1. Learning Outcomes
Distinguish between Power and Energy. And use the equation Energy = Power x Time
Use the equations:
Units Used (kWh) = Power (kW) x Time (h)
Cost (p) = Units used x Cost per unit (p)
Use data to make comparisons of different sources of domestic energy.

2. Key Words:
Power: The rate of flow of energy
Energy: The ability to do work

3. Others 2%
Fridge/ freezer 15%
Television 2.5%
Water heater25%
Oven 5%
Drag the appropriate labels to the correct boxes in order to complete the graph showing the percentage use of electricity in the home.
Washing machine
<1%
Computer <1%
Tumble dryer3.5%
Dish washer1.5%
Bulbs
25%

4. Remember that there are1000W for every 1kW. So, 2kW = 2000W = 2000J/s
The power rating of a bulb is 60W.
This means that 60 Joules of electrical energy is used every second to produce light and heat energy.
The power rating of a kettle is 2kW.
This means that 2000 Joules of electrical energy is used every second to produce heat energy and to boil the water.
Remember that there are1000W for every 1kW.
So,
2kW = 2000W = 2000J/s
0.1kW = 100W = 100J/s
3.7kW = 3700W = 3700J/s
0.02kW = 20W = 20J/s
The unit of power is the Watt (W), and 1W is equivalent to 1J of energy per second.

5. You will need to be able to use: Power (W) = voltage (v)  current (A)
HIGHER
Electrical power is the measure of how fast electrical energy can be transformed into other energies.
You will need to be able to use:
Power (W) = voltage (v)  current (A)
If the mains voltage is 240V, what is the power of a sound system, if there is 0.4A of current flowing through it?
The current flowing through a toaster is 4.2A,
The mains voltage is 240V.
What is the power of the toaster?
Answer
Answer
Power (W) = current (A)  voltage (V)
= 4.2  240
= 1008W
( about 1kW )
Power(W) = current (A)  voltage (V)
= 0.4  240
= 96W

6. This small power station is producing 250kW of electricity.
HIGHER
This small power station is producing 250kW of electricity.
This step-up transformer is increasing the voltage to 25,000V.
What will the current be through the cables?
Answer
Step-up-transformer
Power (W) = current (A)  voltage (V)
250,000 = current  25,000
250,000 = current = 10A
25,000

7. I pay my electricity bill every month by direct debit
I pay my electricity bill every quarter
How much energy do we use?

8. Power 100W There are 1000W in every 1kW
Power is the rate of doing work, or the rate of energy transfer and is measured in Watts.
There are 1000W in every 1kW

9. Click here for the answer
Highest value
100W
0.05kW
0.4kW
1kW
30W
20kW
650W
Click here for the answer
Lowest value

10. Highest value
20kW
20,000W
1kW
1,000W
650W
650W
0.4kW
400W
100W
100W
0.05kW
50W
30W
30W
Lowest value

11. energy transfer = power  time
There are 1000W in every 1kW
energy transfer = power  time
So, a 100W light bulb will transfer 100J of electrical energy into a total of 100J of light and heat energy per second.
And a 2kW kettle will transfer 2,000J of electrical energy into a total of 2,000J of heat energy per second.
In fact,
1W = 1J/s

12. Units used (kWh) = power (kW)  time (h)
This was the type of electric meter that would have measured the amount of electricity used in your home.
It measured the electrical energy used in units of 1kWh (equivalent to a 1000W device running for 1 hour). You are then billed on the number of units used in total for a given period.
kWh
391M
Units used (kWh) = power (kW)  time (h)

13. 0567 Units used (kWh) = power (kW)  time (h)
These days, digital electric meters are the norm.
But although it looks different, the amount of electricity used is still measured in kWh.
0567
kWh
Units used (kWh) = power (kW)  time (h)
cost = units used  cost per unit

14. Below are the meter readings taken at particular times during the year for a 3 bedroom house.
0567
kWh
2207
kWh
3519
kWh
4517
kWh
September 9th (2010)
January 9th
(2011)
May 9th (2011)
September 9th (2011)
1. Calculate the number of units (KWh) used during each period.
2. Calculate the total number of units (KWh) used during the year.
3. During what period was the most electricity used, and can you explain why?
4. If one unit (1kWh) costs 8.3p, what is the average cost per day for this house?

15. Units used (kWh) = power (kW)  time (h)
If a single 100W light bulb was switched on for a total of 4 hours, and the cost of a unit of electricity (1kWh) is 8p then…..
Units used (kWh) = power (kW)  time (h)
= 0.1 4
= 0.4
cost = units used  cost per unit
= 0.4 8
= 3.2p
This doesn’t seem much, but this is for only one 100W light bulb that is on for 4 hours – how many light bulbs do you have switched on in your home during the day?

16. Different Shoes
How would today’s lesson be different if it was taught by:
A third world (LEDC) farmer
A climate change scientist
A money saving expert