Electrostatic sparks and shocks

• Charged objects can discharge suddenly, forming an electrostatic spark.

• Electrostatic sparks can happen when a charged object is brought close to an earthed conductor.

• Normally electrostatic sparks are very small. If the object has a lot of charge, then these sparks can be hundreds of metres long.

• Sparks can occur between charged objects and people.

Objects can be charged by friction.

Over time, the charge that builds up on an object will 'leak away' as the charges are transferred through the air. When this happens it means the object has discharged.

Air is not a good conductor so the charges ‘leak away’ slowly.

The rate at which an object becomes discharged depends on a range of factors including humidity, air pressure and temperature.

Find out how objects can discharge by trying the balloon experiment further on in this guide.

A coulombmeter shows the amount of charge, and whether it has a positive or negative charge.

The amount of charge is measured coulombs C.

One coulomb is a very large amount of charge.

Did you know?

It would take approximately 6,000,000,000,000,000,000 electrons to have a charge of -1 C.

We often need to use much smaller amounts of charge than 1 Coulomb

Example 1

1 milli coulomb (1 mC ) is a thousandth of a coulomb.

There are 1000 milli coulombs in 1 coulomb.

1 mC = 0.001 C

or

1 x 10 ⁻³ C.

Example 2

1 micro coulomb (1 𝜇C) is a millionth of a coulomb.

There are 1,000,000 micro coulombs in 1 coulomb.

This can be written as:

1 𝜇C = 0.000 001 C

or

1 x 10 ⁻⁶ C

Example 3

1 nano coulomb (1 nC) is a billionth of a coulomb.

This can be written as:

There are 1,000,000,000 nano coulombs in 1 coulomb. So a nano coulomb is a very small amount of charge.

1 nC = 0.000 000 001 C

or

1 x 10 ⁻⁹ C

When an object has enough charge, a spark can occur. This happens when the following three events occur:

1. The earthed conductor approaches the charged object.

2. Charges ’jump’ quickly across the gap between the charged object and the earthed conductor.

3. The object discharges in a fraction of a second.

People sometimes become charged when they scuff their feet on a carpeted floor as they walk.

There is friction between the shoes and the carpet. Charging by friction occurs when insulators are rubbed together.

Electrons are transferred from one object to the other. For example, electrons transfer from the carpet to the shoe.

The shoe has gained electrons and so becomes negatively charged. The carpet has lost electrons and so becomes positively charged.

The excess negative charges on the shoe are able to spread through the body because the body is a conductor.

When the person moves near to an earthed conductor, like a metal door handle, the electrons jump to the conductor. This is a spark. The greater the charge build up on the person, the further the spark can jump.

If the person touches the conductor, any remaining charge in their body flows to the ground and they become discharged. This process is called earthing - the charged person or object has been earthed.

Look through the slide show below to see how electrons move from a charged person to a door handle.

Machines designed to generate static electricity, like Van de Graaff generators or Wimshurst machines can produce large electrostatic sparks. These sparks can reach around 10cm in length.

Lightning bolts are an example of a very large electrostatic spark and they transfer enormous amounts of charge.

The sparks that jump between the thundercloud and the ground are typically around 3 to 5 km long and the air locally can reach temperatures of around 30 000 °C.

Try this experiment at home to find out how objects can discharge, as the charge 'leaks away'.

1. Inflate a balloon.

2. Charge the balloon by rubbing it vigorously against your hair or a jumper.

3. Put the rubbed surface of the balloon against a wall - the electrostatic force should keep it there. If not then charge it more by rubbing it on your hair or jumper and then try again.

4. Measure the time taken for the balloon to fall from the wall. You can use a watch or a stop-watch (eg on a phone) to do this. Be patient! It is sometimes possible to get a balloon to stay on the wall for several hours.

5. Write down your result in a table.

6. Repeat the experiment to get three readings.

7. Once you have recorded each time taken calculate a mean average. Find out how to calculate a mean average in this Working Scientifically guide.

8. The whole experiment can then be done again on a different day when the weather has changed.

9. Look at the times you recorded. Can you see a link between the weather conditions and the amount of time taken for the balloon to fall?