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.
Practice questions with answers
A taste of the 81 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
Why can no real machine achieve 100% efficiency?
- •Friction destroys some of the input energy
- •Machines can only transfer half their input energy
- ✓Some energy is always dissipated to surroundings
- •Some of the energy is used up and disappears
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
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