P1: Energy Stores and Transfers
Energy stores, systems and energy transfers — understanding the eight energy stores, four pathways of transfer, conservation of energy, dissipation and Sankey diagrams.
📋 Key Definitions
Energy store: A way in which energy can be stored within a system. Energy is never created or destroyed — it is only transferred between stores.
System: An object or group of objects. You define the system to describe what is happening with energy.
Conservation of energy: Energy cannot be created or destroyed, only transferred usefully, stored or dissipated (wasted).
Dissipation: Energy that is transferred to a thermal store of the surroundings that is not useful. This is often called wasted energy.
⚡ The Eight Energy Stores
| Energy Store | Description | Example |
|---|---|---|
| Kinetic | Energy stored in a moving object | A car travelling along a road |
| Gravitational potential | Energy stored in an object raised above the ground | A book on a high shelf |
| Elastic potential | Energy stored in a stretched or compressed object | A stretched spring or squashed ball |
| Thermal (internal) | Energy stored in a hot object due to particle vibrations | Hot water in a kettle |
| Chemical | Energy stored in chemical bonds between atoms | Fuel, food, batteries |
| Nuclear | Energy stored in the nucleus of an atom | Uranium in a nuclear reactor |
| Magnetic | Energy stored in two separated magnets that attract or repel | Two magnets held apart |
| Electrostatic | Energy stored in two separated charges that attract or repel | Two opposite charges held apart |
🔄 Energy Transfer Pathways
Energy is transferred between stores by four pathways: mechanically, electrically, by heating (particles), and by heating (radiation).
| Pathway | How it works | Example |
|---|---|---|
| Mechanically | A force doing work on an object | Pushing a box along the floor — chemical store of person → kinetic store of box + thermal store (friction) |
| Electrically | Charge doing work (current flowing through a component) | Current through a lamp — chemical store of battery → thermal store of lamp + light |
| Heating by particles | Energy transferred from a hotter object to a colder one via conduction or convection | A kettle heating water — thermal store of element → thermal store of water |
| Heating by radiation | Energy transferred by electromagnetic waves (infrared, light) | Sun warming the Earth — nuclear store of Sun → thermal store of Earth via radiation |
⚖️ Conservation of Energy
Conservation of energy principle: Energy can be transferred usefully, stored or dissipated, but it can never be created or destroyed. The total energy input always equals the total energy output.
This means that in any process:
Total energy in = Total energy out = Useful energy + Wasted energy
Wasted energy is energy transferred to stores where it is not wanted or needed. Most wasted energy ends up in thermal stores of the surroundings.
💨 Dissipation
Dissipation: When energy is transferred to the thermal store of the surroundings, it cannot easily be used again. This wasted energy is called dissipated energy.
Common causes of dissipation:
- Friction between moving parts → thermal store of surroundings
- Air resistance (drag) → thermal store of surroundings
- Sound from vibrating objects → thermal store of surroundings
- Electrical resistance in wires → thermal store of surroundings
Dissipated energy spreads out and becomes less useful — it cannot be recovered easily.
📊 Sankey Diagrams
Sankey diagrams: Show the energy transfers in a system. The width of each arrow is proportional to the amount of energy it represents. Useful energy goes one way; wasted energy goes another.
- The total width of arrows going in = the total width of arrows going out
- Useful output arrows point in the main direction
- Wasted output arrows point downwards or away
- The thicker the arrow, the more energy it represents
Example — Reading a Sankey Diagram
An electric kettle has 2000 J of electrical energy input. The Sankey diagram shows 1600 J as useful thermal energy in the water and 400 J as wasted energy (sound and heat to surroundings).
Total in = 2000 J. Useful out = 1600 J. Wasted = 400 J. 1600 + 400 = 2000 J ✓
📝 Worked Examples
Worked Example 1 — Identifying Energy Stores
Question: A ball is thrown upwards. Describe the energy stores involved as it rises and falls.
Solution:
- When first thrown: kinetic store (moving) is at its maximum
- As it rises: kinetic store decreases, gravitational potential store increases
- At the highest point: kinetic store = 0 (momentarily stopped), gravitational potential store is at its maximum
- As it falls: gravitational potential store decreases, kinetic store increases
- Some energy is dissipated to thermal stores due to air resistance throughout
Worked Example 2 — Energy Transfers in a System
Question: Describe the energy transfers when a car brakes to a stop.
