Energy stores and transfersIB MYP Sciences: Revision notes
Section 1
Energy stores
Energy is measured in joules (J). We think of energy as being held in stores. A store only changes when energy is transferred in or out of it.
- Kinetic: in anything that is moving
- Gravitational potential: in anything that is lifted above the ground
- Elastic potential: in a stretched, squashed or twisted object such as a spring
- Chemical: in fuel, food and batteries
- Thermal: in the particles of a hot object (they move and vibrate more)
- Nuclear: in the nucleus of atoms
- Electrostatic: between electric charges that attract or repel
- Magnetic: between magnets or magnetic materials that attract or repel
Say that energy is transferred between stores. Do not say that energy is 'used up' or 'made'.
Section 2
Transfer pathways
Energy moves from one store to another by a pathway:
- Mechanical: a force moves an object (pushing, lifting, stretching)
- Electrical: a current in a circuit
- Heating: thermal energy moves from a hotter to a colder place because of a temperature difference
- Radiation: energy carried by waves such as light, infrared or sound waves
For example, a kettle transfers energy electrically to its element, which transfers energy by heating to the water.
Heat is not a store. Thermal energy is stored, and heating is the pathway.
Section 3
Describing energy changes
To describe a change, name the store that decreases and the store that increases, then name the pathway.
- A ball falls: gravitational potential store decreases, kinetic store increases (mechanical)
- A torch: chemical store of the battery decreases, energy is transferred electrically, then by radiation (light) to the surroundings
- A wound-up toy: elastic store decreases, kinetic store increases
In many changes some energy ends up in the thermal store of the surroundings because of friction or air resistance.
Section 4
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, so the total energy stays the same.
Useful energy is the part transferred to where we want it. The rest is wasted energy. It usually ends up in the thermal store of the surroundings, where it spreads out (dissipates) and is hard to use again. Wasted energy is not destroyed.
'Energy is lost' is wrong. Say it is wasted, or transferred to the surroundings.
Section 5
Sankey diagrams
A Sankey diagram shows energy transfers using arrows. The width of each arrow is drawn to scale with the amount of energy in joules.
- The input arrow comes in from the left
- The useful output arrow carries on straight
- Wasted energy arrows bend away, often downwards
Worked example: a motor takes in 200 J electrically and transfers 150 J as kinetic energy. The wasted energy is 200 - 150 = 50 J (as thermal). The useful arrow is three times as wide as the wasted arrow. The outputs (150 + 50) add up to the input, which shows energy is conserved.
Must know
- Eight stores: kinetic, gravitational potential, elastic, chemical, thermal, nuclear, electrostatic, magnetic
- Four pathways: mechanical, electrical, heating, radiation
- Energy is measured in joules and is always conserved
- Wasted energy dissipates into the thermal store of the surroundings
- In a Sankey diagram arrow width shows the amount of energy, and outputs add up to the input
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Energy stores and transfers
- A child in Nairobi winds up a clockwork toy car, puts it on the floor and lets it go. The toy speeds across the floor, then gradually slows down and stops.Describe the energy transfers that happen from the moment the spring is released until the car stops.2 marks
- A hydroelectric power station in Norway has a reservoir high up a mountain. Water runs down large pipes to turbines at the bottom, and the turbines turn a generator that supplies electricity to nearby towns.Describe the energy transfers from the water at the top of the pipes to the electricity leaving the generator.2 marks
- A class in Melbourne compares three bouncing balls, X, Y and Z. Each ball is dropped from a height of 1.00 m onto the same hard floor and the height it rebounds to is measured. The students predict that a ball that rebounds lower transfers more energy to the surroundings. The mean rebound heights were 0.80 m for ball X, 0.55 m for ball Y and 0.30 m for ball Z.Identify the independent variable, the dependent variable and one control variable in this investigation.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).