Electric Energy

Electric Energy

Using Electric EnergyEnergy can be neither created nor destroyed,…but it can be transformed from one kind to another

Objectives

Explain how electric energy is converted into thermal energy

Determine why high-voltage transmission lines are used to carry electric energy over long distances

Define kilowatt-hour Perform energy and power

calculations

Power and Energy

Power is the time rate of doing work or expending energy Unit of work or energy is the joule Unit of power is the watt (1 joule per

second)

Energy

Work is equal to the product of the power consumed and the time of charge flow

PW

tW P t

PW

tW qV

Iq

tt

q

I t

I

q

PW

tqV

I

q

P V I

1

W qV

Iq

tq I t

W V I t

W V I t I RtV t

R

b gb g 2

2

Equivalent Expressions

W V I t I RtV t

R

PW

tVI I R

V

R

b g 22

22

Thermal Energy Current through a resistor generates

heat Light bulbs Space heaters Stoves, etc.

Voltage “drops” across a resistor Energy converted to heat is lost

“Joule heating” or I2R lossW q V I Rt 2

Example

A heater has a resistance of 10 Ω It operates on 120.0 V

What is the current through the resistor?

What thermal energy is supplied by the heater in 10.0 seconds?

Example Sketch the

problem List the knowns

and unknowns Strategy

Use I = V/R to find the current

Use W=I2Rt to find the energy*

Knowns

R

V V

t s

Unknowns

I A

W J

10 0

120 0

10 0

.

.

.

?( )

?( )

*The textbook uses “E” for energy

See solution on Page 521 of textbook

Where Does Electric Energy Come From?

Chemical energy (battery) Mechanical energy (generator) Gravitational energy (water falls) Steam turbines & wind turbines

(heat) Solar panels (light) Nuclear energyEnergy can be neither created nor destroyed,…but it can be transformed from one kind to another

Where Does Electric Energy Go?

Chemical energy (making batteries)

Mechanical energy (motor) Heat (intentional or byproduct) LEDs (light) Nuclear energy (particle

accelerators)Energy can be neither created nor destroyed,…but it can be transformed from one kind to another

Transmission of Electricity Power is lost as heat in transmission

line Reduce the loss

Low resistance wire High conductivity Large diameter

Lower current Why high-voltage?

Increase voltage and reduce current

P I Rloss 2

P V Itransmitted

Units of Electrical Energy Your electric bill is for energy, not

power (even though we call the utility, “the power company”)

Electric meters measure the power consumption in kilowatts and the time in hours 1 kWh = (1000 J/s)(3600 s) = 3.6 x 106 J

PW

tW P t

Example

The resistance of a stove element is 11Ω at operating temperature with a voltage of 220V. What is the current through the stove

element? How much energy is used in 30

minutes? What would the cost be at 12 cents per

kilowatt-hour?

Extra Credit

Find the name of each person for whom the following units are named Watt Joule Coulomb Ampere Volt

Practice Problems

Textbook, Page 522 #14 – 17 Textbook, Page 525 #18 – 20

Two people do #20