1. Energy transfers and efficiency

2. Energy cannot be created or destroyed
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.

3. Learning objectives
Must: Describe what is meant by the word “efficiency”. {C}
Should: Calculate the efficiency of a device using the efficiency equation. {B}
Could: Interpret and draw a Sankey diagram. {B-A}

4. Learning outcomes
To carry out efficiency calculations and interpret Sankey diagrams
To explore exam style questions on energy and efficiency.

5. 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.
The Sankey diagram for an electric lamp below shows that most of the electrical energy is transferred as heat rather than light.

6. Efficiency?
How do the Sankey diagrams show which light bulb is more efficient?
Describe what each Sankey diagram tells you about useful and wasted energy and whether it is efficient or not?
The first Sankey diagram shows that …..
The seconds Sankey diagram shows that …..

7. Accurate Sankey Diagrams
Torch with Chemical -> Light 5J & Heat 95J
Car with Chemical -> Kinetic 25J & Heat 75J
Bunsen Burner with Chemical -> Heat 85J, Light 10J, Sound 5J
Hairdryer -> Electrical -> Sound 5J, Heat 20J, Kinetic 75J
Notice how this diagram is completed to scale. The vertical part of the scale is 2 squares = 10J of energy. Try and draw these 4 in a similar way on one sheet of graph paper.

8. This diagram represents energy transfer by the drill motor
This diagram represents energy transfer by the drill motor. The manufacturer claims the drill motor has an efficiency of The drill motor is supplied with 3000 J of electrical energy when the hole is drilled.
What form of energy does A represent?
What does B represent and what forms of energy are included in it?
What is the useful energy transferred by the drill motor used for?
What is the useful energy from the drill motor?
What is the energy wasted by the drill?

9. Electrical wasted thermal / sound Kinetic 0.35x 3000J = 1050J
This diagram represents energy transfer by the drill motor. The manufacturer claims the drill motor has an efficiency of The drill motor is supplied with 3000 J of electrical energy when the hole is drilled.
What form of energy does A represent?
What does B represent and what forms of energy are included in it?
What is the useful energy transferred by the drill motor used for?
What is the useful energy from the drill motor?
What is the energy wasted by the drill?
Electrical
wasted thermal / sound
Kinetic
0.35x 3000J = 1050J
(1-0.35) x 3000J = 1950J

10. Summary Questions – Learning point
Energy can be _________ from one form into other forms.
Energy can be ___________ from one place to another place.
Energy cannot be ________ or __________.
_______ or _______ energy is energy in the place we want it and in the form we need it.
_______ energy is energy that is not useful.
Useful energy and wasted energy both end up being transferred to the ____________ which become warmer.
As energy _______ out, it gets more and more difficult to use for further energy transfers.
created
destroyed
useful
transformed
spreads
surroundings
wasted
Stored
transferred