P5.2 Power and efficiency
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P5.2 Power and efficiency
Mathematical learning outcome:
PM5.2i recall and apply: efficiency = useful output energy transfer (J) / input energy transfer (J)
Assessable content statements:
P5.2a describe, with examples, the process by which energy is dissipated, so that it is stored in less useful ways
P5.2b describe how, in different domestic devices, energy is transferred from batteries or the a.c. from the mains
P5.2c describe, with examples, the relationship between the power ratings for domestic electrical appliances and how this is linked to the changes in stored energy when they are in use
P5.2d calculate energy efficiency for any energy transfer
P5.2e describe ways to increase efficiency
P5.2f explain ways of reducing unwanted energy transfer
P5.2g describe how the rate of cooling is effected by the thickness and thermal conductivity of its walls (qualitative only)
A variety of engaging and exciting quizzes to test learner’s knowledge on the topic.
There are premade quizzes and learners compare their scores with others.
When approaching P5.2b a complete circuit comprising of a bulb connected to a battery can be shown. As the light comes on ask the learners where the energy has come from. The same can be done for a kettle again asking learners where the energy has come from. Get learners to discuss how a battery and the mains are both similar.
To introduce P5.2c use two different kettles, each with a different power rating, fill each one with a litre of water. Give one learner a stopwatch and time how long it takes to boil the water in each kettle. Get learners to discuss why the kettle with a higher power rating boiled quicker. Get learners to write on a post it note another appliance, which will work differently depending on their power rating.
Show learners a number of energy efficiency ratings of different appliances and get them to make suggestions of what they show. Once learners have identified it as efficiency introduce the equation for calculations.
Learners can be provided with the energy efficiency rating of their school and asked to think of ideas on how they can improve the energy efficiency of their school. Ideas of lighting, water, windows etc. Learners can either present their findings or make a poster. An alternative approach is to show learners a series of different appliances and ask them how they can make it more efficient.
A practical approach to P5.2g is to look at the rate of cooling. Learners are provided with a beaker and paper towels. Learners fill the beakers up with hot water and then wrap around the beaker different amounts of paper towels e.g. 1,2,3, etc. Learners record the cooling rate of each beaker to see how it is affected.
Common misconceptions or difficulties students may have
Learners may muddle up power and energy, as they believe it to be the same. To demonstrate the difference get learners to transfer 5 balls from 2 trays placed 10 meters apart. On the first run get learners to pick the balls and walk across and drop the ball in to the second bucket. After a while get learners to pick up the balls and run to the second bucket. Ask learners which one was quicker. Get the idea across of the balls representing energy moving to the appliance and we were transferring the energy (power), the quicker we move the more power it has and the quicker the energy is transferred.
Learners may find it difficult to calculate the amount of energy transferred from domestic mains electricity using the formula. Practice is the best method. Get learners to practice as many questions as possible.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course
The mathematical skills and understanding of key concepts of efficiency are useful for topics including ‘work done’ (P5.1). The knowledge and understanding of ‘Power & efficiency’ builds on the work covered previously in sections P1.2 c,d,e (heat) and P5.1 (work done).
Approaches to teaching the content
Learners should be encouraged to explain how devices can be made more efficient. This could be taken further as higher ability groups should be able to draw sankey diagrams to illustrated where energy is wasted.
Learners to be paired or grouped and giving the scenario of designing an appliance for comet which has a grade A rating. By looking at the purpose of the device they will realise that the heat loss needs to be reduced, which can be done through insulation/ lubrication. Learners to present their ideas in a mini presentation (2 minutes).
A very good work booklet, which looks at electrical equipment and appliances and calculates the efficiency and power.
It even stretches to the cost of electricity.
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