Potential energy facts for kids

Kids Encyclopedia Facts

Quick facts for kids Potential energy
Imagine an archer pulling back a bowstring. The energy from the archer's body is stored in the bent bow as elastic potential energy. When the string is let go, this stored energy turns into the kinetic energy (motion energy) of the arrow as it flies.
Common symbols	PE, U, or V
SI unit	joule (J)
Derivations from other quantities	U = m g h (gravitational) U = 1/2kx² (elastic) U = 1/2CV² (electric) U = −m ⋅ B (magnetic) U = ${\textstyle \int F(r) \, dr}$

In physics, potential energy is stored energy that an object has because of its position or arrangement. Think of it as energy waiting to be used! This energy is stored when a force, like gravity or a spring, is worked against.

The idea of potential energy was first named by a Scottish scientist named William Rankine in 1853. However, the basic concept goes back to the ancient Greek philosopher Aristotle.

Some common types of potential energy include the energy stored in a stretched spring (elastic potential energy), the energy of an object held high above the ground (gravitational potential energy), and the energy stored between electric charges (electric potential energy). We measure all forms of energy in units called joules (J).

What is Potential Energy?
Different Kinds of Potential Energy
Who Discovered Potential Energy?
How Work and Potential Energy are Connected
Gravity's Stored Energy Near Earth
Energy in Stretched Springs
Gravity Between Planets and Stars
Electric Energy in Charges
Energy in Magnets
Energy Inside Atoms
Real-World Uses of Potential Energy
Related pages
See also

What is Potential Energy?

Potential energy is the energy an object has due to its position or how its parts are arranged. It's like a battery, holding energy until it's needed. For example, a ball held high above the ground has potential energy because of its height. A stretched rubber band also has potential energy because of its stretched shape.

This stored energy is equal to the work done to put the object in that position or configuration. This work is done against forces like gravity or the restoring force of a spring.

Different Kinds of Potential Energy

There are many types of potential energy, each linked to a specific kind of force. For example:

Elastic potential energy is stored in stretchy things like springs or rubber bands.
Gravitational potential energy is stored when an object is lifted against gravity.
Electric potential energy is stored between electric charges.
Chemical potential energy is stored in the bonds of atoms and molecules, like in food or fuel.
Nuclear potential energy is stored inside the nucleus of atoms.

Forces that create potential energy are called conservative forces. This means the amount of energy stored only depends on the starting and ending positions, not the path taken. When you do work against a conservative force (like lifting a ball), you increase its potential energy. When the force does work (like the ball falling), the potential energy decreases.

A simple way to think about gravitational potential energy is with this formula: Here, U_g is the gravitational potential energy, m is the object's mass, g is the strength of gravity (about 9.8 m/s² on Earth), and h is its height.

Who Discovered Potential Energy?

Scientists started to really understand energy and work around the 1840s. The term 'potential energy' was created by William Rankine, a Scottish engineer and physicist, in 1853. He wanted to give a clear name to this type of stored energy.

Rankine's idea was inspired by the ancient Greek philosopher Aristotle, who talked about things having 'potential.' Later, in 1867, another famous scientist, William Thomson, introduced the term 'kinetic energy' for energy of motion. These two terms, potential and kinetic energy, help us describe all the different ways energy exists.

How Work and Potential Energy are Connected

Potential energy is very closely related to forces and work. When a force does work on an object, it can change that object's energy. For example, when you lift a heavy box, you are doing work against the force of gravity. This work doesn't make the box move faster (kinetic energy), but it stores energy in the box because of its new, higher position. This stored energy is potential energy.

Forces like gravity and the force from a spring are called 'conservative forces'. This means that the amount of work done by these forces only depends on where an object starts and where it ends up, not the path it took. Imagine climbing a hill: the energy you gain depends on how high you go, not whether you took a winding path or a steep one.

When a conservative force does work, the potential energy of the object decreases. If you do work against a conservative force, the potential energy increases. It's like saving up energy for later!

Gravity's Stored Energy Near Earth

A trebuchet uses the gravitational potential energy of the counterweight to throw projectiles over two hundred meters

When you lift an object, you give it gravitational potential energy. The higher you lift it, the more energy it stores. This energy is stored because you are working against the Earth's gravity.

