Energy flashcards
13 cards on Energy: the key points, worked examples and the slips to watch. Swipe through them, or print them.
Card 1 of 13
Energy
Energy stores and transfers
What are the energy stores, and how does energy move between them?
Answer it in your head, then turn the card over.
Energy stores and transfers
Key points
- The stores: kinetic, gravitational potential, elastic potential, thermal, chemical, nuclear, magnetic and electrostatic.
- Energy moves between stores by a force doing work, by heating, by an electric current or by waves.
- A pendulum swinging down moves energy from its gravitational store to its kinetic store, so it is fastest at the bottom.
- A stretched or squashed spring holds energy in its elastic potential store.
Watch out
Light and sound are ways energy is transferred, not energy stores.
Energy
Conservation of energy
What does the law of conservation of energy say, and where does wasted energy go?
Answer it in your head, then turn the card over.
Conservation of energy
Key points
- Energy can be transferred usefully, stored or dissipated, but it can never be created or destroyed.
- In a closed system the total energy stays the same.
- Dissipated energy spreads into the surroundings, usually warming them, and is no longer useful.
A motor takes in 120 J, 30 J usefully
Dissipated = 120 − 30 = 90 J, mostly heating the motor and the air
Watch out
Energy is never used up: the wasted part is still there, spread out as a slight warming.
Energy
Specific heat capacity
What is specific heat capacity, and how do you work out the energy needed to warm something?
Answer it in your head, then turn the card over.
Specific heat capacity
Key points
- Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C.
- Water's is high, so it takes a lot of energy to warm up.
- Required practical: heat a metal block, measure the energy supplied and the temperature rise, then work out c.
Formula
ΔE = mcΔθ: energy (J) = mass (kg) × s.h.c. (J/kg °C) × temperature change (°C)
Warming 2 kg of water by 10 °C
c for water = 4200 J/kg °C
ΔE = 2 × 4200 × 10 = 84,000 J
Watch out
Use the temperature change, not the final temperature: 20 °C to 30 °C is a rise of 10 °C.
Energy
Kinetic energy
How do you calculate the energy in a moving object's kinetic store?
Answer it in your head, then turn the card over.
Kinetic energy
Key points
- Kinetic energy depends on the mass and the speed.
- Only the speed is squared, so doubling the speed makes the kinetic energy four times bigger.
- Mass in kilograms and speed in m/s give the energy in joules.
Formula
Ek = ½mv²: kinetic energy (J) = 0.5 × mass (kg) × speed² (m/s)
A 4 kg ball moving at 5 m/s
5² = 25
Ek = 0.5 × 4 × 25 = 50 J
Watch out
Square the speed and halve the mass: forgetting either is the classic slip.
Energy
Gravitational potential energy
How do you calculate the energy an object gains when it is lifted?
Answer it in your head, then turn the card over.
Gravitational potential energy
Key points
- Lifting an object fills its gravitational potential store.
- On Earth g is about 9.8 N/kg: use the value the question gives you.
- As it falls, the energy moves to its kinetic store. Ignoring air resistance, the kinetic energy gained equals the GPE lost.
Formula
Ep = mgh: GPE (J) = mass (kg) × g (N/kg) × height (m)
A 3 kg box lifted 2 m
Ep = 3 × 10 × 2 = 60 J
Dropped, it reaches the floor with 60 J of kinetic energy, if there is no air resistance.
Watch out
Put the mass in kilograms and the height in metres: grams and centimetres give answers far too big.
Energy
Work done
What is work done, and how do you calculate it?
Answer it in your head, then turn the card over.
Work done
Key points
- Work is done when a force moves an object: it is energy transferred, measured in joules.
- 1 J of work is done when a 1 N force moves something 1 m. A joule is a newton metre.
- Work done against friction transfers energy to thermal stores, warming the surfaces.
Formula
W = Fs: work done (J) = force (N) × distance moved along the force (m)
Pushing a box 4 m with a 50 N force
W = 50 × 4 = 200 J
Watch out
Use the distance moved in the direction of the force, in metres.
Energy
Power
What is power, and how do you calculate it?
Answer it in your head, then turn the card over.
Power
Key points
- Power is the rate at which energy is transferred, or work is done.
- It is measured in watts: 1 watt is 1 joule per second.
- A more powerful kettle needs the same energy to boil the water, but transfers it in less time.
