Let’s Review:

Which of the objects has a “Total” negative charge?

Which of the objects has a “Total” positive charge?

Which of the objects has a “Total” neutral charge?

How did “B” become positive?

A. Gaining protonsB. Losing protonsC. Gaining electronsD. Losing electrons

How did “C” become negative?

A. Gaining protonsB. Losing protonsC. Gaining electronsD. Losing electrons

Electric Power(P)• What is Power?

– Power is defined as the time rate of doing work.• It is how fast we do work!!• What is work?

– Work is the transfer of energy through motion.

• Power is the rate of transfer of energy through motion.• Measured in Watts(W)

• Electric Power is also the time rate of doing work. – It is how fast electrical energy can be transferred to

other forms of energy.

Consider where we have heard the word “Watts” used in electricity!!

• Light bulbs

• What does a light bulb do?

What happens

in terms of energy?

Electric Power(P)

• Calculating Electric Power

P = V I

P –Power(in Watts(W))V—Voltage(in Volts(V))I--Current(in Amps(A))

Ex) How much power is consumed when a current of 12 A flows when connected to a 9 V battery?

Given:V = 9VI = 12 AP = ?

P = V IP = 9(12)P = 108 W

Ex) A CD Player with a power rating of 10 W needs 1.11 A to work properly. What size battery is required?

Given:V = ?I = 1.11 AP = 10 W

P = V IV = P/IV = 10/1.11V = 9 V

Ex) A 60 W light bulb is connected to a 110 V electric outlet. How much current flows through the light bulb?

Given:V = 110VI = ?P = 60 W

P = V II = P/VI = 60/110I = .54 A

A 3 A current flows through a CD Player when connected with a 9 V battery. How much power is consumed by the CD player?

A. 3 WB. .33 WC. 12 WD. 27 W

What current will flow through a 40 W light bulb when connected to a 110 V outlet?

A. 3 AB. .36 AC. 12 AD. 27 A

Electrical Energy• Measure of the amount of power actually

consumed over time.– Measured in kW * hr

• All electrical appliances have a Power Rating(an amount of power they consume “while on”)– So electrical energy depends on how long you use

a device.

Electric Energy• Determining electric energy

E = P*tE = electrical energy(kW*hr)P = Power(kW)t = time (hrs)

Electrical Energy(cont.)

• You have an electric meter outside of your house somewhere.

• What does it tell you?• Does it always move at the same rate?• Is it always moving?• Two Types of Meters:

1. Analog2. Digital

Analog

What’s the reading on the meter above?

19696

Digital

What is the reading on the meter below?

A. 4,925 kW*hrB. 9,925 kW*hrC. 14,925 kW*hrD. 84,925 kW*hr

Power Bills

• You can determine how much a power bill is by multiplying the electrical energy used by the cost per kilowatt-hour.

• If electrical energy costs $.10 per kW*hr and you consume 500 kW of power, your electric bill will be $50.00.

Fuse Boxes and Circuit Breakers

• Offer protection from over-heating circuits.• Fires are bad!!• No fires are good!!

•Fuses Blow•Circuit Breakers Trip

Why do Fuses “blow” and Circuit Breakers “Trip”?

• All fuses(and circuit breakers) have a maximum amount of current they can have passing through them.

• If the maximum is exceeded, they blow!!

fuse

Circuit Breaker

Example of Blowing a Fuse• A fuse is rated at 20 A. • Will it “blow” if the following are hooked up?

– A TV drawing 4 A.– A hair dryer drawing 7 A.– A curling iron drawing 6 A.– A “hair straightener” drawing 5 A.

Total Current = 22 A

Fuse will only allow 20 A, fuse BLOWS!!

The following devices are connected to a 110 V circuit. Will a 12 A fuse connected to the circuit blow?

Radio—40 ohmsComputer—60 ohmsHair dryer—120 ohmsIpod charger—25 ohms

A. YesB. NoC. Not at first, but it will after

19 seconds