For objects near the Earth's surface, we can calculate this energy with a simple formula:

U_g is the gravitational potential energy (measured in joules).
m is the object's mass (in kilograms).
g is the acceleration due to gravity (about 9.8 meters per second squared on Earth).
h is the object's height above a chosen reference point (in meters).

This formula works well for small changes in height, like lifting a book or climbing a ladder. The work done by gravity only depends on how much an object moves up or down.

Energy in Stretched Springs

Springs are used for storing elastic potential energy

Archery is one of humankind's oldest applications of elastic potential energy

A spring stores energy when it is stretched or compressed. This is called elastic potential energy. The more you stretch or compress a spring, the more energy it stores. This happens because the spring tries to return to its original shape.

The formula for the potential energy stored in a linear spring is:

U is the elastic potential energy.
k is the spring constant, which tells you how stiff the spring is.
x is how much the spring is stretched or compressed from its normal length.

When you let go of a stretched spring, its stored elastic potential energy turns into kinetic energy (energy of motion). This is how a toy dart gun or a bow and arrow works!

Gravity Between Planets and Stars

For very large distances, like between planets or stars, the simple mgh formula for gravity doesn't work. Instead, we use a more general formula for gravitational potential energy:

U is the gravitational potential energy.
G is the gravitational constant, a special number in physics.
M and m are the masses of the two objects.
r is the distance between the centers of the two objects.

This formula gives a negative value for potential energy. This might seem strange, but it's a way scientists make calculations easier. It means that objects are "bound" together by gravity. To separate them completely, you would need to add energy.

Electric Energy in Charges

Plasma formed inside a gas filled sphere

Objects with an electric charge can also have potential energy. This is called electric potential energy. It's the energy stored because of their position relative to other charged objects.

If you have two electric charges, they will either attract or repel each other. To move them closer if they repel, or pull them apart if they attract, you have to do work. This work is stored as electric potential energy.

The formula for the electric potential energy between two charges Q and q separated by a distance r is:

U is the electric potential energy.
Q and q are the amounts of the two electric charges.
r is the distance between the charges.
ε₀ is a constant called the vacuum permittivity.

A related idea is electric potential (also known as voltage), which is the electric potential energy per unit of charge.

Energy in Magnets

Magnets also have potential energy, called magnetic potential energy. This energy depends on how far apart magnets are and how they are lined up. For example, a compass needle has the lowest magnetic potential energy when it points north, aligning with Earth's magnetic field. If you push it away, it will try to snap back into alignment.

If you try to push two like poles of magnets together (north-north or south-south), you feel a strong push. The energy you use to force them together is stored as magnetic potential energy. When you let go, they spring apart, and this stored energy turns into motion.

Energy Inside Atoms

Nuclear potential energy is the energy stored within the tiny particles inside an atomic nucleus. These particles, like protons and neutrons, are held together by a very strong force called the strong nuclear force.

When atoms undergo certain changes, like in nuclear fission (splitting atoms) or nuclear fusion (joining atoms), some of this nuclear potential energy can be released. This is how nuclear power plants generate electricity and how the Sun produces its enormous energy. For instance, the Sun fuses hydrogen into helium, releasing a huge amount of energy every second.

Real-World Uses of Potential Energy

Gravitational force keeps the planets in orbit around the Sun

Potential energy is used in many ways in our daily lives:

Hydroelectric Power: In places like Dinorwig Power Station in Wales, water is pumped to a higher lake when there's extra electricity. This stores energy as gravitational potential energy. When electricity is needed, the water flows down through turbines, turning the potential energy back into electricity.
Clocks: Many old clocks are powered by falling weights. As the weights slowly drop, their gravitational potential energy is converted to the energy that moves the clock's gears.
Counterweights: Elevators and cranes use heavy counterweights. As the elevator goes up, the counterweight goes down, helping to balance the load and save energy.
Roller Coasters: A roller coaster is pulled up a big hill, gaining lots of gravitational potential energy. Then, as it races down, this potential energy changes into thrilling kinetic energy.
Transportation: Cars, trucks, and trains use gravitational potential energy when going downhill. This can help them save fuel or even generate electricity (like in some electric trains).