+

Measuring Electrical Energy Mrs. McLean – Science 300

Do different appliances use

different amounts of electricity?

How much does electricity cost?

+ So how much does it cost every

time I watch TV?

+ Today, we will learn:

1. How to calculate how much electrical energy a device uses over a

period of time

2. How to calculate how much a device costs to run for an amount of

time

3. That different devices use different amounts of electrical energy

This is important because:

When you leave home, you will need to know how your electricity bill is worked out.

When you buy new electrical appliances, you will know about power ratings.

We need to make our fossil fuels last as long as possible. By knowing about how much electricity we use, we can achieve this.

+ Energy

Energy is the ability to do work

+ Electrical Energy

Electrical energy is the energy transferred

to a load by moving electrical charges.

+ Joules

Energy is measured in Joules (J)

We need to know how much energy something uses over time = a rate

We can use Joules per second (J/s)

1 Joule / sec = 1 Watt (W)

A 100W light bulb uses 100J of energy every second

+ Voltage, Current, and Time

Using voltage, current and time elapsed, one can

discover how much energy is being used.

V x I x T = E (joules)

+ Two Circuits…

So if I have two different circuits, each with 3V

originating from cells…

One has one bulb

The other has two bulbs

Which set of batteries will die faster?

How much ENERGY is used in each?

Look at page 334

+ Let’s do the math…

First Circuit:

Voltage: 3 V

Current: 0.2 A

Time: 5 hrs

Second Circuit:

Voltage: 3V

Current: 0.4 A

Time: 2.5 hrs

They each use up

10 800 Joules.

+ Watt hours and KiloWatt Hours

Wh and kWh are a larger unit used to

measure electrical energy.

Why would we not just use joules?

Power Rating

Different electrical devices have different power ratings

They use different amounts of electrical energy over a given time.

15W 100W

VS

+ What? Hours?

A 100W light bulb on for 1 hour will use

100 Watt hours (Wh) of electricity

This is usually measured in Kilowatt Hours (kWh)

0.1 kWh

On an electricity bill,

electrical energy = power rating x time

1 unit of electricity = 1 kilowatt hour

+ Electricity Meters

+ Cost of Electricity

At the moment, one unit (1 kWh) of electricity costs ¢12.78

To run a normal 100W light bulb for 5 hours costs:

100W x 5hours = 500Wh = 0.5kWh

0.5kWh x 12¢ = 6¢

To run a 15W eco bulb for 5 hours costs:

15W x 5hours = 75Wh = 0.075kWh

0.075kWh x 12¢ = 0.9¢

+ What about for a year?

To run a normal 100W light bulb for 5 hours a day all year costs:

100W x 5hours = 500Wh = 0.5kWh

0.5kWh x 12¢ = 6¢

6¢ x 365 days = 2190¢ = $21.90

To run a 15W eco bulb for 5 hours a day all year costs:

15W x 5hours = 75Wh = 0.075kWh

0.075kWh x 12¢ = 0.9¢

0.9¢ x 365 days = 328.5¢ = $3.29