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Section 8 Static electricity: Part 8.1 Introduction and making and simply demonstrating static electricity - a tale of friction between materials !!!

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INDEX of physics notes on static electricity, electrostatics & electric fields


8.1A Introduction and making and simply demonstrating static electricity - a tale of friction between materials!!!

 This page, I hope, will answer many questions on static electricity e.g.

 What is static electricity?    What is an electric field?

 How can we produce examples of static electricity?

 Where in the house or everyday life do you encounter examples of static electricity?

 Can static electricity be dangerous?   Can static electricity be useful?

All matter is made up of atoms, which in turn are made of a nucleus of positive protons and neutral neutrons and a surrounding cloud of negative electrons in their specific energy levels ('shells').

Most of matter, most of the time, contains equal numbers of positive and negative charges so there is net charge of zero and their effects cancel out.

However in some situations, you can get a build up of negative or positive charge that cannot flow away like a normal electric current to restore the 'local' zero balance. This is what we call static electricity.

If a material readily allows the passage of an electrical current, it is called a conductor.

If a material is poor at conducting an electrical current, it is referred to as an insulator.

Remember that electrons carry a negative electrical charge (-).

'Static effects'

We've all noticed at some time the 'crackling' or tiny spark effect when taking off or putting on an article of clothing, particularly with nylon materials, which can also give a similar effect when used as bed clothes.

Well, these are examples of what we call 'static electricity' and you may even experience a small electric shock as the static electricity is discharged - through you!

The crackling is the discharge of tiny pockets of static electricity produced by the friction of clothes or bed clothes rubbing against each other.

The electric charge flow, the current, is so small it is highly unlikely to cause you any harm, but you might experience a little shock - a little prick of pain!

Notice that the materials involved like nylon, are electrical insulators, not metals, so what is going on?

What is static electricity?

Static electricity is a build up of electric charge that cannot move or flow as a normal electric current in a conducting material.

Therefore static electricity usually forms in or on insulating materials like plastics (polymers) and glass.

Any static charge produced on an electrically insulating material cannot immediately flow away,

- unless a conductor is brought near the material to discharge the static electricity,

- or it discharges, heating and ionising air, causing a spark if the potential difference ('voltage') becomes high enough.

A static electricity spark is an electrostatic discharge caused by the sudden flow of an electric current across an air gap. This heats up and ionises the air, causing light to be emitted. The size of the spark depends on the distance between the (+ and -) sources of electrical charge and the potential difference in voltage between them. An extreme example is lightning, with a p.d. of up to a billion volts (p.d. of 108 V !!!)

The heating causes a rapid expansion and vibration of the air causing the sound of a thunder clap.
 

Static charge is produced when two electrically insulating surfaces are rubbed together.

The static charge is results from the movement of electrons due to the friction effect when two insulating material surfaces rub against each other.

This produces an excess of electrons (negative area of static charge) in one material and deficiency of electrons (positive area of static charge) in the other material.

Note that the positive charge does NOT move, only the negative electrons move in the transfer of charge.

The excess of positive charge created will be numerically equal to the excess of negative charge created.

The question of which material is positive and the other negative, from the direction of electron transfer, depends on the chemical constitution of the two materials.

If the electrons can 'leap' back from a negative region to a positive region, the static electricity is discharged and you see a spark or get a tiny electric shock!


8.1B Simple demonstration and explanation of static electricity

 - creating an electrostatic energy store that can do work by picking up bits of paper!

charging a plastic rod with static electricity by rubbing with a cloth

You cannot produce static electricity by rubbing a metal rod with a cloth because any static electricity formed would immediately run through the rod into your body or vice versa! However, you can demonstrate static electricity in a very simple way by rubbing a plastic rod with a clean dry cloth and getting to pick up little pieces of paper in which a charge is induced by the electric field of the rod.

You need a bone dry cloth because water, although a very poor electrical conductor (due to minute concentrations of H+ and OH- ions), any static charge would drain away.

I used a polystyrene biro case and a small duster cloth used for cleaning my camera lenses (illustrated above).

