Electric Fields and Particle Detectors

A field is an area in space where a Force is experienced. For example, in a Gravitational field a Mass will experience a Force called Weight.

In an electric or magnetic field a charged particle will experience a Force. There are two types of charges (positive and negative).

As seen in previous courses, like charges repel and unlike charges attract, so the Forces can act in one direction or the other, as shown:-

If two conduction plates have an applied Voltage then an electric field will exist between them, as shown:-

Work Done

If we exert a Force to raise a Mass in a Gravitational field we have to do work and so energy is transformed. If an electric field exerts a Force on a charged particle then Work will be done in accelerating the particle.

When Charge, Q, is moved through a Voltage, V, the Work Done, W, is given by:


W = Q x V


The term Potential difference is often used when discussing Voltages. So, the definition of a Potential difference is "the work done moving a Coulomb of charge between two points".

But can we use this to understand the motion of a particle in a field??


Kinetic Energy

The Kinetic Energy equal to the Gravitational field the work done by the field on the particle. So, if an Electron is accelerated between two charged plates it will gain Kinetic Energy, such that:


Ek = W

1/2 m v2 = Q V

If an Electron is moving at right angles to an electric field it will be deflected from it's course (just like a projectile). Therefore it will either be deflected upwards or downwards, depending on the polarity (direction) of the field. This applies for all charged particles.

Particle Detectors

The video below demonstrates the construction of a simple Cloud Chamber.

A cloud chamber is a container filled with a supersaturated vapour. The molecules in this vapour are Polar (meaning they have a positive end and a negative end). When a charged particle passes through the vapour, the vapour can become ionised along the path of the particle. The ionised particles act as condensation nuclei, causing the vapour to condense into a cloud along the path of the particle. This creates a vapour trail similar to the trail from a high altitude aircraft.