Static Electricity

Question 1

Define electrostatic charge

Answer 1

The study of electric charges which are not moving

Question 2

2 types of electrostatic charge

Answer 2

Positive charges – consists of protons and positive ions, formed by atoms losing electrons
Negative charges – consists of electrons and negative ions, formed by atoms gaining electrons

Question 3

S.I. unit of charge

Answer 3

Coulomb (C)

Question 4

Charge carried by protons and electrons

Answer 4

Protons: 1.6 x 10^-19 C
Electrons: -1.6 x 10^-19 C

Question 5

Number of electrons in 1C of charge

Answer 5

1/1.6 x 10^-19 C = 6.25 x 10^18

Question 6

Law of electrostatics

Answer 6

Like charges repel and unlike charges attract

Question 7

The force of attraction/repulsion increases when distance between charges ___________?

Answer 7

Decrease

Question 8

What interaction between a specimen and a charged object is a sure test to conclude that the specimen is charged (not neutral)?

Answer 8

Repulsion only. The specimen should have a similar charge as the object.

Question 9

Factors that affect the strength of forces between 2 objects

Answer 9

1. Amount of charge in both objects
2. Distance between the centres of both objects

Question 10

Define electrical conductors

Answer 10

Electrical conductors allow electric charges to flow through them easily.

Question 11

Examples of electrical conductors

Answer 11

Metals, acids, graphite, (impure) water, salt solutions and ionized air

Question 12

Why are metals good electric conductors

Answer 12

Metals are good electric conductors due to the presence of free electrons which can move freely when there is an electric force.

Question 13

Can positive charges move in a metal conductor?

Answer 13

No, because protons are found tightly bounded within the nucleus of the atoms.

Question 14

Define electrical insulators

Answer 14

Electrical insulators do not allow electric charges to flow through them easily. The electrons are tightly bound to their own atoms and not free to move between atoms

Question 15

Examples of electrical insulators

Answer 15

Rubber, glass, plastic.

Question 16

Can positive charges move in a metal conductor?

Answer 16

No

Question 17

Define electric field

Answer 17

An electric field is a region where an electric charge experiences an electric force.

Question 18

What is the direction of the electric field?

Answer 18

It is the direction of how a positive test charge would move.

Question 19

A stronger electric field is denoted as having more/less lines of force which are drawn closer/further to/away each other?

Answer 19

More, closer to

Question 20

Electrostatic charging by friction (when rubbing an amber rod with fur)

Answer 20

1. When 2 different materials, especially insulating materials are rubbed together, negative charges (electrons) are transferred from one object to another.
2. Electrons from fur are deposited onto amber when fur rubs amber rod.
3. The material that loses electrons will become positively charged while the material that gains electrons will be negatively charged
4. Amber becomes negatively charged while fur becomes positively charged

Question 21

Define electron affinity

Answer 21

The tendency for an atom to acquire/gain electrons

Question 22

What is the triboelectric series?

Answer 22

Different substances with different electron affinities are arranged according to their affinity for electrons. This is known as the triboelectric series

Question 23

Negative affinity of a material means that it will most probably be a receiver/donor of electrons

Answer 23

Receiver

Question 24

Positive affinity of a material means that it will most probably be a receiver/donor of electrons

Answer 24

Donor

Question 25

Common materials that can be electrostatically charged by friction (negatively charged)

Answer 25

- Polythene (rubbed with wool) - Ebonite/amber/rubber (rubbed with fur)

Question 26

Common materials that can be electrostatically charged by friction (positively charged)

Answer 26

- Perspex (rubbed with wool) - Glass (rubbed with silk)

Question 27

How to neutralize electric charges for insulators?

Answer 27

Pass the apparatus quickly through a Bunsen flame

Question 28

How to neutralize electric charges for conductors?

Answer 28

Touch the apparatus to allow excess negative charges (electrons) to flow through our body to the ground. This process is called earthing.

Question 29

Process of charging by induction (for conductors of electricity) - charging 2 neutral insulated metal spheres

Answer 29

1. 2 neutral metal spheres mounted on insulating stands, P and Q are placed touching each other. 2. A negative polythene strip is brought near to P. Electrons from P are repelled away from the strip and move into Q, leaving behind excess positive charges on the end of P nearer to the negatively charged strip. 3. Sphere Q is then separated from P while the negatively-charged polythene strip is kept in position 4. Polythene strip is removed. Sphere P becomes positively charged while sphere Q becomes negatively charged. Since spheres P and Q are conductors, they have equal amounts of opposite charges which are redistributed throughout P and Q.

