成人快手

Key points

  • Charged objects can suddenly, forming an electrostatic .

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

  • 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.

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Video - Static electricity

Can you answer these questions based on the video?

  1. What happens when you rub a balloon on a jumper?

  2. How can a charged object be discharged?

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Discharging objects

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 鈥榣eak away鈥 slowly.

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

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

A metal rod being held in the right hand and being rubbed by a cloth held in the left hand.
Figure caption,
A metal rod can be charged by rubbing a cloth against it
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How much charge?

A coulombmeter showing a 0.11 charge

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

A coulombmeter showing a 0.11 charge

The amount of charge is measured coulombs C.

One coulomb is a very large amount of charge.

Three question marks

Did you know?

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

Three question marks

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 鈦宦 C.

Example 2

1 micro coulomb (1 饾渿C) is a millionth of a coulomb.

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

This can be written as:

1 饾渿C = 0.000 001 C

or

1 x 10 鈦烩伓 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 鈦烩伖 C

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Electrostatic sparks

An image of lightning striking a metal conductor rod.
Image caption,
Lightning striking a metal conductor rod. The rapid flow of charges heats the air to reach very high temperatures. The bright flash we see is usually blue, purple or white in colour.

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 鈥檍ump鈥 quickly across the gap between the charged object and the earthed conductor.

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

An image of lightning striking a metal conductor rod.
Image caption,
Lightning striking a metal conductor rod. The rapid flow of charges heats the air to reach very high temperatures. The bright flash we see is usually blue, purple or white in colour.
A spark between a earthed conductor and a charged dome of a Van de Graaf generator
Image caption,
A spark between the earthed conductor (left-hand small sphere) and the charged dome of a Van de Graaf generator.
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How people can become charged

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

There is between the shoes and the carpet. Charging by friction occurs when 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 .

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.

Image gallerySkip image gallerySlide 1 of 3, A man is walking towards a door with this left hand near the metal door handle. The majority of electrons are shown in his left leg which is nearest the door., A person can accumulate electrostatic charge if they scuff their feet as they walk
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Machines that generate static electricity

A Van de Graaff generator creates electrostatic sparks
Image caption,
A Van de Graaff generator (pictured) or a Wimshurst machine can create electrostatic sparks

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.

A Van de Graaff generator creates electrostatic sparks
Image caption,
A Van de Graaff generator (pictured) or a Wimshurst machine can create electrostatic sparks
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Lightning

Lightning strikes over a city

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 掳C.

Lightning strikes over a city
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The balloon experiment

A floating, inflated blue balloon on a string.

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.

Weather conditionsTime for balloon to fall (minutes) 1st readingTime for balloon to fall (minutes) 2nd readingTime for balloon to fall (minutes) 3rd readingMean average
1.
2.
3.
A floating, inflated blue balloon on a string.
  1. Once you have recorded each time taken calculate a mean average. Find out how to calculate a mean average in this Working Scientifically guide.

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

  3. 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?

It is important to make your experiment as fair and accurate as possible. When measuring the effect of weather on the balloon's charge, what steps could you take to ensure the results aren't affected by anything else?

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Test your knowledge

Quiz

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Play the Atomic Labs game! game

Try out practical experiments in this KS3 science game.

Play the Atomic Labs game!
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More on Electricity

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