Chapter 16: Static Electricity Flashcards
State the amount of negative charge that is carried by an electron.
1.6 x 10^-19 C
Define conductors.
Conductors are materials that allow electric charges to flow through them easily. The electrons are held loosely to their atoms, hence charged particles are free to move about.
Define insulators.
Insulators do not allow electric charges to flow through easily. the electrons are held tightly and are not free to move, hence charged particles are not free to move about.
State and explain the respective methods for charging and discharging an insulator.
1) Electrostatic charging by friction
- When two different materials are rubbed together, negative charges (electrons) will transfer from one object to the other. (important: state direction of travel! only electrons can move.)
- One object will be positively charged while the other will be negatively charged.
2) Discharging an insulator through candle flame
- Bring the insulator near a candle flame
- Intense heat from the flame ionises the nearby air particles. Ionised air: pool of positive and negative charges
- These ions will neutralise the excess charges on the glass rod and discharge it.
State the respective methods for charging and discharging a conductor.
1) Charging
- Induction: the process of charging a conductor without contact between the conductor and the charging body
- Earthing
2) Discharging
- Earthing
Define electric field.
A region where an electric charge experiences an electric force.
Define direction of the field.
The direction of the force that would act on a small positive charge.
How is the strength of an electric field represented?
It is indicated by how close the field lines are to one another.
Hazards and Applications of Electrostatics - Lightning
Describe how lightning forms and how lightning conductors work.
1) A thundercloud becomes electrically charged due to friction between water molecules in the thundercloud and air particles in the atmosphere.
2) Negative charges tend to gather at the bottom of the cloud.
3) The negatively-charged underside of the cloud repels the electrons near the surface of the Earth. The surface of the Earth becomes positively charged.
4) When the accumulation of charges is large enough, it can ionise the air particles nearby. The ionised air particles provide a conducting path for the electrons in the cloud to reach the Earth. Electrons that follow this discharge path to the Earth form lightning.
5) Lightning tends to strike tall or isolated structures. Thus, lightning conductors protect tall buildings, by providing a conducting path for electrons in the air to flow from the top of a tall building to the Earth. This prevents lightning from damaging tall buildings.
Hazards and Applications of Electrostatics - Electrostatic discharge
Describe and explain why vehicles which carry fuel can pose as an electrostatic hazard.
1) Electric charges can accumulate on trucks due to friction between the road and the rotating tyres of the trucks.
2) Tires acquire negative charges, while the parts of the metal body of the truck become positively charged.
3) Sudden discharges (sparks) may occur, which may ignite flammable fumes (or vapour) that the vehicle is carrying.
Describe the process of photocopying with reference to practical applications of electrostatics.
1) Inside the photocopier, there is a metal drum, which is coated with selenium (photoconductor, i.e. only conducts electricity in the presence of light.)
2) The drum is charged positively on its entire surface. This is done by rotating the drum near a highly-charged wire.
3) The original image to be photocopied is placed on a sheet of clear glass above the drum.
4) An intense light beam is shone on the image.
5) The darker areas of the image reflect less light. The corresponding regions on the drum remain insulating, and the positive charges remain on the surface of the drum.
6) The lighter areas of the image reflect more light onto the drum below. These areas of the drum become conducting and are discharged.
7) As the drum continues rolling, the positively-charged image on the drum attracts the negatively charged toner powder in the toner cartridge.
8) A positively-charged sheet of paper is passed over the drum surface. Negatively-charged toner is attracted away from the drum and onto the sheet of paper.
9) The sheet of paper is heated and pressed. This fuses the toner to the sheet of paper permanently.
Describe the process of removing fly ash in electrostatic precipitators.
1) The waste gases and fly ash are passed through a negatively-charged wire grid. This makes the fly ash particles negatively charged.
2) The fly ash particles are then routed past metal plates that are positively charged or earthed.
3) The metal plates attract the negatively-charged fly ash particles, so the gases leaving the plate are free of fly ash particles.
Describe the process of electrostatic spray painting.
1) Paint droplet becomes charged when rubbed against the nozzle of the spray
2) Car body earthed/ has opposite charge
3) Paint droplets attached to metal body
4) All droplets have same charge, so they repel each other
5) Only 1 droplet attracted to each point on the body → once it is neutralised, other drops will not be attracted → results in even coating
