Energy
What's covered
Key facts
Efficiency
A machine with efficiency greater than 1.0 (more than 100%) is impossible — it would create energy.
Energy Calculations
Doubling the speed of an object quadruples its kinetic energy, because v is squared.
Energy Resources
The main disadvantage of wind and solar versus fossil fuels is that they are intermittent — they cannot generate on demand.
Energy Stores and Transfers
Energy cannot be created or destroyed, only transferred between stores.
Power
A higher-power device uses more electrical energy per hour than a lower-power one, so costs more per hour at the same price per kWh.
Specific Heat Capacity
Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C.
Thermal Energy Transfers
Black matt surfaces emit and absorb infrared radiation better than white shiny surfaces, which reflect.
Efficiency
As a decimal, efficiency = useful output energy transfer ÷ total input energy transfer.
Energy Calculations
Elastic potential energy stored in a spring: Ee = ½ke², with k = spring constant and e = extension.
Energy Resources
The main advantage of nuclear power over burning fossil fuels is that it produces large amounts of electricity without directly emitting CO₂ during generation.
Sample questions
A taste of the 83 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
Why can no real machine achieve 100% efficiency?
- •All machines lose mass during operation
- •Friction can be eliminated only with lubricant
- •Machines need batteries that drain energy
- ✓Some energy is always dissipated to surroundings
What is the equation for gravitational potential energy?
- •GPE = ½mgh
- •GPE = mg/h
- ✓GPE = mgh
- •GPE = mh
What is the main advantage of nuclear power over burning fossil fuels?
- ✓It produces large amounts of electricity without directly emitting CO₂ during generation
- •Nuclear fuel is a renewable resource that replenishes naturally within the rocks of the Earth's crust over time
- •Nuclear power plants are cheaper to build and faster to commission than coal-fired power stations of similar output
- •Nuclear power produces no waste at all during electricity generation — only harmless water vapour is released
Which energy store increases when an object is lifted to a greater height?
- •Chemical energy store (bonds)
- •Elastic potential energy store (stretch)
- ✓Gravitational potential energy store
- •Kinetic energy store (movement)
A device transfers 6000 J in 2 minutes. What is its power in watts?
- •12 000 W
- •3000 W
- ✓50 W
- •500 W
Which equation correctly describes specific heat capacity?
- •E = ½mc²Δθ where the ½ accounts for average particle KE
- •E = m/(cΔθ) — energy divided by capacity × temperature change
- ✓E = mcΔθ (E in J, m in kg, c in J/kg°C)
- •E = mcΔθ where m is moles and c is concentration in mol/dm³
Try it for four weeks. Free.
One school. Unlimited classes. No card limit. No teacher limit. If your students aren't practising daily by the end of the trial, you owe us nothing.