Fields in Physics

Fields

A field is the general idea of something having values through space (and time).

In physics it describes a force or influence that extends through space and time.

Example: Gravity

Objects with mass, like the Earth or an apple, are pulled towards each other over a distance.

You can jump really high but won't escape Earth's gravity field, you merely move within it and then get pulled back down.

Gravity changes through space: it gets weaker with distance.

Gravity changes over time: The Moon creates tides. The Sun, Planets, Moons, Comets, etc, are all in a gravitational dance.

Play with Gravity Freeplay to see the movements.

Types of Fields

The main types of field in physics are:

Gravitational Field (mentioned above)

Electric Field: there is an electric field around any charged object. This field exerts force on other charged objects. The strength of the force depends on the size of the charges and the distance apart.

Magnetic Field: If you've played with magnets, you've felt a magnetic field.

You can move a paperclip without touching it! The region around the magnet exerts a force on the paperclip
A compass needle shows the direction of the magnetic field

Electromagnetic Field: A combination of Electric and Magnetic fields: light, radio and microwaves are examples of electromagnetic waves.

You can feel the warmth from a fire without touching the flames
your phone gets reception even though you are far from the nearest tower

$wave-diffraction-radio$

Electric and Magnetic Fields

Here's how it works:

Electric Fields are created by electric charges.

A stationary electric charge generates an electric field around it. This field exerts force on other charges in the vicinity, which is why charged objects can attract or repel each other even without touching.

Magnetic Fields are created by moving electric charges.

Magnetic fields are created by moving electric charges. Specifically, they are produced by electric currents, which are flows of electric charge, or by changing electric fields. Electric motors rely on this and use electricity to drive magnets that rotate a central shaft, often with great force.

Electromagnetic Fields are a combination of electric and magnetic fields

They are created when electric charges move back and forth (oscillate). An excellent example is the fields produced by alternating current (AC) in wires, or those generated by radio transmitters.

When an electric charge accelerates (not just moves at a constant speed, but changes its velocity) it produces electromagnetic waves.

These waves are ripples in the electromagnetic field that carry energy from one place to another, see Electromagnetic Spectrum to learn more.

Electromagnetism and Gravity are two of the four fundamental forces of nature.

Why Fields?

Let's look at charged particles: we know there is an interaction between them: Opposites attract and like charges repel.

We can model that as a simple line between two charges.

But with many charges, all the interactions become hard to deal with:

But a model where each charge contributes to a field is easier to use, and gives a better feel for what is happening:

We can now imagine being anywhere in that field, with the lines pointing in the direction we feel force.

Play with Electric Field Animation.

Summary

Gravity attracts masses together and creates a gravitational field
Static electric charges create an electric field which can exert force on other charges.
Moving electric charges (electric current) create a magnetic field which can influence magnets and other currents.
Accelerating electric charges create waves in the electromagnetic field which carry energy of different frequencies from radio through light on to X-Rays and Gamma rays.