Measuring Electrical Energy

Lesson 11

Measuring Electrical Energy

What is Energy?

• Energy- The ability to do work

• Electrical Energy- The energy transferred to an electrical load by moving charges.

• The symbol for electrical energy is E and the SI unit for measuring energy is called the joule.

• 1 joule is a very small amount of energy so we use Watt hour and Kilowatt hour.

• 1 watt hour = 3600 joules.

• 1 kilowatt hour = 1000 watt hours.

Observing sample circuits

• Circuit 1 has two 1.5 volt batteries in series producing a total of 3.0 volts. The circuit can keep the bulb lit for 5 hours.

• Circuit two has two 1.5 volt batteries in series producing a total of 3.0 volts. There are two bulbs in parallel in this circuit which can stay lit for 2.5 hours.

Why does circuit two 2 last half of the time that circuit 1 does?

• It is using more energy.

Calculating Electrical Energy

• We can use a formula to calculate the amount of energy used in household appliances.

• Energy = Power x Time

• kWh kW h

• Total Energy Used = Speed That Electricity is used x How long electricity is used

How much energy is that?

• Examples :• How many kilowatt hours of electrical

energy are used by a kettle that has a power rating of 1.5kW and is operated for 1.0 hours?

• Energy = Power x Time • E = P x T• E = 1.5 kW x 1.0 hour• E = 1.5 kWh

• Using a vacuum cleaner runs 2 for hours with a power rating of 1.6 kW.

• E = P x t

• E = 1.6 kW x 2 h

• E = 3.2 kWh

• Mowing a lawn for 3.5 hours with a mower that has a rating of 1.65 kW.

• E = P x t

• E = 1.65 kW x 3.5 h

• E = 5.78 kWh

Something to remember

• Power must be in kW

• 1000 W = 1 kW

• To change from W to kW divide by 1000

Example

• Example 1

• 400 W = ? kW

• 400 ÷ 1000

• = 0.4 kW

Example

• Example 2

• 15 W = ? kW

• 15 ÷ 1000

• = 0.015 kW

Example

• Example 3

• 3750 W = ? kW

• 3750 ÷ 1000

• = 3.75 kW

• Time must be in hours

• 60 minutes = 1 hour

• To change from min to hours divide by 60

• Example 1

• 45 min = ? h

• 45 ÷ 60

• = 0.75 hours

• Example 2

• 5 min = ? h

• 5 ÷ 60

• = 0.08 hours

• Example 3

• 90 = ? h

• 90 ÷ 60

• = 1.5 hours

More examples

Using a stereo with a rating of 30 W for 2 hours.

• E = P x t

• E = (30W ÷ 1000) x 2 h

• E = 0.03 kW x 2h

• E = 0.06 kWh

• Using a blender with a rating of 390 W for 20 minutes.

• E = P x t

• E = (390 W ÷ 1000) x (20 min ÷ 60)

• E = 0.39 kW x 0.33 h

• E = 0.13 kWh

Word Problems I (5)