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### 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 Eand the SI unit for measuring energy is called the joule.

- 1 joule is a very small amount of energy so we use Watt hourand 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

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