The duster must be dry so that any static charge created cannot drain away - water is a very poor conductor, but it does conduct a little!

Image 1a and Image 1b illustrate the two possibilities of creating static charge by removing electrons with a force of friction acting between the plastic surface and cloth surface,

but, depending on the two plastic and cloth materials involved, the electrons can move one way or another between them.

Image 1a assumes negative electrons are rubbed off by the cloth making it negative and the plastic rod becomes positive (deficient in negative charge - electrons). This tends to happen with acetate plastic rods.

Image 2a assumes negative electrons are rubbed onto the plastic rod by the cloth making it negative and the cloth becomes positive (deficient in negative charge - electrons). This tends to happen with polythene plastic rods.

I don't know which situation applies to this rod and cloth combination, but that's not the point here. The point is that only one of two possible static electricity creation situations can result from the rubbing of two insulating surfaces together and the resulting electron transfer from the force of friction.

The excess of positive charge created is numerically equal to the excess of negative charge created, I've marked 8 + and 8 - on each image!

In reality we are dealing with many electrons, not just the transfer of 8 of them, but it simplifies the explanation picture!

The direction of electron transfer depends on the nature of the two materials.

You charge plastic rods of cellulose acetate, Perspex and polythene by rubbing them with a dry dusting cloth.

Polythene becomes negatively charged - gains electrons, so the duster becomes positively charged - lost electrons.

Perspex and cellulose acetate becomes positively charged - the plastic loses electrons, so the duster becomes negatively charged - gained electrons.

You can then suspend the rods, finely balanced with a fine thread and are perform simple attraction and repulsion experiments.

e.g. you should be able to demonstrate that:

two rods of the same plastic repel - like charges repel,

polythene rod (-) should attract either a Perspex rod (+) or an acetate rod (+), unlike charges attract,

a Perspex rod should repel a cellulose acetate rod.

attraction and repulsion rules of static electricity positive and negative charge effects

Think of the circles as a cross section of the plastic rod and the + and - signs as the static charge on the surface, (+) from electron loss and (-) from electron gain.


8.1C Three more simple demonstration methods of detecting static electricity on a charged plastic rod

(1) Getting the plastic rod to pick up tiny bits of paper.

 I used a few bits of very fibrous kitchen role (a large surface area which helps) and a biro case as the plastic rod.

It works even better with a more solid rod of poly(ethene) ('polythene') or Perspex.

Image 2a: The plastic rod (biro) is charged by rubbing in vigorously with a clean dry cloth to charge it up with static electricity.

Image 2b: If the charged rod is carefully held close to some small bits of uncharged paper, they are picked up and sometimes as a string.

The charge on the rod induces the opposite charge on a bit of paper by pulling or pushing electrons on the paper surface.

The opposite charges then attract each other, so the bits of paper are attracted to the rod.

The effect can be transmitted to a second bit of paper giving a little paper chain held together by static charges.

The plastic rod - paper chain will hold together because both materials are electrical insulators and the electrical charge is slow to drain away.

You need to imagine the following:

either: plastic - ... + paper bit - ... + paper bit -

the negative plastic rod repels the electrons inducing a positive charge on the paper surface

or: plastic + ... - paper bit + ... - paper bit +, where the positive plastic rod attracts the electrons on the paper surface

where the dotted line ... represents the electric field attractive force.

This is actually a simple test to show the plastic rod was charged up with static electricity.

 

(2) Another simple demonstration of an electric field effect

A simple kitchen sink experiment!

A simple 'kitchen sink' experiment that clearly shows the effect of an electric field.

deflecting a stream of water with a charged plastic rod of static electricity

Image 3a: You get the thinnest continuous stream of water descending from a tap.

Image 3b: You then charge up the plastic rod (biro) and carefully hold it quite close to the water stream, BUT not touching (this discharges the static charge into the water). The plastic rod is to the right of the stream. The electric field around the plastic rod attracts the water molecules attracting the water stream towards the plastic rod.

Technical points of explanation (its more A level chemistry, but no matter!):

Water is an neutral covalent molecule (not ionic).

However, water is known as a 'polar molecule' and one end is naturally slightly positive and the other end slightly negative (electrically neutral overall).

