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What is a plasma?

In this video, Dr Kate Lancaster explains what a plasma is, and how it's different from the other states of matter.

Let’s get started! Plasma is often referred to as the fourth state of matter. So how is it different from the other states of matter – solids, liquids, and gases? Take a look at Kate’s introductory video and then read on for a bit more detail.

Solids, liquids and gases are made up of atoms. The atoms have a nucleus in the middle, made of positive protons and neutral neutrons. Negative electrons ‘orbit’ around this nucleus. Because the number of negative electrons is the same as the number of positive protons, atoms are electrically neutral.

What is plasma?

Plasmas are most similar to gases in the way that the particles behave. However, in a plasma, some or all of the electrons have been separated from the nucleus in a process called ionisation.

This leaves behind positively charged ions and negatively charged electrons. This has some important consequences for the plasma behaviour. Unlike the neutral atoms in a gas, the charged particles in a plasma can be moved around by electric and magnetic fields.

Electric fields

Electric fields can be formed by applying a voltage across two metal plates on either side of the plasma. The field causes the electrons and ions to move towards the plates, with electrons moving in the opposite direction to ions.

Magnetic fields cause charged particles to spiral around them and, in many cases, particles will follow the magnetic field lines like a train follows a track. This gives us a way of guiding the plasma.

Charged particles

But it is not just external fields that affect plasmas. Charged particles also create electric fields and, if they are moving, magnetic fields. These internal fields mean that the particles can interact strongly with one another, leading to so-called ‘collective effects’.

Complex behaviours

This really just means that the behaviour of one particle affects the others around it, leading to complex behaviours. This often takes the form of waves that travel through the plasma.

So, where the particles in a gas are pretty passive, just moving around until they find something to collide with, the particles in a plasma can be influenced. This ability to control and guide the plasma lies at the heart of many of its most interesting applications.

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Frontier Physics, Future Technologies

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