GCSE Science/Advanced static electricity topics
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Where charge concentrates
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| The charge spreads out on this uneven conductor unevenly. The two electrons that are circled in red cannot see each other. The body of the conductor gets in the way and shields them. This is why the charges congregate around the highly curved areas. |
When an object is charged up with electrons, the electrons try to spread out over the object to be as far apart as possible. This means they go to the surface rather than spread throughout. On a metal object the material of which the object is made shields the electrons from one another. They do not ‘’see’’ the electrons round the back. This means that highly curved objects can hold more electrons than flat objects. For a complicated shape, the electrons tend to congregate on the more highly curved areas, and to desert the flatter areas.
Inducing charge separation on neutral objects
[edit | edit source]Consider a crystal of gold for example. {This argument works for all materials}. Normally the atoms are perfectly spherical, and completely neutral.
Q1) Why are the atoms neutral?
Remember that neutral atoms contain positive charges in the nucleus and negative electrons orbiting in their shell. Normally the center of the positive charges and the center of the negative electrons is in the same place: the exact center of the atom. If we bring up a charged object to them however, something important happens. Look at the diagram on the right.
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A charged rod is brought near the surface of the crystal. {Note: the rod is not shown, just the charges on it}. The electrons in the atoms try to get away from the negative charge because they are repelled. The positive charges in the nucleus try to get nearer to the negative rod because they are attracted to it. The result is the atom slightly changes shape. The center of the negative charges goes to the right of the center of the atom and the center of the positive charges goes to the left. We say there is a separation of charges
Q2) What would happen if we brought up a positively charged rod?
There is always an attractive force between the rod and the crystal no matter what the charge on the rod.
To see why this is so, we need to know one more thing about electrostatics. The force of repulsion or attraction falls off with distance. In fact, rather like gravity it falls off as the square of the distance. This means if you go to twice the distance the force becomes only a quarter as much. If you go three times the distance it becomes only one ninth as great and so on. Now look at the diagram again. The negative charges in the atoms are repelled by the rod, the positive ones are attracted, but the positive charges are closer, so overall there is a force of attraction.
Q3) Repeat the above argument with a positively charged rod to convince yourself that there is still an attractive force.
Lightning
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| Time-lapse photography captures multiple cloud-to-ground lightning strikes during a night-time thunderstorm. |
Lightning is basically a great big spark. In a thunderstorm, separation of charge occurs in big rain clouds. No one is really sure of how this occurs, although it appears to be caused by ice crystals rubbing together and becoming charged. but what is known is that storm clouds have a negative bottom and a positive top. The bottom of the cloud is nearer to the ground so it induces a positive charge on the ground. When the voltage becomes high enough a spark flies between the cloud and the earth; this is lightning. The spark is so hot, it causes the air to rapidly expand then collapse; this causes thunder.
Q4) Why does the voltage need to be very high before lightning can occur?
Lightning rods
[edit | edit source]Lightning rods are used to protect buildings from lightning strikes. They are usually made from a length of copper rod with a pointed end. They are attached to tall buildings and the lightning strikes them rather than the building itself.
Q5) Why do you think the rods are made of copper rather than iron which is cheaper?
Q6) Why do the rods have a pointed end?
Q7) It is standard advice that if caught out in a storm you should never stand under a lone tree. Why is this advice good?
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Summary
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