1. Write down the energy transfers in the following:
Speakers
Wind turbines
Fan
Hydroelectric power station

2. Efficiency Learning objectives
To be able to collect and use measurements of V and I to calculate power.
To be able to calculate energy used from measurements of power and time.
To be able to state what energy is ‘wasted’

3. Energy cannot be created or destroyed,
Law of conservation of energy
All energy transfers follow the law of conservation of energy:
Energy cannot be created or destroyed,
just changed in form.
This means that energy never just ‘disappears’.
The total amount of energy always stays the same,
i.e. total input energy = total output energy.
In most energy transfers, the energy is transferred to several different forms, which may or may not be useful.
Energy that is transferred to unwanted forms of energy is wasted.

4. useful output energy total input energy
Efficiency
Efficiency is a measure of how good a device is at changing energy from one form to another.
All devices waste energy, so the efficiency of a device is never 100%.
Calculating Efficiency:
energy efficiency =
useful output energy
total input energy
X 100

5. Calculating Efficiency
This filament bulb is supplied with 100 J of electrical energy, which it converts to 45 J of light energy.
How much energy
is wasted?
In what form is the energy wasted?
What is the efficiency
of the bulb?
Wasted energy = Total – Useful
= 100 J – 45 J
= 55 J
55 J
heat
Efficiency = Useful
Total
= 45 J
100 J
= 0.45 or 45%
0.45 or 45%

6. Energy Transfer diagrams
Sankey diagrams
Energy Transfer diagrams
Consider a light bulb. Let’s say that the bulb runs on 100 watts (100 joules per second) and transfers 20 joules per second into light and the rest into heat. We can show how energy is transferred into various forms using Sankey diagrams. Draw this as a diagram:
“Input” energy
“Output” energy
20 J/s light energy
100 J/s electrical energy
80 J/s heat energy (given to the surroundings)

7. Example questions Consider a computer: Consider a kettle:
150 J/s electrical energy
10 J/s wasted sound
20 J/s wasted heat
Useful light and sound
How much energy is converted into useful energy?
What is the computer’s efficiency?
Consider a kettle:
Work out each energy value.
What is the kettle’s efficiency?
Sound energy
Wasted heat
Heat to water
2000 J/s electrical energy

8. In which two forms is energy usually wasted?
Sound and light
Heat and sound
Heat and light
Light and chemical 

9. What is the intended energy transfer for a Bunsen burner?
Sound  light
Heat  chemical
Chemical  light
Chemical  heat 

10. What is the energy change for a falling rock?
Kinetic  gravitational
Gravitational  kinetic
Kinetic  speed
Mass  speed 

11. In what form is energy wasted for all electrical devices?
Heat
Electrical
Sound
Light 

12. The cost of electricity
Learning objectives
Understand that Power is calculated from IV and that energy consumed is calculated from:
Energy = Power x time.
Know that mains electricity is charged for in units of kilowatthours.
Use the equation to calculate the cost of electricity: cost = power x time x cost of 1kWh.

13. Buying electricity
In your home will be an electricity meter.
It records how much electricity you use in units.
From the number of units you use your electricity bill is calculated. 26
How many units have been used here? ______
Demo to look at meter and different appliances

14. electrical energy = power x time
Calculating the units of electricity
The amount of electrical energy (i.e. the amount of electricity) used by an appliance depends on its power and how long
the electricity is used for.
electrical energy = power x time
Power is measured in kilowatts (kW) and the time is measured in hours (h), so what are the units of electricity measured in?
1 unit of electricity = 1 unit of electrical energy
= 1 kilowatt hour (kWh)

15. Cost of electricity = Number of units x cost per unit
The cost of electricity is the number of units multiplied by the cost per unit:
Cost of electricity = Number of units x cost per unit
which is the same as……
Cost of electricity = Power (kW) x time (h) x cost per kWh (p)
Example:
How much would 10 units of electricity cost at a price of 9p per unit?
Cost = 10 x 9p
= 90 p

16. Buying electricity – question 1:
A kettle uses 45.2 kWhr of energy.
If electricity costs 10p per unit, how much does it cost to use the kettle?
Number of units:
Number of units of electricity is 45.2 units
Cost of electricity:
Cost = 45.2 x 10 p
= 452p or £4.52

17. Buying electricity - question 2:
An iron that operates at a power of 3 KW for 4 hours uses electricity that costs 8p per unit.
How much does it cost for the electricity that it uses?
Number of units:
kWhr = 3kW x 4hr
= 12 kWhr
Number of units of electricity is 12 units
Cost of electricity:
Cost = 12 x 8 p
= 96p

18. Amount of electrical energy
Electricity is measured in units called “kilowatt hours” (kWh). For example…
A 3kW fire left on for 1 hour uses 3kWh of energy
A 1kW toaster left on for 2 hours uses 2kWh
A 0.5kW hoover left on for 4 hours uses __kWh
A 200W TV left on for 5 hours uses __kWh
A 2kW kettle left on for 15 minutes uses __kWh

19. The Cost of Electricity
To work out how much a device costs we do the following:
Cost of electricity = Power (kW) x time (h) x cost per kWh (p)
For example, if electricity costs 8p per unit calculate the cost of the following…
A 2kW fire left on for 3 hours
A 0.2kW TV left on for 5 hours
A 0.1kW light bulb left on for 10 hours
A 0.5kW hoover left on for 1 hour
48p 8p 8p 4p

20. The cost of electricity
Learning outcomes
Use the equation to calculate the cost of electricity:
cost = power × time × cost of 1kWh
where power is measured in kilowatts and time is measured in hours.