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# Energy

## Energy definition

Energy $(E)$ is a quantity that describes the ability of an object to do work on other objects.

The energy that an object has indicates how much work it could do.

A battery with $10 \text{ kJ}$ of energy can do $10 \text{ kJ}$ of work when connected to a motor.

Energy cannot be measured directly and must be determined using other properties of the system (e.g. velocity, position or temperature).

The SI unit of energy is the same as the SI unit for work, the joule $(\text{J})$.

Example Energy (estimation)
Camera flash $10\text{ J}$
Chocolate bar $1.2\text{ MJ}$
Lightning bolt $1\text{ GJ}$
Fuel carried by an aircraft $5\text{ TJ}$
Lightning transfers electrical energy from the clouds to the ground

## Chemical, thermal and elastic energy

Chemical potential energy is the energy stored inside molecules.

When coal is burned, the chemical potential energy stored in a bond between two carbon atoms is released as thermal energy.

Thermal energy refers to the vibrations of the particles inside an object. Hot objects that have lots of thermal energy are used to do work.

In many power stations, steam at very high temperatures does work on turbines making them rotate at high speeds.

Elastic potential energy is the energy stored inside a solid that is compressed, stretched or deformed.

When you stretch a rubber band you are increasing its elastic potential energy.

The elastic energy stored in the stretched bow is used to make the arrow fly through the air

## Potential energy

Potential energy $(E_{\text{p}})$ or $(U)$ is the energy stored in a system due to the arrangement of objects or particles within it.

Subcategories of potential energy include gravitational potential energy, electric potential energy and elastic potential energy.

• The gravitational potential energy is the energy possessed by a mass due to its position in the gravitational field of other masses.

Picking up a glass from the floor and placing it on a table increases its gravitational potential energy.

• Similarly the electric potential energy is the energy possessed by a charge due to its position in the electric field of other charges.

An electron that is bound to an atom has electric potential energy.

• The elastic potential energy is the energy stored in a system (typically a solid) due to its deformation (i.e. change in volume or shape).

A stretched rubber band stores elastic potential energy.

## Gravitational potential energy

The gravitational potential energy $(E_{p}$ or $GPE)$ on Earth is the energy stored by an object that has been raised from the surface of the Earth.

When you carry a ball up some stairs you are increasing the ball's gravitational potential energy. This energy can be converted to work when you let the ball drop down the stair.

A ball that is at the top of a hill has the potential to roll down the hill, converting its gravitational potential energy into kinetic (movement) energy.

Objects that have more mass have more gravitational potential energy.

The energy $\Torange{(E_{\text{p}})}$ required to raise a mass $\Tblue{(m)}$ up a vertical height $\Tred{(h)}$ is equal to $$\Torange{E_{\text{p}}} = \Tblue{m} \times g \times \Tred{h}$$ $g$ is the gravitational field strength near Earth's surface ($10 \text{ N/kg})$

A mass of $10 \text{ kg}$ at a height of $10\um$ above the Earth's surface has a gravitational potential energy of $1000 \text{ J}.$