Edexcel GCSE Physics (1PH0) · Topic 3 — Conservation of energy
Mini-Lesson
Conservation of Energy
This mini-lesson walks you through the whole of Edexcel Topic 3 — Conservation of energy: the gravitational and kinetic energy equations, how energy is transferred and conserved in a closed system, how it gets wasted, how to use it more efficiently, and the energy resources that power the world.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Stores & transfers
Energy is shifted between stores
We never "use energy up" — we shift it from one store to another along a pathway. Edexcel wants you to draw and interpret diagrams that show the energy going in and the energy coming out (including the wasted output).
Stores hold energy — e.g. gravitational, kinetic, chemical, thermal, elastic.
Pathways move it — mechanically (a force), electrically, by heating, or by radiation (light/sound).
A transfer diagram shows the input, the useful output and the wasted output. (Spec 3.3)
Watch out: "heat", "electrical", "light" and "sound" are pathways (ways energy is transferred), not stores. Don't write "light energy is stored".
Quick check
Which is a store?
?On a transfer diagram for a falling ball, which of these correctly labels a store of energy (not a pathway)?
Equation · spec 3.1
Change in gravitational PE
Raise an object above the ground and you fill its gravitational store. Edexcel gives you the equation for the change in this store:
ΔGPE = m × g × Δhchange in g.p.e. (J) = mass (kg) × gravitational field strength (N/kg) × change in vertical height (m)
Δh is the vertical height change. Use g = 10 N/kg on Earth in Edexcel calculations.
Worked example
A 2 kg book is lifted 1.5 m onto a shelf (g = 10 N/kg).
ΔGPE = 2 × 10 × 1.5 = 30 J
Calculate · spec 3.1
Your turn — gravitational store
1A 50 kg climber goes 8 m higher up a cliff. Using g = 10 N/kg, how much energy is transferred to her gravitational store?
J
Hint: ΔGPE = 50 × 10 × 8.
Equation · spec 3.2
Kinetic energy of a moving object
Every moving object has energy in its kinetic store. It depends on mass and, much more strongly, on speed:
KE = ½ × m × v²kinetic energy (J) = ½ × mass (kg) × (speed)² ((m/s)²)
Because speed is squared, doubling the speed gives four times the kinetic energy — which is why a fast car carries so much more energy than a slow one.
Worked example
A 1000 kg car travels at 20 m/s.
KE = ½ × 1000 × 20² = ½ × 1000 × 400 = 200 000 J (200 kJ)
Calculate · spec 3.2
Your turn — kinetic energy
2An 8 kg bowling ball rolls at 5 m/s. Calculate the energy in its kinetic store.
J
Hint: KE = ½ × 8 × 5². (5² = 25)
Conservation · spec 3.4 & 3.6
Energy is never created or destroyed
The principle of conservation of energy: energy can be transferred, stored or dissipated, but it cannot be created or destroyed.
A closed system is one with no energy transferred in or out. The spec is precise: where there are energy transfers in a closed system there is no net change to the total energy of that system. On a swing, energy moves between the gravitational store (top) and the kinetic store (bottom) — but the total stays the same:
At the bottom the swing moves fastest — its kinetic store is largest there.
In an ideal closed system the totals balance: g.p.e. lost = k.e. gained. In real life some energy is dissipated by air resistance and friction — but it isn't destroyed, it just spreads out into the surroundings (next screen).
Quick check
Reading the swing
?Ignoring friction, at which point of its swing does the pendulum bob have the most energy in its kinetic store?
System changes · spec 3.5
Tracking the stores when a system changes
Edexcel names specific situations and expects you to say how the stored energy changes. Learn this set:
Object thrown upwards / up a slope:kinetic → gravitational (and some to heat by friction on a slope).
Moving object hitting an obstacle:kinetic → sound, heat (and kinetic of the obstacle if it moves).
Object accelerated by a constant force: the force does work, filling the kinetic store.
Vehicle slowing down:kinetic → thermal (heat in the brakes).
Water brought to the boil in an electric kettle: electrical pathway → thermal store of the water.
Object projected upwards: kinetic store empties into the gravitational store, then refills on the way down.Sort it · spec 3.5
Where does the energy end up?
For each system change, tap the store the energy is mainly transferred to.
