Measuring Electrical Energy Lesson 11 Measuring Electrical Energy

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 • k. Wh k. W 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. 5 k. W and is operated for 1. 0 hours? • Energy = Power x Time • E=Px. T • E = 1. 5 k. W x 1. 0 hour • E = 1. 5 k. Wh

• Using a vacuum cleaner runs 2 for hours with a power rating of 1. 6 k. W. • E=Pxt • E = 1. 6 k. W x 2 h • E = 3. 2 k. Wh

• Mowing a lawn for 3. 5 hours with a mower that has a rating of 1. 65 k. W. • E=Pxt • E = 1. 65 k. W x 3. 5 h • E = 5. 78 k. Wh

Something to remember • Power must be in k. W • 1000 W = 1 k. W • To change from W to k. W divide by 1000

Example • • Example 1 400 W = ? k. W 400 ÷ 1000 = 0. 4 k. W

Example • • Example 2 15 W = ? k. W 15 ÷ 1000 = 0. 015 k. W

Example • • Example 3 3750 W = ? k. W 3750 ÷ 1000 = 3. 75 k. W

• 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=Pxt • E = (30 W ÷ 1000) x 2 h • E = 0. 03 k. W x 2 h • E = 0. 06 k. Wh

• Using a blender with a rating of 390 W for 20 minutes. • E=Pxt • E = (390 W ÷ 1000) x (20 min ÷ 60) • E = 0. 39 k. W x 0. 33 h • E = 0. 13 k. Wh

Word Problems I (5)