# Static Electricity

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## Static Electricity

Static electricity is caused by a build up of electric charges that are not free to move. This can often result in an electric shock or a spark when the built-up charges eventually move.

## Build-up of Static

When two insulating materials are rubbed against each other, electrons move from one material onto the other. This causes one material to be positively charged because it has lost electrons. The other material is negatively charged because it has gained electrons. The direction that the electrons move depends on the materials.

One example is to rub a cloth on a polythene rod. This is shown in the image on the right. Electrons move from the cloth to rod, causing the end of the rod to be negatively charged and the cloth to become positively charged.

Recall that positive and negative charges attract. This is known as electrostatic attraction and it is a non-contact force. This means that the positively charged cloth and the negatively charged polyethene rod attract one another. The closer they are together, the  stronger the force of attraction between them. Also, if you used two cloths to rub the polythene rod, the cloths would both be positively charged and hence would electrostatically repel one another.

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## Sparking

As electric charge builds up on a material, the potential difference between the object and the Earth (which has potential difference of $0\text{ V}$ increases. If the potential difference gets big enough, electrons may jump across the gap to the Earth – this is called a spark

Sparks can also jump to an earthed conductor, which is a conducting material connected to the Earth. This is why during a thunderstorm you should stay away from tall metal objects such as electricity pylons. Lightning is an example of very large spark caused by a potential difference between charged storm clouds and the Earth. The electrons from the clouds may jump to a metal object such as pylons causing a very large current.

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## Electric Fields

Electric fields are created by electrically charged objects. The closer you get to the object the stronger the electric field.

Electric fields are shown using field lines. Field lines always flow from positive to negative electrical charge. The more electric field lines in an area, the stronger the electric field.

When an object is placed in an electric field, it experiences a force. This force is caused by the interaction of the electric field of the object and the electric field it has been placed in. This may cause attraction or repulsion. The stronger the electric field, the greater the force.

We can also explain the production of sparks using electric fields. A high potential difference between a charged object and the Earth produces a strong electric field. This electric field removes electrons from the air within the electric field. This is called ionisation. The ionised air becomes an electrical conductor and so a large current may flow through it. This current is the spark.

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## Static Electricity Example Questions

A build up of electrical charges that are not free to move

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The acetate rod is positively charged because it has lost electrons

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Repel.

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B.

The field lines are closer together, this represents a stronger electric field.

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When charged particles build up on an object, this creates a large potential difference between the object and the Earth. This potential difference produces a strong electric field.  This electric field removes electrons from the air within the electric field. This is called ionisation. The ionised air becomes an electrical conductor and so a large current may flow through it. This current is the spark.

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