Sustainable Energy

Lesson 5

Learning objective: To explain the link between the power rating and energy use

of an electrical appliance.• Must: Recall the definition for power and give its

unit of measurement – Grade D• Should: Be able to calculate power rating and

energy used by an electrical appliance. - Grade C• Could: Calculate the power of a range of

appliances using an ammeter and a voltmeter – Grade B.

• Keywords: Power, current, voltage, watt and energy transfer.

Starter-Label the power station

How many did you get?

• 1. Furnace and boiler.• 2. Turbine.• 3. Generator.• 4. Transformer.

Mains supply.

• What is the mains supply voltage to our homes?

• 230 V (volts).

• You need to remember this.

Do different appliances use different amounts of electricity?

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

Power

When an electric current is passed through a device electric current is transferred from the supply to the device.

The rate at which this energy is transferred is called POWER.

Power is measured in WATTS (W).

Power Rating• Different electrical devices have different power ratings• They use different amounts of electrical energy over a

given time.

15W 100W

VS

P = I x VPower (Watts) = Current (amps) x Voltage (volts)

Voltage = Potential Difference

Pee on the Ivy

P = I x V

Task- Complete the table using the equation P=IVAppliance Power (w) Voltage (v) Current (A)

Kettle 3000 13.0

Toaster 1550 230

Microwave 1300 240

Drill 240 1.5

Door Lift 2300 10

Fax Machine 230 1.0

CD player 15 0.06

TV 59 230

Vacuum Cleaner 240 2.7

Hairdryer 800 3.5

Desk Lamp 12 4.2

Sewing Machine 230 0.17

Possible homework task

Task- Complete the table using the equation P=IVAppliance Power (w) Voltage (v) Current (A)

Kettle 3000 230 13.0

Toaster 1550 230 6.7

Microwave 1300 240 5.4

Drill 360 240 1.5

Door Lift 2300 230 10

Fax Machine 230 230 1.0

CD player 15 250 0.06

TV 59 230 0.26

Vacuum Cleaner 648 240 2.7

Hairdryer 800 229 3.5

Desk Lamp 50 12 4.2

Sewing Machine 39 230 0.17

The Watt

1 Watt = 1 Joule transferred in 1 second

Power (W)= Energy (J) / Time (s)

Device Power Rating (Watt)

Time (sec) Energy (J)

Kettle 2400 180

Energy = Power x TimeEnergy = 2400W x (180s)Energy = 432000J

432000

Electric Cooker: 10kWFridge: 50WFreezer: 300WWashing machine: 500WDishwasher: 1300W

Electric Cooker: 10kW. 45minsFridge: 50W. 24hoursFreezer: 300W. 24hoursWashing machine: 500W. 1 hourDishwasher: 1300W. 45mins

Device Power Rating (Watt)

Time (sec) Energy (J)

Electric Cooker

10000 2700 27000000

Fridge 50 86400 4320000

Freezer 300 86400 25920000

Washing Machine

500 3600 1800000

Dishwasher 1300 2700 3510000

Total energy use in one day:

63036000J or63 million and 36 thousand Joules

and Total Energy for 3 months:

5673240000J or5 billion, 6 hundred and seventy three million, 2 hundred and forty thousand Joules!

Plenary- True or False

1. Power is measured in watts?2. Power= voltage / current?3. Current is measured in volts?4. Voltage is measured in volts?5. Potential difference is another word for voltage?6. I is the symbol for current?7. The mains supply voltage to our homes is 240V?8. The higher the power rating the more energy is used?9. 10 watts means 10 joules of energy every minute?