Example 1: efficiency
A kettle is given 200 kJ of energy, and 160 kJ heats the water. Find its efficiency.
- Efficiency = useful ÷ total × 100.
- 160 ÷ 200 × 100 = 80%.
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IB DemystifiedMYP Sciences
Energy stores and transfers
Everything that happens, from a ball bouncing to a light switching on, involves energy moving from one store to another. Energy is never used up, but it can be wasted, and choosing efficient machines saves money and fuel.
Recommended for MYP 1 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Energy stores and transfers
- Energy in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- identify the main energy stores
- describe how energy is transferred between stores
- apply the law of conservation of energy
- identify useful and wasted energy using Sankey diagrams
- calculate simple efficiencies
- discuss energy-saving choices at home
Before you start
You will use these skills. If any feel shaky, review them first.
- forces and motion (see Forces and their effects)
- simple circuits
- percentages
Key vocabulary
- Energy store
- A way energy is held, such as kinetic, gravitational potential, elastic, thermal or chemical.
- Energy transfer
- Energy moving between stores: mechanically, electrically, by heating or by radiation.
- Joule (J)
- The unit of energy; 1 kJ = 1000 J.
- Conservation of energy
- Energy cannot be created or destroyed, only transferred.
- Wasted energy
- Energy transferred to stores that are not useful, often heat.
- Efficiency
- How much of the energy put in ends up as useful energy.
Understanding the ideas
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What is it?
Energy is stored in different ways: moving objects have kinetic energy, raised objects have gravitational potential energy, stretched springs have elastic energy, hot objects have thermal energy, and food, fuel and batteries have chemical energy.
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Why does it happen?
Energy is transferred between stores mechanically (by forces), electrically (by currents), by heating, and by radiation such as light and sound. The total amount of energy never changes, but some always ends up in stores we did not want, usually as heat.
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How do we know?
We measure energy in joules. Sankey diagrams show where energy goes, and efficiency tells us what fraction is useful. James Joule showed in the 1840s that mechanical work could be turned into heat, helping to establish conservation of energy.
-
Why does it matter?
Using efficient devices such as LED bulbs and better stoves saves money, reduces fuel use and cuts pollution.
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What does it connect to?
Energy links to forces, circuits, heating and electricity generation in physics, to food and respiration in biology, and to fuels in chemistry.
Energy stores and transfers
- Stores: kinetic, gravitational potential, elastic, thermal, chemical, nuclear, magnetic and electrostatic.
- Transfers: mechanically (forces), electrically (current), by heating, and by radiation (light, sound).
- Conservation: total energy in = total energy out.
- Efficiency = useful energy out ÷ total energy in × 100%.
- Reducing waste: lubricating moving parts, insulating, and using efficient devices.
Energy in the real world
LED bulbs have replaced filament bulbs in many homes, cutting electricity bills. Solar cookers are used in sunny regions to cook without wood. Hydroelectric dams transfer the energy of falling water into electricity.
Worked examples
Example 2: wasted energy
A motor receives 1000 J and gives 700 J of kinetic energy. How much is wasted?
- Energy is conserved: 1000 − 700 = 300 J.
- This is mostly wasted as heat and sound.
Assessment tips
Energy questions often use Sankey diagrams and everyday examples. Expect to:
- Identify energy stores and transfers.
- Use conservation of energy to find wasted energy.
- Calculate efficiency as a percentage.
- Plan fair tests and analyse data.
Common mistakes: saying energy is used up or destroyed; mixing up stores and transfers; forgetting units (J or kJ); and calculating efficiency using the wasted energy.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: elastic band launchers
Partially guided investigation · about 40 minutes · pairs
- Research question
- How does the distance an elastic band is stretched affect how far it launches a folded paper pellet?
- Scientific background
- A stretched elastic band has an elastic store of energy. When released, energy is transferred to the kinetic store of the pellet. More stretch stores more energy.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Elastic band, ruler with a groove or two pegs, folded paper pellets of the same size, metre rule or tape measure, masking tape to mark the floor.
- Method
- Fix the elastic band between two pegs on a table edge.
- Pull the pellet back 2 cm, release it and measure how far it travels.
- Repeat for stretches of 4, 6, 8 and 10 cm.
- Do three launches for each stretch and find the mean distance.
Safety. Wear eye protection. Never aim at people; launch towards an empty area of floor. Do not overstretch the band.
Then evaluate: which energy stores and transfers happen during a launch, and where is energy wasted?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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