Formulae
P = E ÷ t: power (W) = energy transferred (J) ÷ time (s)
P = W ÷ t: power (W) = work done (J) ÷ time (s)
A 2 kW kettle on for 3 minutes
2 kW = 2000 W and 3 minutes = 180 s
E = P × t = 2000 × 180 = 360,000 J
Watch out
Change kilowatts to watts and minutes to seconds first: 2 × 3 = 6 J is far too small.
Energy
Efficiency
How do you calculate the efficiency of a device?
Answer it in your head, then turn the card over.
Efficiency
Key points
- Efficiency is the fraction of the input energy that is transferred usefully.
- Multiply by 100 to give it as a percentage.
- No device is more than 100% efficient: it cannot give out more energy than goes in.
Formulae
efficiency = useful output energy ÷ total input energy
efficiency = useful power output ÷ total power input
400 J in, 100 J out usefully
Efficiency = 100 ÷ 400 = 0.25
0.25 × 100 = 25%
Watch out
Useful over total, never total over useful: an efficiency above 1 means the division is upside down.
Energy
Wasting less energy
How can you reduce the energy a device wastes, and so raise its efficiency?
Answer it in your head, then turn the card over.
Wasting less energy
Key points
- Wasted energy is dissipated, usually by heating the surroundings.
- Lubricate moving parts to reduce friction, so less energy goes into thermal stores.
- Insulate to reduce heating losses, and streamline to reduce air resistance.
- Less energy wasted means more of the input becomes useful output.
Watch out
Insulation and lubrication slow down the waste: they never make a device more than 100% efficient.
Energy
Conduction and convection
How is energy transferred by conduction and by convection?
Answer it in your head, then turn the card over.
Conduction and convection
Key points
- Conduction: heated particles vibrate more and pass energy to their neighbours as they collide. It happens mainly in solids.
- Metals conduct best, because their free electrons carry energy too.
- Convection: in a liquid or gas, heated fluid expands, becomes less dense and rises, and cooler, denser fluid sinks.
- This makes a convection current. It cannot happen in a solid, where the particles cannot move about.
Watch out
Say why warm fluid rises: it is less dense. "Heat rises" earns nothing.
Energy
Infrared radiation and insulation
How does infrared radiation transfer energy, and how do homes reduce heat loss?
Answer it in your head, then turn the card over.
Infrared radiation and insulation
Key points
- Infrared radiation needs no particles, so it is the only transfer that can cross a vacuum, like space.
- Dark matt surfaces are the best emitters and absorbers. Light, shiny surfaces reflect infrared.
- Loft insulation, cavity wall foam and double glazing trap air, a poor conductor, so less energy escapes.
- Thicker walls with a lower thermal conductivity make a house cool more slowly.
Watch out
A shiny surface is a poor emitter as well as a poor absorber, which is why a silvery flask keeps drinks hot.
Energy
Renewable and non-renewable resources
Which energy resources are renewable, which are not, and what is the difference?
Answer it in your head, then turn the card over.
Renewable and non-renewable resources
Key points
- A renewable resource is replenished as it is used, so it will not run out.
- Renewable: wind, solar, hydroelectric, tidal, wave, geothermal and bio-fuel.
- Non-renewable: coal, oil, gas and nuclear fuel. Once used, they are gone.
- Resources are used for transport, heating and generating electricity.
Watch out
Renewable is about supply, not reliability: wind is renewable but only works when the wind blows.
Energy
Comparing energy resources
What are the advantages and disadvantages of the main energy resources?
Answer it in your head, then turn the card over.
Comparing energy resources
Key points
- Coal, oil and gas are reliable, but burning them releases carbon dioxide, a greenhouse gas that adds to global warming.
- Nuclear power is reliable and releases no carbon dioxide, but leaves radioactive waste that must be stored safely.
- Wind and solar are renewable but depend on the weather, so they are less reliable.
- Hydroelectric and tidal schemes are dependable, but flood or change habitats.
Watch out
Nuclear fuel is non-renewable even though it releases no carbon dioxide as it runs.
Print these cards
Four cards to an A4 sheet, each the size of a standard A6 revision card.
One-sided
Each card prints on one face: the question at the top, the notes under it.
Two-sided
Questions on the front, notes on the back. Print double-sided and flip on the long edge, then cut along the lines.
Put it into practice
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