AC.

• Alternating current constantly switches direction.

### 21.3 Electricity in homes

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

### 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.

### 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.

### 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.

Bright Ideas

• What 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?