Dissipation · spec 3.7 & 3.8
Why energy gets wasted
In every system change some energy is dissipated — it spreads out and ends up stored in less useful ways (usually heating the surroundings).
The spec singles out mechanical processes (where forces act on objects): friction causes a rise in temperature, so energy is dissipated by heating the surroundings — making the process wasteful.
Friction warms the surfaces and the air — that heat can't easily be recovered, so it is "wasted".
Key idea: the energy isn't destroyed (conservation still holds) — it is just spread thinly into the surroundings where it is too dilute to be useful.
Quick check
What "wasted" means
?A box is pushed across a rough floor and stops. Which statement best describes what happens to the energy?
Reducing waste · spec 3.9 & 3.10
Wasting less energy
Two ways the spec wants you to know for reducing unwanted energy transfer:
Lubrication — oil between moving parts cuts friction, so less energy is dissipated by heating.
Thermal insulation — slows energy transfer out of a warm thermal store.
For a building, the rate it cools depends (qualitatively) on its walls:
Thicker walls → slower energy transfer → cools more slowly.
Lower thermal conductivity walls → slower energy transfer → cools more slowly.
Quick check
Keeping the heat in
?Two houses are identical except for their walls. Which wall will let the building cool down most slowly?
Efficiency · spec 3.11
Useful vs total energy
A device's efficiency is the fraction of the supplied energy that comes out usefully. A Sankey diagram shows it: the wider the arrow, the more energy.
An old filament lamp: only 20 J of every 100 J becomes useful light.
efficiency = useful energy transferred ÷ total energy supplied(× 100 for a percentage) — always between 0 and 1 (0–100%)
That lamp's efficiency = 20 ÷ 100 = 0.2 = 20%. An LED reaching 90% wastes far less. (This is the only equation in Topic 3 about waste — there is no power or specific-heat equation here.)
Calculate · spec 3.11
Your turn — efficiency
3An electric motor is supplied with 1200 J and transfers 360 J usefully to a kinetic store. Calculate its efficiency as a percentage.
%
Hint: (360 ÷ 1200) × 100.
Increasing efficiency · spec 3.12
Making devices more efficient
You can raise efficiency by cutting the wasted output, or by recapturing it:
Reduce the waste output — e.g. lubricate moving parts or add thermal insulation so less energy is dissipated.
Recycle the waste output — capture the dissipated energy (e.g. waste heat) and feed it back in as useful input.
?A factory machine wastes a lot of energy as heat from its bearings. Which change would increase its efficiency?
Energy sources · spec 3.13
Where our energy comes from
Edexcel lists the main energy sources available on Earth: fossil fuels, nuclear fuel, bio-fuel, wind, hydroelectricity, the tides and the Sun. The big divide is renewable (won't run out on a human timescale) vs non-renewable (finite):
Coal, oil and natural gas are the three fossil fuels. Bio-fuel is renewable but still releases CO₂ when burned.
Comparing them: non-renewables (fossil & nuclear fuel) give a large, reliable energy output and are used for big, steady supplies — but fossil fuels are finite and release CO₂. Many renewables (solar, wind, tidal) are cleaner but less reliable — they depend on weather, time of day or the tide.
Sort it · spec 3.13
Renewable or not?
Tap a resource, then tap the box it belongs in.
♻️ Renewable
⛽ Non-renewable
Patterns & trends · spec 3.14
How our energy use has changed
The spec asks you to explain patterns and trends in the use of energy resources:
Since the Industrial Revolution, fossil fuels dominated — easy to mine and packed with energy.
More recently, use of renewables has grown, because fossil fuels are finite and burning them releases CO₂, and because technology to harness renewables has improved.
?Why has the use of renewable energy resources increased in recent decades?
Recap
Edexcel Topic 3 in a nutshell
Gravitational store (3.1): ΔGPE = m × g × Δh
Kinetic store (3.2): KE = ½ × m × v²
Transfer diagrams (3.3): show input, useful & wasted output
Conservation (3.4, 3.6): energy can't be created/destroyed; no net change in a closed system
System changes (3.5): thrown up, collision, accelerating, braking, boiling kettle
Dissipation (3.7, 3.8): waste heats the surroundings; stored in less useful ways