Sparks in the air
Teaching Guidance
for 14-16
A high voltage will produce a spark in an air gap. This is easily demonstrated with the spheres of a Van de Graaff generator, see the experiment:
Showing that a spark can pass through air
A spark will spontaneously jump across a gap of 1 cm if there is a potential difference, of about 10,000 V across it. A larger gap needs a bigger potential difference so a spark will jump across a 3 cm gap if there is a potential difference of 30,000 V and so on.
Advanced students will understand the idea of an electric field. A spark is produced by a field strength of 10,000 V cm-1. The force on particles will increase if the charge on the spheres increases OR if the spheres are closer together.
The spark discharge process
When a positive ion is produced between the spheres of the Van de Graaff generator, it is accelerated towards the negative sphere, gaining kinetic energy. The bigger the force on the ion (or field strength), the bigger the acceleration. The ion collides with a neutral atom (after, on average, one mean free path). If it is going fast enough, it will knock an electron off the neutral atom turning it into another ion (this is an inelastic collision – kinetic energy is not conserved). If it is not going fast enough, the two will bounce away from each other with some sharing of energy.
Although it may be slowed down, the first ion will be accelerated again and make another ion in its next inelastic collision. Each new ion will also accelerate towards the negative sphere, producing new ions when they collide with air atoms.
The spark is a cascade or avalanche of ions – like a chain reaction. The picture below shows a simple experiment (possibly a thought experiment) that illustrates this.
The gradient of the ramp represents the electric field strength. If the field strength is too small, then it will not accelerate the ions enough to produce an avalanche. Any ions that are produced in the field will be drawn to the side but won’t cause a spark. [The actual mechanism that produces a spark is more complicated: there are negative ions as well as electrons; there will be excited atoms producing light; and there will be ultraviolet radiation and even X-rays produced by decelerating ions and electrons.]
Initiating a spark
At 10,000 V cm-1, the air breaks down and a spark is produced spontaneously. The field strength is enough to pull some molecules apart and produce ions that start the avalanche. It is sometimes useful to reduce the field strength slightly so that the air isn’t breaking down. Then you can initiate the avalanche by producing ions in the electric field. The two most common ways of producing ions are with a flame or a radioactive source (usually an alpha source like radium).
