P2: Static Electricity

When certain insulating materials are rubbed together against eachother they become electrically charged

This is similar to ionic bonding in chemistry, where the material that gains electrons becomes negatively charged and the material that loses electrons becomes positively charged.

When two electrically charged bodies are brought together they exert a force on eachother:

Charges that are the same repel ( + , + ) or ( - , - ) but charges that are opposite attract (+ , - ) or ( - , + )

Electrical charges can move easily through some substances.

Metals allow electrical charges to pass through easily as they contain delocalised electrons which move about freely inside the metal. The delocalised electrons are not confined to one atom.

Insulators can't conduct electricity as all the electrons are held in atoms.

The rate of flow of electrical charge is called the current.

A conductor can only hold charge if it is insulated from the ground. If it is not insulated,the electrons would transfer from the conductor to the ground.

To charge an insulated conductor, it needs to be brought into contact with a charged object:

If the object is positively charged, electrons transfer from the conductor to the object, so the conductor becomes positive as it loses electrons.

If the object is negatively charged electrons transfer to the conductor from the object, so the conductor becomes negatively charged as it gains electrons.

A charged body can be discharged by connecting it to earth with a conductor. Charge then flows through the conductor.

The greater the charge on an isolated body, the greater the potential difference (voltage) between the body and earth.

If the potential difference becomes high enough a spark may jump across the gap between the body and any earthed conductor which is brought near it.

Static electricity can create a spark or make people experience a mild electric shock when certain objects are touched.

Certain materials, including dry human skin, can build up charges which then create a spark when something else is touched.

These can be slightly painful but they are not dangerous.

In some cases static electricity can be very hazardous and precautions need to be taken to ensure that it is safely charged.

Note: You need to be able to state the dangers and precautions taken for your exam

Example: Petrol Stations

When petrol is transferred, either from a truck to the petrol station or from the petrol pumps to cars, a lot of friction is created. A spark could lead to a serious explosion if there was static charge.

So trucks transferring petrol use a grounding device on the hose that draws the electrical charges away from the petrol.

When filling a car with petrol we should turn the engine off, touch something metal when we get out of the car to get rid of any excess charges, avoid using mobile phones, place petrol containers on the ground when filling them up.

Example 2: Refuelling Planes

During refuelling, the fuel gains electrons from the fuel pipe, making the pipe positively charges and the fuel engatively charged.

The resulting voltage can cause a spark which could lead to a major explosion.

To prevent this the fuel tank can be earthed with a copper conductor or the tanker and the plane can be linked with a copper conductor

Example 3: Working on Computers

The internal electronics of a computer can be easily damaged by a spark.

Technicians working on the inside of computers use a special pad on the floor and a grounded ******** their wrist.

This takes away any static charges from their bodies before any damage is done.

Lightning and Buildings

During a thunderstorm, lightning bolts are attracted to the highest point in the area.

To control the force of the electricity, lightning rods are attached to the top of a building (a rod is a piece of metal which runs form the top of the building to the ground)

If lightning does strike, it would hit the metal rod and the current would flow down through the rod into the ground

It would be safely discharged with minimal damage to the building.

Lightning and People

Getting hit by lightning is rare but can cause serious injury and death.

To reduce the chance of lightning during a thunderstorm:

• avoid holding anything metal as this can attract lightning
• try to be inside something that is grounded e.g. a house or car
• avoid standing under a tree during a stomr (it amy be the highest point in the area)

The electrostatic paint sprayer

The nozzle of the spray is connected to a positive terminal

The paint droplets become positively charged as they emerge from the nozzle

Repulsion between the similarly charged droplets keeps the paint as fine spray

The object to be painted (usually a car) is connected to a negative terminal

The paint is attracted to the oppositely charged object, and no paint is wasted with all awkward spots covered too.

The electrostatic precipitator

As the dust/ash particles pass through the first set of grids, they become positively charged

As they pass through the second set (which has a negative charge) they are attracted by the opposite charge, making the ash stick to the negative plates

The cleaner smoke continues up the chimney

Every so often the plates are shaken to remove the ash

The photocopier

The surface of a rotating drum is charged

A bright light is shone on the page to be copied

the light parts of the paper reflect light onto the drum; the dark parts do not

Fine charged powder (toner) is blown across the drum only sticking to the charged parts

A sheet of paper is pressed against the drum and picks up the pattern of the carbon powder which is then fixed in place by a heater

Antistatic Floors - Hospitals

Anaesthetic gases are used by surgeons during operations. Some of these gases are explosive and if they escape into the air there could be an explosion.

To eliminate static charge an antistatic material is used for the floor surface.

This material is a poor electrical insulator and so it conducts the charge to Earth.