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 (e.g. light). Sound waves also carry energy away.

Watch out: "heat", "electrical", "light" and "sound" are pathways (ways energy is transferred), not stores. Don't write "light energy is stored".

Which is a store?

Change in gravitational PE

Raise an object above the ground and you fill its gravitational store. You must recall this equation for the change in this store (it is not on the equation sheet):

Your turn — gravitational store

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:

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.

Your turn — kinetic energy

Which car holds more energy?

Why must the first hill be the tallest?

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:

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).

Reading the swing

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 thermal stores by friction on a slope).
• Moving object hitting an obstacle: kinetic → thermal stores, by heating and by sound (and the kinetic store 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.

Where does the energy end up?

For each system change, tap the store the energy is mainly transferred to.

Why must the first hill be the tallest?

Where does the bulb's extra energy go?

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.

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.

What "wasted" means

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.

Keeping the heat in

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.

That lamp's efficiency = 10 ÷ 100 = 0.1 = 10%. An LED is several times more efficient, so it wastes far less. (This is the only equation in Topic 3 about waste — there is no power or specific-heat equation here.)

Your turn — efficiency

Where does the bulb's extra energy go?

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.

Why not run everything on wind?

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):

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. Some renewables (solar, wind) are cleaner but less reliable — they depend on the weather or the time of day.

Renewable or not?

Tap a resource, then tap the box it belongs in.

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 not run everything on wind?

Edexcel Topic 3 in a nutshell

You've covered every statement in Edexcel Topic 3 — Conservation of energy. Press Finish to see your score.