1. Learning Objective Describe and calculate efficiency To be able to:
Key Words:
Input, output, useful, wasted

2. Exam question Thursday, 23 May 2019 Efficiency
Date and title in books. You need a pen, pencil & ruler.
Starter Activity – 5 minutes
Exam question

3. Learning Outcomes Recall the efficiency equations
Target 1-9
Recall the efficiency equations
(F/H)
Target 4-9
Use the equations to calculate efficiency in decimals and percentages
(H)
Target 5-9
Rearrange the equations to calculate useful and total input energy/power
Extension: Application of Knowledge
Target 8/9
Describe ways to increase efficiency (HT only)
Learning Outcomes

4. The more energy that is used usefully, the better the efficiency.
Wasted Energy
“ooh its freezing in here, put the light on” said no person, ever!
When we put the light on, we want light energy. The thermal energy produced is waste energy.
The more energy that is used usefully, the better the efficiency.

5. Calculating Efficiency
Efficiency = Useful Energy ÷ Total Energy (x 100)
Efficiency = Useful Power ÷ Total Power (x 100)
Efficiency is given as a decimal or as a percentage.
The higher the efficiency, the more energy the appliance is using usefully.

6. Calculating Efficiency
Efficiency = (Useful Energy ÷ Total Energy) x 100
Efficiency is given as a percentage.
Example: A motor converts 2000J of energy into 400J of useful energy. Calculate its efficiency.
Efficiency = (400/2000) x100 = 20%
So 80% of its energy is wasted.

7. Your Task You have been given some questions. Answer them in your book
Remember to label them properly and show your working out.

8. Extension: Calculating Efficiency
Two light bulbs use 90J of energy.
Light bulb A gives out 9J of light
Light bulb B gives out 45J of light.
Calculate the efficiency of both bulbs and identify which you think is an energy saving light bulb.

9. HT Only Question
On cold days, some of the energy transferred from a hot car engine is used to warm the air inside the car. This is a useful energy transfer.
What effect, if any, does this energy transfer have on the overall efficiency of the car engine?
Select the correct answer.
decreases the efficiency
does not change the efficiency
increases the efficiency
Give a reason for your answer.

10. Answer (d) increases the efficiency less (input) energy is wasted or
accept some of the energy that would have been wasted is (usefully) used or
more (input) energy is usefully used
accept heat for energy