Energy Stores, Transfers & Efficiency

Energy Stores and Transfers

Energy is one of the most important ideas in physics. It cannot be created or destroyed, but it can be stored in different ways and transferred from one store to another. Whenever something happens — a ball falls, a kettle boils, a car speeds up — energy is being shifted between stores.

We describe a situation by naming the energy stores that fill up or empty, and the transfer pathways that move the energy between them.

Key terms

Energy store — a way energy is held in a system (e.g. kinetic, chemical).

Energy transfer — the movement of energy from one store to another by a pathway.

The unit of energy is the joule (J). 1 J is roughly the energy to lift a small apple by 1 m.

The Eight Energy Stores

Energy moves between these stores by four transfer pathways:

1. Mechanically — by a force moving an object (doing work). Example: pushing a box.
2. Electrically — by a current flowing in a circuit. Example: a charging phone.
3. By heating — energy flows from hot to cold by conduction, convection or radiation. Example: a saucepan on a hob.
4. By radiation — energy carried by waves, mainly light and infrared. Example: the Sun warming your skin.

Watch out

"Heat", "light", "sound" and "electrical" are transfers or pathways, not stores. In exam answers, store names end in kinetic / chemical / thermal, etc. Do not write "the energy is stored as heat" — write "the thermal store increases".

Conservation of Energy

The principle of conservation of energy states:

Energy cannot be created or destroyed. It can only be transferred from one store to another, or dissipated to the surroundings.

This means the total energy before a change always equals the total energy after. When energy seems to "disappear" — such as a bouncing ball that bounces lower each time — it has actually been transferred to the thermal store of the surroundings (by friction and air resistance) and dissipated, spread out so thinly it can no longer be usefully recovered.

Work Done and Power

When a force moves an object, energy is transferred mechanically. The energy transferred is the work done:

$$W = F \, d$$

where $W$ is work done in joules (J), $F$ is the force in newtons (N), and $d$ is the distance moved in the direction of the force in metres (m). One joule is the work done when 1 N moves something 1 m, so $1\ \text{J} = 1\ \text{N m}$.

Power is the rate of transferring energy — how much energy is transferred each second:

$$P = \dfrac{W}{t}$$

where $P$ is power in watts (W), $W$ is energy transferred (or work done) in joules, and $t$ is time in seconds. One watt is one joule per second.

Worked example

A weightlifter raises a 60 kg barbell, doing 720 J of work, in 1.5 s. Find the power.

$P = \dfrac{W}{t} = \dfrac{720}{1.5} = 480\ \text{W}$.

Kinetic Energy

Any moving object has energy in its kinetic store:

$$KE = \tfrac{1}{2}mv^2$$

where $m$ is mass in kg and $v$ is speed in m/s. Notice the speed is squared — doubling the speed gives four times the kinetic energy. This is why fast-moving vehicles need much greater braking distances.

Worked example

Find the kinetic energy of a 1200 kg car travelling at 20 m/s.

$KE = \tfrac{1}{2}mv^2 = \tfrac{1}{2} \times 1200 \times 20^2$

Gravitational Potential Energy

When an object is lifted, energy is transferred to its gravitational potential store:

$$GPE = mgh$$

where $m$ is mass in kg, $g$ is the gravitational field strength ($g = 10\ \text{N/kg}$ on Earth in 4PH1) and $h$ is the change in height in m.

Exam tip

Use $g = 10\ \text{N/kg}$ unless the question says otherwise. $h$ is the vertical height gained, never the distance along a slope.

Transfers Between KE and GPE

For a falling object, GPE is transferred to KE. If we ignore air resistance, energy is conserved, so all the GPE lost becomes KE gained:

$$mgh = \tfrac{1}{2}mv^2$$

The mass cancels, which is why all objects fall at the same rate in the absence of air resistance.

A pendulum swings the same energy back and forth: GPE is greatest at the top of each swing (where it is momentarily still) and KE is greatest at the lowest point (where it moves fastest).

Worked example

A 0.5 kg ball is dropped from a height of 1.8 m. Find its speed just before it lands ($g = 10\ \text{N/kg}$).

GPE lost $= mgh = 0.5 \times 10 \times 1.8 = 9\ \text{J}$.

Efficiency

No real device transfers all its input energy usefully — some is always dissipated to the surroundings, usually as thermal energy from friction. Efficiency measures how good a device is at transferring energy to where we want it:

$$\text{efficiency} = \dfrac{\text{useful energy output}}{\text{total energy input}} \times 100\%$$

You can also use power in place of energy. Efficiency is always less than 100% and has no units (it is a ratio or a percentage).

Worked example

A motor takes in 500 J of electrical energy and usefully transfers 350 J to the kinetic store. Find its efficiency.

$\text{efficiency} = \dfrac{350}{500} \times 100\% = 70\%$.

The other 150 J is wasted, mostly to the thermal store.

Sankey Diagrams

A Sankey diagram shows energy transfers as arrows whose width is proportional to the energy. The input enters from the left; useful output continues to the right; wasted (dissipated) energy branches off, usually downwards. The total width stays constant — a visual reminder that energy is conserved.

For this lamp, efficiency $= \dfrac{10}{100} \times 100\% = 10\%$ — most of the energy is wasted as heat, which is why filament bulbs have been replaced by efficient LEDs.

Reducing Unwanted Energy Transfers

Wasted energy is nearly always reduced by tackling friction and heat loss:

Lubrication with oil reduces friction between moving parts, cutting the thermal store losses.
Streamlining vehicles reduces air resistance, so less energy is wasted.
Thermal insulation (loft insulation, cavity walls, double glazing) slows heat transfer from a warm house. Materials with low thermal conductivity and trapped air are good insulators.
Thicker or lower-conductivity walls keep buildings warm for longer.

Energy Resources

We generate electricity from energy resources, split into two groups:

Fossil fuels release a lot of energy on demand but produce carbon dioxide (a greenhouse gas) and other pollutants. Renewables are cleaner but can be unreliable (no wind, no sun) or depend on location.

Exam tip

"Renewable" does not mean "clean" or "non-polluting" — it means the resource will not run out. Burning biofuel is renewable but still releases CO₂. Be precise about which property a question is testing.