1. Energy transformations Monday, 31 July 2017

2. Learning objectives
Explain that energy can be transformed from one form to another and draw energy transformations in an energy flow diagram
Recall and apply the law of conservation of energy, that energy can never be created nor destroyed, but only changed from one form to another.
Recognise that energy transformations produce useful energy and wasted energy
Define and calculate energy efficiency
Explain that energy can be transferred from one object to another and draw energy transfers in an energy flow diagram

3. Key words: Energy transformation Energy transfer Sankey diagram
Law of conservation of energy

5. https://youtu.be/LEMK_v-p70Q?t=7m

6. Transfer or Transformation?
Energy transfers are energy changes where the energy just moves to a different place, such as an electric current carrying electrical energy from a cell to a component.
Energy transformations are energy changes where the energy changes from one form (or type) to a different form (or type).

7. Energy Transformation
Most machines transform (change) energy into a different form of energy.
For example, a torch transforms stored chemical energy into electrical energy and then light energy.

8. A ball will bounce to a lower height than it is dropped from because kinetic energy is transformed to thermal (heat) energy.

9. 'Wasted' energy (read, don’t write this down!)
Energy cannot be created or destroyed. It can only be transferred from one form to another or moved.
Energy that is 'wasted', like the heat energy from an electric lamp, does not disappear. Instead, it is transferred into the surroundings and spreads out so much that it becomes very difficult to do anything useful with it.

10. 'Wasted' energy (write this down!)
The heat energy transferred to the surroundings is considered to be wasted because nothing useful can be done with it .
Remember energy cannot be created/destroyed, only transferred from one form to another.

11. Velvety feathers help absorb noise
Velvety feathers help absorb noise. The feathers on the edge of the wing are tattered: this helps to reduce noise.
Owls can fly almost silently, and aircraft engineers are trying to mimic owl wings to see if they can make aircraft less noisy. They could design aircraft which convert less of their input energy into sound energy or design aircraft parts which will convert any sound energy produced into other forms of energy.

12. Energy Input = Energy Output
Energy cannot be created or destroyed: it can only be transferred from one place to another or transformed from one form to another.
This is called the Law of Conservation of Energy.
(The total amount of energy does not change. If you could measure the total energy stored in the petrol used by a car, it would be exactly the same as the total heat, sound and kinetic energy produced by the engine.)

13. Sankey diagrams
Sankey diagrams summarise all the energy transfers taking place in a process.
The thicker the line or arrow, the greater the amount of energy involved.
This Sankey diagram for the lamp shows that it transfers most of the energy by heating, rather than by light:

14. efficiency = useful energy output/ total energy input
The efficiency of an appliance is calculated using this equation:
The units can be J or kJ, as long as both numbers are in the same units.
Efficiency is sometimes given as a percentage by multiplying the answer by 100%.
efficiency = useful energy output/ total energy input

15. Efficiency – why is it important?
We can compare the efficiency of machines or devices to help us decide which is the best one to buy.
People like to buy efficient devices because they cost less to run.
They use less energy to do the same amount of work.

17. efficiency = useful energy output/ total energy input
Example
What is the efficiency of an electric kettle if it uses 500kJ of electrical energy and transfers only 400kJ of energy to the water in the kettle?
Useful energy output = 400kj
Total energy input = 500kj
400/500 = 0.8 (to convert a decimal to percent x100)
0.8 x 100 = 80%
efficiency = useful energy output/ total energy input

18. If you calculate an efficiency that is bigger than 1 (or bigger than 100%) you have probably put the numbers into the equation the wrong way round!

19. 1. The useful energy transfer that happens in an electric lamp is:
electricity → light
electricity → sound
electricity → heat
2. ‘Wasted energy’ becomes less useful because:
it is destroyed after use
it becomes increasingly spread out
it makes the surroundings become cooler
3. Which equation can be used to calculate the efficiency of an energy transfer?
efficiency = useful energy out – total energy in
efficiency = useful energy out ÷ total energy in
efficiency = useful energy out × total energy in
4. In a certain device, 25% of the energy is usefully transferred. The energy efficiency of this device is:
25%
75%
125%
5. A coal-fired power station is 40% efficient. This means that:
40% of the chemical energy stored in coal is transferred to electrical energy
60% of the chemical energy stored in coal is transferred to electrical energy
40% of the chemical energy stored in coal is transferred to the surroundings as heat