Conservation of Energy Notes

Edexcel GCSE Physics: Revision notes

Key facts

Energy is never created or destroyed, only transferred between stores.
$\Delta GPE=mg\Delta h$ and $KE=\tfrac12mv^2$ .
Name stores precisely, e.g. "thermal store of the brakes", not "heat" or "lost".
Energy is dissipated, not lost: it spreads to the thermal store of the surroundings.
Lubrication and thermal insulation reduce unwanted energy transfers.

Conservation of energy

Energy cannot be created or destroyed, only transferred from one store to another.

In a closed system there is no net change in the total energy.

Energy is stored as kinetic, gravitational potential, elastic potential, chemical, thermal, nuclear, electrostatic and magnetic energy. A change moves energy between stores, but the total stays the same.

Gravitational potential
Kinetic

A falling object, ignoring air resistance (illustrative values): energy moves from one store to another but the total stays 100 J.

Energycannot be created or destroyed
Closed systemtotal before = total after

A closed system has 500 J of energy before a change. How much does it have afterwards?

GPE and kinetic energy

Two equations give the energy in the gravitational potential store and the kinetic store.

$m$ is mass in kg, $g$ is gravitational field strength in N/kg, $\Delta h$ is the change in height in m and $v$ is speed in m/s.

In a dropped ball with no air resistance, the GPE lost equals the KE gained.

Sankey diagrams show how much energy is usefully transferred and how much is wasted.

Change in GPE $mg\Delta h$
Kinetic energy $\tfrac12mv^2$

1
Held at height
Energy in the GPE store
2
Falling
GPE decreases, KE increases
3
Just before landing
Nearly all in the kinetic store

A dropped ball (no air resistance)

Worked example

A 2 kg ball is dropped from 5 m (g = 10 N/kg). Find its speed just before landing.

1
GPE lost = 2 × 10 × 5 = 100 J.
2
KE = ½ × 2 × v² = 100, so v² = 100.

Which equation gives the energy stored by a moving object?

Energy transfers

Learn the standard transfers and name the stores exactly.

For each type of change, energy moves between named stores.

Examiners want the store named, for example "thermal store of the brakes", not a vague "heat" or "lost".

1
Kinetic store of the vehicle
energy at the start
2
Thermal store of the brakes and tyres
friction does work
3
Thermal store of the surroundings
energy spreads out

A vehicle slowing down: name each store.

Object projected upwards

Kinetic to gravitational potential

Moving object hits an obstacle

Kinetic to thermal (and sound)

Object accelerated by a constant force

Chemical or other store to kinetic

Vehicle slowing down

Kinetic to thermal (brakes, tyres, road, surroundings)

A car brakes to a stop. Which energy transfer happens?

Dissipation

In every real change energy is dissipated: it spreads out to the thermal store of the surroundings, where it is hard to use.

Friction between moving parts becomes wasteful when it raises the temperature. Energy then dissipates to the surroundings instead of doing useful work.

The total energy is unchanged, but it is spread out and less useful.

1
Energy supplied
Into the device
2
Useful transfer
The job you want done
3
Dissipated
Warms the surroundings
4
Total energy
Unchanged

Where the energy goes

What happens to the total amount of energy when it is dissipated?

Reducing unwanted transfers

Reduce friction with lubrication, and slow thermal transfers with insulation.

Unwanted transfers (mostly to the thermal store of the surroundings) can be reduced by lubrication and by thermal insulation.

A building cools more slowly if its walls are thicker or made of a material with lower thermal conductivity. These effects are described qualitatively.

1
Unwanted transfer
friction in moving parts, heat escaping from a building
2
Lubricate or insulate
or use thicker walls of low thermal conductivity
3
Slower transfer to the surroundings
less energy is dissipated

Reducing the unwanted transfer to the surroundings.

Lubrication

Less friction
Less energy dissipated as heat

Thermal insulation

Slower transfer out
e.g. loft or cavity wall insulation

Thicker walls

Slower heat loss

Low thermal conductivity

Slower heat loss

Which wall would keep a house warmest?

Try an exam question

A 2 kg ball is dropped from a height of 5 m. Gravitational field strength is 10 N/kg. Ignore air resistance. (a) Calculate the gravitational potential energy lost. (b) Calculate the speed of the ball just before it lands.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.