Conservation of energy and efficiencyIB MYP Physics: Revision notes
Section 1
Conservation of energy
The law of conservation of energy says that energy cannot be created or destroyed. It can only be transferred from one store to another.
This means that the total energy before a change equals the total energy after it. When a ball rolls to a stop, its kinetic energy has not vanished; it has been transferred to the thermal store of the ball, the floor and the air.
Do not say energy is 'lost' or 'used up'. Say it is wasted, which means transferred to a store that is not useful.
Section 2
Wasted energy
In every device, some energy is transferred to a store we do not want. This is wasted energy. It usually ends up in the thermal store of the surroundings, because of friction, electrical resistance or sound.
The wasted energy spreads out (is dissipated) into the surroundings, so it is too spread out to be collected and used again.
Section 3
Calculating efficiency
Efficiency tells you how much of the total energy input is transferred usefully.
Efficiency = useful energy output ÷ total energy input
To give a percentage, multiply by 100.
Worked example: a motor takes in 200 J and transfers 150 J usefully. Efficiency = 150 ÷ 200 = 0.75, or 75%.
The wasted energy is 200 − 150 = 50 J.
The efficiency is never above 100%, because the useful output cannot be bigger than the input.
Check the answer: the useful output is always smaller than the input, so your answer must be less than 1, or less than 100%.
Section 4
Finding the useful output
Rearrange the equation to find missing values.
Worked example: a lamp has an efficiency of 40% and a total input of 500 J.
Useful output = 0.40 × 500 = 200 J. The wasted energy is 500 − 200 = 300 J.
If you are given the useful output (80 J) and the efficiency (20%), the total input = 80 ÷ 0.20 = 400 J.
Section 5
Improving efficiency
To improve efficiency, reduce the wasted energy.
- Lubrication (oil or grease) reduces friction between moving parts, so less energy is transferred to the thermal store.
- Insulation (loft insulation, double glazing, cavity walls) reduces the thermal energy transferred from buildings.
- Better design, such as streamlined shapes, reduces air resistance.
More efficient devices waste less energy, so they cost less to run.
Must Know
- Energy cannot be created or destroyed, only transferred
- Wasted energy spreads out and ends up in the thermal store of the surroundings
- Efficiency = useful energy output ÷ total energy input (× 100 for a percentage)
- Efficiency is never 100% or more
- Lubrication and insulation reduce wasted energy
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Conservation of energy and efficiency
- A school in Seoul is replacing its filament lamps with LED lamps. Each second a filament lamp takes in 60 J of electrical energy and gives out 3 J as light. An LED lamp takes in 8 J each second and gives out 6 J as light.Show that the LED lamp is more efficient than the filament lamp.2 marks
- A motor in a warehouse in Lagos lifts boxes. Each second the motor takes in 500 J of electrical energy and the boxes gain 350 J of gravitational potential energy. The rest of the energy is wasted because of friction in the bearings.Suggest one way to improve the efficiency of the motor and explain how it works.2 marks
- A student investigates which material is the best insulator for a beaker of hot water. For each test she starts with water at 80 °C, wraps the beaker in a different material and measures the water temperature after 10 minutes. The results are: no insulation 62 °C, bubble wrap 71 °C, cotton wool 73 °C and aluminium foil 66 °C.Identify the independent variable and the dependent variable, and state one control variable.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).