Core Physics Topic 6  Transferring Electrical Energy
Why is Electricity so Useful?
We take electricity for granted in our homes. It is very easy to plug in an appliance or switch on a light. Electricity is clean, and, if used properly, safe. It can do all sorts of jobs that other energies can do, and more. It is possible to heat and light with gas. It could be possible to use a gaspowered hoover, but a TV or computer can only work with electricity.
Doing clever things with electricity is Electronics.
Look at this kitchen:
You can see a range of appliances that all use electricity, each one doing a different job, all easy to use.
We can compare the use of different appliances for a particular job.
What are the advantages and disadvantages of using a microwave oven compared with using an ordinary oven in a cooker? 
Energy Transformations in Everyday Appliances
To do a useful job for us, electrical energy has to be converted into other forms of energy. For example:
A drill:
Electrical Energy ® Movement (Kinetic) Energy
A radio:
Electrical Energy ® Sound Energy
Electrical energy is converted into other energy. Complete this table. One has been done as an example:

The power of an appliance
Electrical energy is measured in joules (J).
The amount of energy an electric appliance uses depends on:
how much power it uses;
how long its on for.
The Power is the rate at which an appliance transforms electrical energy into other forms of energy. Power is measured in Watts (W). 1 watt is 1 joule every second:
1 W = 1 J/s
On every appliance there is a label that shows the power of the appliance.
This motor operates at a voltage of 230 V and at mains frequency of 50 Hz (50 cycles per second). Its power is 250 watts. That means it turns 250 joules of electrical energy into movement energy every second.
The formula that gives you the right answer is shown in the box below:
energy transferred (kWh) = power (kW) × time (h) 
In Physics Code:
E = Pt
How many joules will the motor convert in 10 s? 
Often appliances have their power marked in kilowatts (kW):
1 kW = 1000 W
1 W = 1/1000 kW
So our 250 W motor will have a power of 250 ÷ 1000 = 0.25 kW
Complete the following table of powers of different appliances. One has been done as an example:

How much does it cost to use an appliance?
When we work out the cost of using an appliance, we pay for the energy that has been used. We could pay for the number of joules used, but the joule is only a small unit. So we need a bigger unit. This is called the kilowatthour (kWh) or unit.
1 kilowatt hour is the amount of energy used by a 1 kW appliance running for 1 hour.
A common bear trap is to write "kW/h" (kilowatts per hour). Don't!
To work out the amount of energy used by an appliance:
Work out the power in kilowatts (kW);
Work out the time used in hours (h);
Multiply the two numbers together.
The formula that gives you the right answer is shown in the box below:
energy transferred (kWh) = power (kW) × time (h) 
In Physics Code:
E = Pt
In triangle form:
Worked Example Lorraine uses a 7500 W shower for 20 minutes. How many kilowatt hours has she used? 
Answer 1. Work out the power in kilowatts: Power = 7500 ÷ 1000 = 7.5 kW
2. Convert the minutes to hours: 20 min = 20 ÷ 60 = 1/3 hour
3. Multiply the two numbers together: Electrical energy = 7.5 × 1/3 = 2.5 kWh 
How many units (kilowatthours) are used by a 2.5 kW oven running for 1.5 h? 
A common bear trap is to fail to convert watts to kilowatts and minutes to hours.
To work out the cost we simply multiply the number of kilowatthours by the cost per unit.
total cost = number of kilowatthours × cost per kilowatthour 
Electrical energy typically costs about 11 pence per kilowatthour.
Worked Example How much does Lorraine's shower cost? 
Answer Lorraine has used 2.5 kWh Cost = 2.5 kWh × 11 p = 27.5 pence (28 p) 
How much does it cost to watch 4.5 hours of TV which has a power of 300 watts? Electricity costs 11 p per unit. 
Summary
