Physics Day 23

Aim: How much power does a toaster use?LO: Calculate power use for V, I and RLO: Calculate energy use from powerLO: Kilowatt-hour is a unit of energyAGENDADo NowNotesHW09

Do Now:- How much does electricity cost?- In what units do we measure electricity when we buy it?- How large is the average monthly electricity bill?

What is a kilowatt hour?

• Before we see how much electricity costs, we have to understand how it's measured. – When you buy electricity they charge you by the

kilowatt-hour (kWh). When you use a 1 kilowatt appliance for 1 hour, that's a kilowatt-hour. If you used it for two hours that would be 2 kWh.

• Most things don't draw an even kilowatt, though. – A typical central AC draws about 3.5 kilowatts

What’s a Watt?

• The rate of electrical use at any moment is measured in watts. For example: A 100-watt light bulb uses 100 watts.

• A typical desktop computer uses 65 watts.• A central air conditioner uses about 3500 watts.• If your device lists amps instead of watts, then

just multiply the amps times the voltage to get the watts. For example:

• 2.5 amps x 120 volts = 300 watts

What’s it cost in 2006

Cost Today in NYC

• 20 cents/ kw.hr• Average monthly bill is about $100• Lower in Winter, higher in summer

Electric Power,P.

time

EnergyP

Since the electrical energy is charge times voltage (QV), the above equation becomes,

.t

QVP

Since the current is charge flow per unit time (Q/t), the above equation becomes,

.VIVt

Q

t

QVP

Since V = IR, the above equation can also be written as,

.2

2

R

VRIIVP SI Unit of Power: watt(W)

Killowatt-hour (kWh)

The SI unit of power is watt, after James Watt (1736-1819), who developed steam engines.

Utility companies use the unit kilowatt-hour to measure the electrical energy used by customers. One kilowatt-hour, kWh is the energy consumed for one hour at a power rate of 1 kW.

.sec s

J

ond

jouleWwatt

Electrical Energy

•Our daily life depends on electrical energy.

•We use many electrical devices that transform electrical energy into other forms of energy.

• For example, a light bulb transforms electrical energy into light and heat. •Electrical devices have various power requirements.

Electric Power,P.

time

EnergyP

Since the electrical energy is charge times voltage (QV), the above equation becomes,

.t

QVP

Since the current is charge flow per unit time (Q/t), the above equation becomes,

.VIVt

Q

t

QVP

Since V = IR, the above equation can also be written as,

.2

2

R

VRIIVP SI Unit of Power: watt(W)

Killowatt-hour (kWh)

The SI unit of power is watt, after James Watt (1736-1819), who developed steam engines.

Utility companies use the unit kilowatt-hour to measure the electrical energy used by customers. One kilowatt-hour, kWh is the energy consumed for one hour at a power rate of 1 kW.

.sec s

J

ond

jouleWwatt