Solution:
- Before braking: energy is in the kinetic store of the car
- During braking: friction between brake pads and discs does work mechanically
- Energy transferred: kinetic store of car → thermal store of brakes and surroundings
- Some energy also transferred to thermal store via air resistance (sound)
- After stopping: kinetic store = 0, energy has been dissipated to thermal stores
Worked Example 3 — Energy Stores in a Rollercoaster
Question: At point A on a rollercoaster the carriage is at the top of a hill moving slowly. At point B it is at the bottom moving fast. Compare the energy stores.
Solution:
- Point A (top): mostly gravitational potential store, small kinetic store
- Point B (bottom): mostly kinetic store, gravitational potential store has decreased
- Energy transferred: gravitational potential store → kinetic store (mechanically, by gravitational force doing work)
- Some energy dissipated to thermal store due to friction and air resistance
Worked Example 4 — Describing Energy Pathways
Question: A torch is switched on. Describe the energy transfers and pathways.
Solution:
- Energy starts in the chemical store of the battery
- Transferred electrically by the current through the wires
- At the bulb: chemical store of battery → thermal store of bulb filament (by electrical heating)
- Thermal store of filament → light radiation emitted (useful) + thermal store of surroundings (wasted)
- Some energy also dissipated in wires due to resistance
Worked Example 5 — Sankey Diagram Calculations
Question: A motor takes in 500 J of electrical energy. It transfers 300 J to kinetic energy and the rest is wasted as heat and sound. Draw a Sankey diagram description and state the wasted energy.
Solution:
- Total energy in = 500 J
- Useful energy out (kinetic) = 300 J
- Wasted energy = 500 − 300 = 200 J
- Sankey diagram: one input arrow of width 5 units; two output arrows — useful 3 units wide (kinetic) and wasted 2 units wide (thermal + sound)
Worked Example 6 — Complete Energy Transfer Description
Question: A person uses a bow to shoot an arrow. Describe all energy stores and transfers.
Solution:
- Initially: chemical store of the person
- Person pulls bow string: chemical store → elastic potential store of the bow (transferred mechanically, force doing work)
- String released: elastic potential store → kinetic store of the arrow (transferred mechanically)
- As arrow rises: kinetic store → gravitational potential store (mechanically, by gravity)
- Throughout: some energy dissipated to thermal stores (friction, air resistance, sound)
❓ Practice Questions
Q1: Foundation Name the eight energy stores and give one example of each.
Q2: Foundation A ball falls from a height of 2 m. Describe the energy transfers that take place.
Q3: Foundation State the conservation of energy principle.
Q4: Higher An electric heater takes in 1500 J of energy. 1200 J is transferred as useful heat to the room. The rest is wasted as light and sound. Calculate the wasted energy and describe what a Sankey diagram would look like.
Q5: Higher A car of mass 1000 kg is moving at 20 m/s. The driver brakes. Explain where the energy from the kinetic store goes and why it cannot all be transferred usefully.
Q6: Higher Describe the energy transfers when a phone is being charged, including the pathways.
✅ Answers
- Kinetic — moving car; gravitational potential — book on shelf; elastic potential — stretched spring; thermal — hot cup of tea; chemical — food/fuel; nuclear — uranium in reactor; magnetic — two repelling magnets held apart; electrostatic — two opposite charges separated.
- At the top: gravitational potential store is at its maximum, kinetic store is zero. As the ball falls: gravitational potential store decreases, kinetic store increases. Energy is transferred mechanically (by gravitational force doing work). Some energy is dissipated to the thermal store of the surroundings due to air resistance.
- Energy cannot be created or destroyed. It can only be transferred usefully, stored, or dissipated. The total energy input always equals the total energy output.
- Wasted energy = 1500 − 1200 = 300 J. The Sankey diagram would have one input arrow (representing 1500 J), one useful output arrow (representing 1200 J, 80% of the width) going straight ahead, and one wasted output arrow (representing 300 J, 20% of the width) pointing downwards.
- The kinetic store energy is transferred to the thermal store of the brakes and tyres (by friction doing work mechanically) and to the thermal store of the surroundings (by air resistance). Not all energy can be transferred usefully because friction and air resistance always cause some dissipation to the thermal store of the surroundings — this energy cannot be recovered.