It doesn't matter what the sign of the charge is on the plastic rod.

If the rod is -ve it attracts the +ve end of the water molecules, if the rod is +ve it attracts the -ve end of the water molecule.

Therefore whatever the rod's static charge sign, the stream of water molecules is attracted towards the statically charged rod.

Again, this is a simple test to show that the plastic rod is charged and it doesn't matter whether it holds a positive or negative charge.

The deflection shows up better as the shadow on the wall.

 

(3) The gold leaf electroscope

how a gold leaf electroscope works to detect static electricity How a gold leaf electroscope works

The gold leaf electroscope consists of a zinc plate mounted in an insulated wooden box with windows (so you see what happens!).

Attached to the stem of the zinc plate is a thin sheet of gold (gold leaf).

Both metals should not be carrying a static charge - you ensure any charge has been discharged by touching the top with any conducting rod of metal.

An insulating plastic rod is charged up by rubbing with a dry duster cloth.

Let us assume it carries an excess of electrons i.e. the plastic rod carries a negative static charge.

If the plastic rod is touched onto the zinc plate, you get a charge transfer and some electrons will flow down into the conducting zinc plate AND the equally conducting gold leaf.

Therefore both the zinc rod and the gold leaf have the same negative charge and will repel each other (- -), so the gold leaf moves apart from the zinc rod and curls up.

If you then touch the zinc plate with any other metal rod, the electrons will flow into it and the static electricity is then discharged through your hand and the gold leaf falls back against the zinc rod. This might not work if you are wearing rubber gloves!

Repeat experiment with another plastic rod of different charge:

If the plastic rod is positively charged with static electricity, electrons will drain from both metal strips making them both  positive, repulsion should take place i.e. the gold leaf will be repelled from the zinc surface.

You can also touch the 'uncharged' zinc plate with a positively charged insulator and this time the electrons will flow onto it from the plate making both metals positive. Therefore the gold leaf will rise due to the like charge repulsion (+ +). Look at the diagram and just imagine swapping the -ve signs for positive signs, its the same effect in the end.

Note: Because the zinc and gold leaf are themselves insulated from the 'earth', in this case, you can have an electrically conducting material that will hold a static electric charge. In an exam, never say a metal plate can never hold a static charge!

 

Finally - The spectacular Van de Graaff generator experiment

A Van de Graaff machine involves an insulating belt continuously rubbing over copper contact brushes which are connected to a steel dome.

The friction causes static charge to build up in the belt and metal contacts.

The steel dome, despite being a good electrical conductor, is insulated by the surrounding air (up to a point!).

Therefore there is no complete circuit and static electricity can build up.

The static charge gradually builds up as the machine runs and can be spectacularly released if another conducting metal sphere is brought near the steel dome,

BUT, if you hold the sphere without rubber gloves, the static electricity discharges through you with a spark that jumps across!

BUT, don't worry, although the p.d. voltage is high, the current is very small and harmless.

The potential difference between the sphere and dome is so great that the static charge discharges, jumping through the air to the sphere - the p.d. is so great the hair is heated up and ionises at high temperature, causing a discharge of light energy and sound energy too.

However, before switching on the Van de Graaff generator, you place your hands on the sphere and then switch the machine on.

You need to be insulated by standing on a thick plastic or rubber mat - otherwise the charge leaks away.

The resulting static charge builds up in your body, including your hair.

The hairs on your head have the same positive or negative static charge and so they repel, giving you an impressive 'spaced out' and startling hair style !!!

See later section for full explanation of why you see a spark.

INDEX physics notes on static electricity - electrostatics & electric fields


Keywords, phrases and learning objectives on static electricity

Be able to describe and explain, with the aid of diagrams, what causes static electricity.

Explain how friction produces static electrically charged objects.

Be able to describe simple demonstrations of static electricity with plastic rods rubbed with a cloth to pick up  bits of paper, bending a stream of water.

Given a diagram, explain how gold leaf electroscope detects electric charge.

Appreciate a Van de Graaff generator produces a great charge flow build up to produce very voltage discharges.


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