Question 30

Use of insulating stand

Answer 30

It is to prevent excess charges from flowing down to earth or flow from earth to neutralise the metal sphere

Question 31

Advantage of charge by induction

Answer 31

Net charge of original charged object remains unchanged

Question 32

Is it possible to charge 2 metal spheres by rubbing

Answer 32

No, because they are connected to earth by our hands, excess charges on conductor in hands will flow and conductor will be neutralised, excess negative charges would flow from one conductor to another such that both are neutralised

Question 33

Is it possible to charge an insulator by induction?

Answer 33

No, because only surface electrons can move through a small distance, insulator remains overall neutral as the negative charges cannot leave the surface.

Question 34

Process of charging by induction (for conductors of electricity) - charging a single insulated metal sphere to obtain negative charges

Answer 34

1. Positively-charged acetate strip is brought near a neutral conductor (metal sphere) mounted on an insulating stand. (Electrons of the conducting sphere are attracted to the positively charged strip at the end, leaving behind positive charges at the other side of the conductor.) 2. The conductor is then earthed by touching it with a finger. (As the negative charges are attracted to the positive charges of the strip, this allows electrons to flow up the earth to neutralize the positive charges) 3. Finger is then removed from the conductor 4. Acetate strip is then removed and there is a re-distribution of excess negative charges on the sphere due to law of electrostatics and the sphere is left negatively charged.

Question 35

Process of charging by induction (for conductors of electricity) - charging a single insulated metal sphere to obtain positive charges

Answer 35

1. Negatively charged polythene strip is brought near a neutral conductor mounted on an insulating stand. Electrons of the conducting sphere are repelled away from the negatively-charged strip, leaving negative charges at the other side of the conductor 2. The conductor is then earthed by touching it with a finger. As positive charges are attracted to negative charges in the strip, this allows electrons to flow down the earth to neutralise the negative charges. 3. Finger is then removed from the conductor 4. Polythene strip is then removed and there is a redistribution of positive charges on the sphere due to law of electrostatics and the sphere is left positively charged

Question 36

Name 3 electrostatic hazards

Answer 36

Lightning, fires due to sparks caused by sudden electrical discharge, damage to electronic equipment

Question 37

How is lightning produced

Answer 37

- Thunderclouds are charged by friction between the water molecules in the thunderclouds and the air molecules - It then ionises the air and the ionised air provides a conducting path for electric charge to be discharged to the nearest or sharpest object on the ground. Hence lightning is produced

Question 38

How to overcome lightning

Answer 38

Lightning conductors/rods on buildings can provide a conducting path for electrons in the air to discharge safely onto the ground

Question 39

How to overcome the electrostatic hazard of fires due to sparks caused by sudden electrostatic discharge

Answer 39

- Excess charges on a petrol tanker flow safely through a metal chain to the ground - Synthetic rubber tires which contains graphite can aid in electrical discharge to the ground

Question 40

How to overcome damage to electronic equipment

Answer 40

- Electrical components are stored in antistatic packaging (metallic film)

Question 41

2 applications of electrostatics

Answer 41

Electrostatic paint spraying and photocopier

Question 42

How does electrostatic paint spraying work

Answer 42

1. Paint droplets from an aerosol become charged from rubbing against the nozzle of the spray 2. Droplets are attached to the metal body of the car which is earthed 3. Since droplets have the same charge, they repel each other in the spray and spread out evenly, resulting in an even coat of paint

Question 43

How does a photocopier work

Answer 43

1. Photocopiers make use of the metal selenium which conducts when it is exposed to light and is an insulator when not exposed to light 2. The whole surface of the drum is charged positively by rotating it near a highly charged metal wire 3. When a printed page is photocopied, light is reflected off the page and projected onto the drum 4. Other parts of the drum correspond to the black parts of the page, these areas of the drum receive no light and remain insulated, so charges remain. 5. Fine particles of the carbon powder (toner) are attracted to the charged areas of the drum and presses against the copy paper 6. Toner is transferred onto the photocopy as the drum rotates and presses against the copy paper 7. Heat is then supplied to melt the toner powder and fix it onto the paper surface.

Question 44

Purpose of an electrostatic precipitator

Answer 44

It is used to remove fine ash and dust from chimneys

Question 45

How does an electrostatic precipitator work

Answer 45

1. It consists of metal plates that are positively charged and a number of metal rods running through them which are negatively charged. 2. Air molecules around rods are ionised. Positive ions are attracted back to the rods while negative ions are picked up by tiny particles of ash and dust 3. Negatively charged particulates are attracted to the metal plates where they are collected.