Static Electricity

Question 1

What are the two types of electricity?

Answer 1

Static and current

Question 2

What is static electricity?

Answer 2

Electricity that involves ‘unmoving’ electrons.

Question 3

When a body loses electrons…

Answer 3

…it becomes positively charged.

Question 4

When a body gains electrons…

Answer 4

…it becomes negatively charged.

Question 5

What is charge?

Answer 5

Denoted by Q or q, it is a quantity based on the number of electrons missing or present. It is a scalar quantity measured in coulombs (C).

Question 6

How would you demonstrate repulsion and attraction of charges?

Answer 6

Use a perspex rod, while one is on a watch glass on oil and the others are charged perspex and polythene.

Question 7

What is a conductor?

Answer 7

A medium that allows electric charge to flow through it easily.

Question 8

What is an insulator?

Answer 8

A medium that does not allow electric charge to flow through it easily.

Question 9

What does charging objects by induction mean?

Answer 9

When charging objects by induction, we cause materials to become charged by forcing the electrons to move a certain way.

Question 10

How would you demonstrate the separation of charges in two insulated spheres? / How would you charge two insulated spheres by induction?

Answer 10

Place two insulated spheres in contact, hold a negatively charged rod near one of them, separate the spheres while keeping the rod in place, remove the rod.

Question 11

How would you demonstrate the separation of charges in one insulated sphere? / How would you charge an insulated sphere by induction?

Answer 11

Hold a negatively charged rod near an insulated sphere, earth the sphere (by touching it), remove the earth and remove the rod.

Question 12

What are uses of the gold leaf electroscope?

Answer 12

• To detect and identify charge
• To approximate the size of applied charge
• To test whether a material is an insulator or a conductor

Question 13

How do electrons distribute themselves on a conductor in static electricity?

Answer 13

All static charge resides on the outside of a conductor (discovered by Faraday) and static charge tends to accumulate at the point of a conductor.

Question 14

What are the only environments completely secure from lightning?

Answer 14

Faraday cages

Question 15

What led to the invention of lightning rods and who invented them?

Answer 15

Knowledge about the accumulation of charge at a point, Benjamin Franklin.

Question 16

What is the purpose of a Van de Graaff generator?

Answer 16

It is designed to generate and demonstrate effects of static electricity.

Question 17

How does a Van de Graaff generator work?

Answer 17

• As the bell rotates, metal combs transfer charge to the metal dome. Charge spreads out equally on the outside of the dome. The more the bell rotates, the stronger the charge built up.
• If an earth is brought near the dome, a static discharge will ‘arc’ to it.
• If a person is in contact with the dome, they recieve the charge as it tries to go to earth. As hair is negatively charged, the electrons repel each other and the hair stands up.

Question 18

How would you demonstrate that all static charge resides on the outside of a conductor?

Answer 18

Touch a proof plane on the outside of the dome and test with an electroscope to show it has gained charge. Earth it and repeat on the inside of the dome to show absence of charge.

Question 19

How would you demonstrate that static charge tends to accumulate at a point?

Answer 19

Touch a pear shaped conductor to a charged Van de Graaff generator. Test the rounded and pointed side on a gold leag electroscope.

Question 20

State Coulomb’s law.

Answer 20

The electrostatic force between two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them.

Question 21

What is permittivity?

Answer 21

A measure of the ability of a material to resist the formation of an electric field within it. Measured in farads per metre

Question 22

What are electric field lines?

Answer 22

They show the direction and strength of a force due to a positive charge in the field.

Question 23

How would you demonstrate electric field lines?

Answer 23

Using magnets, paper and iron shavings.

Question 24

What is electric field strength?

Answer 24

The force at any point in an electric field that a 1C charge would experience at that point.

Question 25

What are the uses of static electricity?

Answer 25

• Electrostatic precipitation
• Xerography (photocopying)

Question 26

Explain electrostatic precipitation.

Answer 26

As potentially polluting smoke passes through an industrial chimney, it is charged by an EHT of around 50-60kV. This negatively charges the particles in it. The charged smoke passes a positively charged set of plates. The ionised particles are attracted to the plates and the smoke is released into the atmosphere after being ‘cleaned’.

Question 27

Explain xerography.

Answer 27

As light is incident on the drum in a photocopier, the places at which the light hits lose their charge. This photoelectric effect enables the application of xerography. The drum is negatively charged. The og being copied is placed on the glass plate and light is reflected off it. The non print shape loses charge. The drum is exposed to fine toner dust. This sticks to the print image. The paper rolls over the drum and picks up the part that has toner. The toner is fused to the paper by a pair of heated rollers.

Question 28

What are protections against hazards of static electricity?

Answer 28

• Protecting integrated circuits
• Discharging vehicles before re-fuelling
• Fine dust in industry

Question 29

Explain protecting integrated circuits.

Answer 29

Charge can damage integrated circuits. All electronic engineers have to wear an earthed bracelet to discharge them while working on sensitive equipment.

Question 30

Explain discharging vehicles before re-fuelling.

Answer 30

Ships and planes experience huge friction when travelling. They need to be neutralised before being re-fuelled. Copper strips in the rubber tyres ensure aircraft discharge as they land.

Question 31

Eplain fine dust in industry.

Answer 31

Given the density of airborne dust, a static discharge can ignite the material in the air and cause an explosion. All electric equipment is earthed and no naked flames are allowed to lower explosion chances and fire hazard. Good ventilation is also required.