Types of Energy
Gravitational Potential Energy
State the Law of Conservation of Energy.
Energy cannot be created or destroyed, only transformed or transferred.
What is the formula for energy efficiency?
Efficiency% = (total input / useful output) × 100
In a Sankey diagram, what does arrow width represent?
Arrow width shows the amount of energy transferred (or energy per second if it’s power).
Name two renewable energy sources.
Any two: e.g., solar, wind, hydro, geothermal, wave/tidal, biomass.
Name 2 of the energy(s) present in a moving bicycle.
E.g., mechanical, kinetic, heat, sound
In a torch: chemical → electrical → ? (include wasted energy)
Chemical → electrical → light (+ thermal wasted).
A device takes in 800 J and produces 600 J useful energy.
Calculate its efficiency.
(600 / 800) x 100 = 75%
What do the sideways arrows and downwards arrows in a sankey diagram represent?
The horizontal arrow represents useful energy output. Downwards arrows represent wasted energy.
Why is solar energy described as intermittent?
It depends on sunlight — night time, clouds, seasons reduce output unpredictably.
Explain why thermal energy is considered kinetic at the particle level.
Thermal energy comes from random motion of particles; since kinetic energy = energy of motion, thermal energy is microscopic kinetic energy.
Why does energy appear to “disappear” in real systems?
It doesn’t disappear; it’s transferred to the surroundings, usually as thermal energy (and some sound), becoming spread out and less useful.
A 15% efficient light bulb takes in 4000 J.
How much energy is wasted?
4000J x 85% = 3400J wasted
If input energy is 500 J and useful output is 150 J, calculate efficiency.
150J / 500J x 100 = 30% efficiency.
Explain one environmental advantage and one disadvantage of coal.
Advantage: high energy output and reliable baseload, often cheap per kWh.
Disadvantage: high CO₂ emissions + air pollution; mining and environmental impacts.
A hot metal block and a moving tennis ball both involve kinetic energy. Explain how they are similar and how they are different.
Similar: both involve kinetic energy (motion).
Different: tennis ball = ordered macroscopic motion of the whole object; hot metal = mainly random microscopic motion of particles (thermal), with no net movement of the object.
A rollercoaster eventually stops due to friction.
Explain how conservation of energy still applies.
The coaster’s mechanical energy (GPE/KE) is gradually transformed into thermal energy in the wheels, track, and air (plus sound). Total energy stays the same.
A motor runs for 1 hour using 10,000 J of input energy and is 75% efficient.
How much energy is transferred to the surroundings?
75% useful → 25% wasted. 25% x 10,000J = 2,500J tranferred to surroundings.
Two machines have identical efficiency, but one produces far more useful energy per second.
Explain how this is possible.
Because efficiency is a ratio, not an amount. A machine can have the same efficiency but a much larger input energy per second (power), so it produces more useful energy per second too.
Even if wind farms produce enough energy annually, why can they not reliably power a country alone without storage?
Wind is variable and doesn’t always match demand. Without storage/backup, you can’t guarantee supply during low-wind periods or peaks.
A book rests on a table.
Using conservation of energy, explain where its gravitational potential energy originally came from and describe what happens to that energy if the book slides off slowly due to friction.
The GPE came from work done lifting the book (chemical energy in muscles → kinetic → GPE). If it slides off slowly with friction: GPE → kinetic (briefly) → mostly thermal energy in the book/table + some sound; energy spreads into surroundings.
Two identical balls are dropped — one lands on concrete, one on sand.
Both stop.
Explain how conservation of energy applies in both cases and why the outcomes differ.
In both cases: initial GPE becomes KE, then becomes other forms when they stop.
Concrete: more energy becomes sound and heat, with less deformation time; some elastic deformation/bounce.
Sand: more energy goes into deforming/moving sand grains, internal friction, and thermal energy; stopping distance/time is larger. Same conservation, different pathways.
A company claims their machine is 110% efficient because it “reuses waste heat.”
Using conservation of energy, explain why this is impossible.
“110% efficiency” means more useful energy out than energy in, which violates conservation of energy. Reusing waste heat doesn’t create extra energy — it just changes what counts as “useful,” but total output still cannot exceed input.
A toaster requires 1500J of energy to operate, only 1000J of which are emitted as thermal energy (the rest is wasted as sound and light). Construct a Sankey diagram to demonstrate this.
A country replaces all coal plants with wind turbines.
Total yearly energy production stays the same.
Explain the major energy system problem that would still exist.
Even if total yearly energy matches demand, the problem is timing and reliability (grid stability): wind output doesn’t align with peak demand; you need storage, backup generation, or demand management to prevent blackouts and keep frequency/voltage stable.