- Chemical store of the power station/battery → transferred electrically through the charger cable → chemical store of the phone battery (useful, storing energy). Some energy is dissipated to the thermal store of the charger and cable (due to electrical resistance) and to the thermal store of the phone battery (resistance during charging).
🎯 Exam Tips
- Always name the energy store, not just the type of energy. Say "kinetic store" not "kinetic energy".
- When describing transfers, state: the store energy goes FROM → the store it goes TO → and the PATHWAY (mechanically, electrically, heating).
- Sankey diagrams: the width of the arrow shows the amount of energy. Total in must always equal total out.
- Exam questions often ask "where does the wasted energy go?" — it is transferred to the thermal store of the surroundings.
- Never say energy is "lost" — it is dissipated or transferred to thermal stores.
- In descriptions of falling objects, always mention dissipation due to air resistance for full marks.
🔢 Maths Skills
Mathematical Skills
Interpreting Sankey diagrams requires you to read proportional arrow widths and calculate wasted energy. You must be able to add and subtract energy values, work with proportions, and calculate percentages from Sankey diagram data.
Maths Example
A Sankey diagram shows an input arrow of width 10 units. The useful output arrow is 6 units wide. What percentage of the input energy is wasted?
Wasted width = 10 − 6 = 4 units. Wasted percentage = (4 ÷ 10) × 100 = 40%. So 40% of the input energy is dissipated to unwanted stores.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Energy is "lost" in transfers Correct: Energy is never lost — it is dissipated (transferred to the thermal store of the surroundings where it is less useful)
2. Wrong: An object has "kinetic energy" Correct: An object has energy in its "kinetic store" — always name the store, not just the type of energy
3. Wrong: Energy is "used up" when a device works Correct: Energy is transferred from one store to another — the total amount of energy stays the same (conservation of energy)
✍️ 6-Mark Question
Extended Answer
6 marks: Describe the energy transfers that take place when a pendulum swings from its highest point to its lowest point and back again. Explain why the pendulum eventually stops.
At the highest point, the pendulum has maximum energy in its gravitational potential store and zero energy in its kinetic store (it is momentarily stationary). As it swings down, energy is transferred from the gravitational potential store to the kinetic store (mechanically, by the force of gravity doing work). At the lowest point, the kinetic store is at its maximum and the gravitational potential store is at its minimum. As the pendulum swings back up, energy is transferred from the kinetic store back to the gravitational potential store. Each swing, some energy is dissipated to the thermal store of the surroundings due to air resistance and friction at the pivot. Because energy is continuously dissipated, less energy is available in the kinetic and gravitational potential stores each swing. Eventually, all the energy has been dissipated to the thermal store of the surroundings and the pendulum stops.
Mark scheme: 1 mark — identifying gravitational potential store at top; 1 mark — transfer to kinetic store as it falls; 1 mark — kinetic store maximum at bottom; 1 mark — transfer back to gravitational potential store on the upswing; 1 mark — dissipation to thermal store due to friction/air resistance; 1 mark — explanation that energy dissipation causes the pendulum to stop
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student investigates two electric heaters. Heater A takes in 2000 J of electrical energy and transfers 1600 J to the thermal store of the room and 400 J is wasted. Heater B takes in 2000 J and transfers 1200 J to the thermal store of the room and 800 J is wasted.
(a) Calculate the percentage of useful energy for each heater.
(b) Draw a description of the Sankey diagram for Heater B, stating the proportions of each arrow.
(c) A student claims "Heater A wastes less energy than Heater B, so Heater A is always the better choice." Evaluate this claim.
Answers: (a) Heater A: (1600 ÷ 2000) × 100 = 80%. Heater B: (1200 ÷ 2000) × 100 = 60%. (b) One input arrow of width 10 units; useful output arrow 6 units wide going straight ahead (thermal store of room); wasted output arrow 4 units wide pointing down (thermal store of surroundings — sound, light). (c) The claim is partially correct — Heater A is more efficient (80% vs 60%) and wastes less energy. However, other factors may matter: Heater B may cost less to buy, may be more portable, or may be designed for a different room size. Also, if Heater B has a higher power rating it could heat a room faster despite lower efficiency. The claim is too simplistic — efficiency is important but should not be the only factor considered.
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