Energy
What's covered
Key facts
A device with efficiency 1.0 (100%) would transfer all input energy into useful output with no waste energy.
Doubling the speed of an object quadruples its kinetic energy, because v is squared.
The main disadvantage of wind and solar versus fossil fuels is that they are intermittent — they cannot generate on demand.
Energy cannot be created or destroyed, only transferred between stores.
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 is the energy needed to raise the temperature of 1 kg of a substance by 1 °C.
Black matt surfaces emit and absorb infrared radiation better than white shiny surfaces, which reflect.
A machine with efficiency greater than 1.0 (more than 100%) is impossible — it would create energy.
Elastic potential energy stored in a spring: Ee = ½ke², with k = spring constant and e = extension.
Nuclear power is low-carbon because reactors don't burn fuel and emit no CO₂ during generation.
Sample questions
A taste of the 96 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 kinetic energy?
- •KE = ½m²v
- ✓KE = ½mv²
- •KE = mv
- •KE = mv²
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
What does specific heat capacity tell us about a material?
- ✓Energy to raise 1 kg of the material by 1°C
- •The energy released when 1 kg of the material cools by 1°C per second
- •The maximum temperature the material can reach before melting
- •The rate at which the material conducts thermal energy
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