Electrostatics
Senior 1 · Physics
Electrostatics
Lesson 5 of 5
25 min
In lesson 1, a metal spoon rubbed with wool while held in the hand did not pick up any paper. Hold the same spoon by a plastic handle and rub it again, and it does pick up paper. The spoon WAS charged by the rubbing the first time, but the charge escaped at once through the metal and through your body into the ground. Metals let charge move through them; plastics do not.
| Material touched on the cap | What the leaf does | Conclusion |
|---|---|---|
| copper wire, aluminium foil, iron nail | falls at once | good conductor |
| pencil lead (graphite) | falls quickly | conductor |
| damp paper, damp cotton thread | falls slowly | poor conductor |
| dry wood | falls very slowly | very poor conductor |
| plastic ruler, glass rod, rubber, dry cloth | stays open | insulator |
Through a conductor, the charge on the electroscope escapes through the material and through your body to earth.
Definition
A conductor is a material that allows electric charge to flow through it easily, because it contains free electrons (or, in liquids, ions) that can move. Examples: metals, graphite, salty water, the human body.
An insulator is a material that does not allow charge to flow through it, because its electrons are held tightly by their atoms. Examples: plastic, rubber, glass, dry wood, dry air.
Figure 5.1: In a metal, the outer electron of each atom is free to wander; in an insulator, every electron stays with its own atom.
Charge given to a conductor spreads over its whole surface at once, because the free electrons push each other as far apart as they can. On an insulator, charge stays where it was put: that is why only the RUBBED end of a polythene rod is charged, and why insulators are the materials we charge by friction.
| Conductors | Insulators | |
|---|---|---|
| Examples | silver, copper, aluminium, iron, graphite, salty or impure water, the human body, the ground | plastic (PVC), rubber, glass, porcelain, dry wood, dry cloth, dry air, oil |
| Electrons | many free electrons | almost no free electrons |
| Where charge sits | spreads over the whole surface | stays where it is placed |
| Can you charge it by rubbing while holding it? | no: the charge flows away through you | yes |
| Uses | wires, plug pins, lightning conductors, earthing straps | covers of cables, plugs and switches; tool handles; the white porcelain or glass insulators on power poles |
Between the two are semiconductors such as silicon and germanium, which conduct a little. They are used to make the chips in phones and the solar panels on roofs.
Definition
Earthing (grounding) a charged conductor means connecting it to the ground with a conductor. The Earth is so large that it can give or take any number of electrons without changing its own charge, so the object becomes neutral.
Worked example 1: earthing a charged sphere
A metal sphere on an insulating stand has a charge of . A wire joins it to earth. Which way do electrons flow, and how many?
The sphere is short of electrons, so electrons flow from the earth into the sphere until it is neutral:
Worked example 2: why is a fuel tanker earthed?
A fuel tanker brings petrol along the road to Kigali. Before the petrol is pumped into a filling station's tank, a worker clips a wire from the tanker to an earthing point. Explain why.
Warning
Common mistakes. (1) Thinking that insulators have no charges in them: they have as many electrons as conductors, but the electrons cannot move freely. (2) Saying that positive charge flows to earth from a positive object: electrons flow the other way, from the earth into the object. (3) Forgetting that water, the human body and damp wood conduct.
Inside a tall thundercloud, strong air currents carry water drops, hail and ice crystals up and down, and they collide millions of times. The collisions charge them by friction: the lighter ice crystals carried to the top of the cloud become positive, and the heavier hail pellets near the base become negative. The negative base induces a positive charge on the ground and on trees and buildings below. Air is normally an insulator, but when the charge becomes large enough the air breaks down and conducts: a giant spark, lightning, carries the charge between cloud and ground in a fraction of a second. It heats the air in its path to about , and the air expands so violently that we hear thunder.
Figure 5.2: A lightning conductor. If lightning strikes the building, the charge runs down the thick copper strip into the ground instead of through the walls.
A lightning conductor is a pointed metal rod fixed above the highest part of a building and joined by a thick copper strip to a metal plate buried in damp ground. The point collects a large induced charge, so a flash is most likely to hit the rod; the copper strip, a good conductor, then carries the charge safely to earth. Schools, churches, mosques, telecommunication masts and tall buildings in Kigali all need them.
Warning
Lightning safety. Rwanda has frequent thunderstorms in its two rainy seasons (March to May and September to December), and lightning kills and injures people every year. When you hear thunder:
Important
Static sparks can start fires. A tiny spark can ignite petrol vapour, gas, or fine flour and grain dust in a mill. That is why smoking is forbidden at filling stations and why fuel tankers and aircraft are earthed before refuelling.
Tip
Metals and water conduct; plastic, rubber, glass and dry air insulate. The Earth is the biggest conductor of all: connecting to it "empties" any charged conductor.
What causes lightning? (NOAA SciJinks, from the US National Oceanic and Atmospheric Administration). Watch how collisions inside the cloud separate the charges, and how the positive charge on the ground meets the leader coming down from the cloud.
Practice
Key points
Further reading
Sources and credits
The experiment write-up, the diagrams, the worked examples, the lightning-safety guidance and the practice exercises are written by Ganzaa.
