# Energy stores and transfers

Revision of Energy stores and transfers for GCSE Physics/Combined Science.

**Energy stores **

There are 8 energy stores where energy can be ‘kept’:

–** chemical store** (in a chemical reaction e.g. fuel + oxygen)

– **kinetic** **store** (in a moving object)

– **gravitational store** (due to the position of an object in a gravitational field)

– **elastic** **store** (e.g. in a stretched or compressed spring)

– **thermal** **store** (in a warm object)

– **magnetic** **store** (in two separated magnets that are attracting, or repelling)

– **electrostatic** **store** (in two separated electric charges that are attracting, or repelling)

– **nuclear store** (released through radioactive decay, fission or fusion.

### Key definition – what is a system?

A **system** is an object or group of objects.

### Examples

An object projected upwards: (e.g. ball thrown upwards):

Kinetic store of ball → Gravitational potential store of ball

A moving object hitting an obstacle: (e.g. car hitting a traffic cone):

Kinetic store of moving object → Kinetic store of obstacle

An object accelerated by a constant force: (e.g. skydiver accelerated by their

weight):

Gravitational potential store of skydiver → Kinetic store of skydiver

A vehicle slowing down (e.g. car applying brakes):

Kinetic store of car → Thermal store of brake pads.

Bringing water to boil in an electrical kettle:

Thermal store of element → Thermal store of water in kettle

**Energy transfers**

There are four pathways along which energy is transferred from one store to another:

– Heating

– Electrical

– Radiation (including light, all electromagnetic waves and sound).

– Mechanical

Quick Check 1

### Example – energy stores and transfers in a roller coater ride

As a roller coaster climbs to it’s highest point, it transfers energy from the electrical supply to a gravitational store. There is a motor doing mechanical work on the roller coaster, applying a force to make it move up. As it goes over the highest point and starts to move downwards, energy is rapidly shifted from this gravitational store to a kinetic store. The force of gravity is doing mechanical work on the roller coaster, pulling it down the slope.

**Energy calculations**

You need to be able to calculate the energy transferred, using the following equations:

**In heating **

(ΔE = mcΔθ)

Change in thermal energy = mass x specific heat capacity x change in temperature

**Mechanical work done by forces**

(W = Fd)

Work done = Force x distance

**Electrical work done when a current flows**

(W = IVt)

Work done = Current x potential difference x time

Quick Check 2**Kinetic Energy**

The equation
for **kinetic energy**:

**Kinetic energy = ½ x mass x velocity ^{2}**

E_{K} = Kinetic energy measured
in joules (J)

m = mass measured in kilograms (kg)

v = velocity measured in metres per second (m/s)

**Elastic Potential Energy**

The equation for elastic potential energy is:

E_{e}=
0.5 x k x e^{2}

**Elastic potential energy = 0.5 x spring constant x extension ^{2}**

E_{e}
= Elastic potential energy (J) k = Spring constant (N/m) e = extension (m)

**Thermal Energy**

The equation for change in thermal energy is:

ΔE = m x c x Δθ

**Change in thermal energy = mass x specific heat capacity x change in temperature **

ΔE = Change
in thermal energy (J); m = Mass (kg); c = Specific heat capacity (J/kg^{o}C)

Δθ = Change
in temperature (^{o}C)

The specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius.

Quick Check 5**Power**

Power is defined as the rate at which energy is transferred or the rate at which work is done.

The equation for power is:

P = E ÷ t

**Power = Energy transferred ÷ time **

P = Power (W); E = Energy transferred (J); t = time (s);

The equation for power can also be written:

P = W ÷ t

**Power = Work done ÷ time **

P = Power (W) W = Work done (J); t = time (s)

Watt is the unit of Power!? The question that answers itself.

An energy transfer of 1 joule per second is equal to a power of 1 watt.

Quick Check 6**Conservation of Energy**

Energy can be transferred usefully, stored or dissipated, but **e****nergy cannot be**

**created or destroyed**.

Sometimes energy is **dissipated**, so that it is stored in less useful ways. This energy

is often described as being ‘wasted’.

Because energy cannot be lost:** Total energy = useful energy + wasted energy**

Unwanted energy transfers can be reduced by a range of methods, for example

through lubrication in car engines and the use of thermal insulation in houses.

The higher the thermal conductivity of a material the higher the rate of energy transfer by conduction across the material. The rate of cooling of a building is affected by the thickness and thermal conductivity of its walls.