8.3 What happens if the static charge builds up on an object? - sparks and ionisation

The full explanation of sparking

(from Van de Graaff generator, hair and comb or 'lightning' - its all the same!)

You cannot get a visible spark in a vacuum, because there are no particles to excite to the point of emitting light.

Ignoring the uncharged neutrons, atoms contain positive nuclear protons and surrounding negative electrons.

At room temperature, unless a chemical reaction is taking place, everything is electrically balanced, and atoms and molecules are stable, including those of air.

Under these circumstances, air is an electrical insulator BUT ....

However, if there is a powerful electrical field (more on fields in previous section), the very high potential difference causes the atoms and molecules in air to ionise and break up into positive ions and negative electrons.

The electric field is so powerful it removes an outer electron from the atom or molecule - the process  of ionisation.

The formation of ions increases the electrical conductivity of air, so the discharge - flow of current, rapidly accelerates.

The charged particles come together discharging the electrical energy, heating the air and exciting the air particles so much they emit visible light.

The electrons of the excited atoms or molecules fall back down to their original energy levels emitting visible photons of light in the process - so we see a spark of light.

You cannot see a visible spark in a vacuum because there are no atoms or molecules to be excited and ionised to the extent that they will emit energy as visible light photons.

Charge movement or not?

Under normal circumstance the potential difference between the earth and any object is 0V (p.d. of zero volts). However, If electric charge increases on an object the p.d. between the object and the earth rises above 0V. If the potential difference is great enough, electrons can leap across the gap between the charged object and the earth.

The electrons, effectively an electrical current, can jump the gap to any conductor in the vicinity that is earthed (in contact with the ground), though for small p.d. voltages the gap must be small.

e.g. (i) If static charge has built up on an article of clothing, as you remove it the electrons can move through your body to earth via your hands and you feel a 'crackling' shock and maybe see some sparks of light!

(ii) As a car is moving along the friction between the car body and the air can cause the build up of static charge because the car body is insulated from the earth by the rubber tyres. As you get out of the car and touch the metal body the static electricity can be discharged through your body to earth and you experience a small electrical shock!

(iii) The Van de Graaff generator experiments have already been described.

 

In the case of lightning you are dealing with a massive build up of static charge in clouds, and the potential difference between the earth and atmosphere is very large. Consequently lightning strikes ('huge sparks') occur across some pretty big gaps between the earth and clouds with spectacular visual results!

The static charge in clouds is caused by ice particles bumping against each other and becoming charged by friction.

Lighter positive ice particles accumulate at the top of the cloud and heaver negative ice particles collect lower down.

When the number of charged particles increases, at some point, you get a massive discharge of electricity as the oppositely charged particles come together to give a giant spark - a flash of lightning.

Sometimes the clouds of negative particles induce a positive charge on the ground and then discharge takes place from cloud (-ve) to ground (+ve), and this is a lightning strike (diagram on the right).

If a negatively charged thunder cloud passes over, it induces a positive charge on the nearest object on the ground e.g. a church spire lightning conductor, and, on the bottom end of the lightning conductor, a negative charge on the copper earth plate.

The electrons travelling down the copper lightning conductor and rapidly dissipated safely into the ground.

Unfortunately this positive charge builds up on any tall objects nearer the cloud's charge e.g. like tall buildings or trees, and that's why in a thunderstorm you should not take refuge under a tall tree - its nearer than you to the charge cloud!

Being inside a car does protect you because the electrical discharge can run through the car body to earth, but avoid touching the car body until the event has passed!

So, sparks occur when there is a big enough potential difference between a statically electrically charged object and any object that is connected to earth (an 'earthed object').

This high p.d. creates a strong electric field between the two objects ('charged' and 'earthed').

 

In industry, and the home to, you can use insulating mats and using shoes with insulating soles if there is a danger of an electric discharge through your body.

 

If the electric field effect is strong enough, that is if the potential difference is great enough, electrons can be remove from particles in the air, a process called ionisation and positive particles are formed (+ve ions).

Air is a good electrical insulator but the presence of these ions makes it a better conductor so an electrical current can flow.

This can be sometimes be seen as a spark because some of the electrical energy is converted to heat and light energy.

 

I've written more about these examples and how to counter the effects of static electricity in the final pages of this in electricity sections 8.4 and 8.5.

 

If any object can be connected to the 'earth', by e.g. a conductor such as a copper wire or strip, then any build up of static electricity can be safely discharged.

This is called earthing the object.

You can get an electrostatic shock if you are electrically 'charged' yourself and you touch something that is earthed, so the static electricity runs through you.

Similarly if you yourself are earthed and you touch something that is charged, you can also get an electric shock.

That's why household circuits and electrical appliances should be earthed for your safety and protection.