Energy transfers and efficiency

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.

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}

Learning outcomes

• To carry out efficiency calculations and interpret Sankey diagrams

• To explore exam style questions on energy and efficiency.

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.

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 …..

Accurate Sankey DiagramsNotice 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.

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

This diagram represents energy transfer by the drill motor. The manufacturer claims the drill motor has an efficiency of 0.35. The drill motor is supplied with 3000 J of electrical energy when the hole is drilled.

1. What form of energy does A represent?

2. What does B represent and what forms of energy are included in it?

3. What is the useful energy transferred by the drill motor used for?

4. What is the useful energy from the drill motor?

5. What is the energy wasted by the drill?

This diagram represents energy transfer by the drill motor. The manufacturer claims the drill motor has an efficiency of 0.35. The drill motor is supplied with 3000 J of electrical energy when the hole is drilled.

1. What form of energy does A represent?

2. What does B represent and what forms of energy are included in it?

3. What is the useful energy transferred by the drill motor used for?

4. What is the useful energy from the drill motor?

5. What is the energy wasted by the drill?

1. Electrical2. wasted thermal / sound3. Kinetic4. 0.35x 3000J = 1050J5. (1-0.35) x 3000J = 1950J

Summary Questions – Learning point

1. Energy can be _________ from one form into other forms.

2. Energy can be ___________ from one place to another place.

3. Energy cannot be ________ or __________.

4. _______ or _______ energy is energy in the place we want it and in the form we need it.

5. _______ energy is energy that is not useful.

6. Useful energy and wasted energy both end up being transferred to the ____________ which become warmer.

7. As energy _______ out, it gets more and more difficult to use for further energy transfers.

transformed transferred

created destroyed

Storedwastedsurroundingsspreads

useful