Static Electricity Flashcards
Define electrostatic charge
The study of electric charges which are not moving
2 types of electrostatic charge
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
S.I. unit of charge
Coulomb (C)
Charge carried by protons and electrons
Protons: 1.6 x 10^-19 C
Electrons: -1.6 x 10^-19 C
Number of electrons in 1C of charge
1/1.6 x 10^-19 C = 6.25 x 10^18
Law of electrostatics
Like charges repel and unlike charges attract
The force of attraction/repulsion increases when distance between charges ___________?
Decrease
What interaction between a specimen and a charged object is a sure test to conclude that the specimen is charged (not neutral)?
Repulsion only. The specimen should have a similar charge as the object.
Factors that affect the strength of forces between 2 objects
- Amount of charge in both objects
- Distance between the centres of both objects
Define electrical conductors
Electrical conductors allow electric charges to flow through them easily.
Examples of electrical conductors
Metals, acids, graphite, (impure) water, salt solutions and ionized air
Why are metals good electric conductors
Metals are good electric conductors due to the presence of free electrons which can move freely when there is an electric force.
Can positive charges move in a metal conductor?
No, because protons are found tightly bounded within the nucleus of the atoms.
Define electrical insulators
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
Examples of electrical insulators
Rubber, glass, plastic.
Can positive charges move in a metal conductor?
No
Define electric field
An electric field is a region where an electric charge experiences an electric force.
What is the direction of the electric field?
It is the direction of how a positive test charge would move.
A stronger electric field is denoted as having more/less lines of force which are drawn closer/further to/away each other?
More, closer to
Electrostatic charging by friction (when rubbing an amber rod with fur)
- When 2 different materials, especially insulating materials are rubbed together, negative charges (electrons) are transferred from one object to another.
- Electrons from fur are deposited onto amber when fur rubs amber rod.
- The material that loses electrons will become positively charged while the material that gains electrons will be negatively charged
- Amber becomes negatively charged while fur becomes positively charged
Define electron affinity
The tendency for an atom to acquire/gain electrons
What is the triboelectric series?
Different substances with different electron affinities are arranged according to their affinity for electrons. This is known as the triboelectric series
Negative affinity of a material means that it will most probably be a receiver/donor of electrons
Receiver
Positive affinity of a material means that it will most probably be a receiver/donor of electrons
Donor
Common materials that can be electrostatically charged by friction (negatively charged)
- Polythene (rubbed with wool)
- Ebonite/amber/rubber (rubbed with fur)
Common materials that can be electrostatically charged by friction (positively charged)
- Perspex (rubbed with wool)
- Glass (rubbed with silk)
How to neutralize electric charges for insulators?
Pass the apparatus quickly through a Bunsen flame
How to neutralize electric charges for conductors?
Touch the apparatus to allow excess negative charges (electrons) to flow through our body to the ground. This process is called earthing.
Process of charging by induction (for conductors of electricity)
- charging 2 neutral insulated metal spheres
- 2 neutral metal spheres mounted on insulating stands, P and Q are placed touching each other.
- 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.
- Sphere Q is then separated from P while the negatively-charged polythene strip is kept in position
- 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.
Use of insulating stand
It is to prevent excess charges from flowing down to earth or flow from earth to neutralise the metal sphere
Advantage of charge by induction
Net charge of original charged object remains unchanged
Is it possible to charge 2 metal spheres by rubbing
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
Is it possible to charge an insulator by induction?
No, because only surface electrons can move through a small distance, insulator remains overall neutral as the negative charges cannot leave the surface.
Process of charging by induction (for conductors of electricity)
- charging a single insulated metal sphere to obtain negative charges
- 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.)
- 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)
- Finger is then removed from the conductor
- 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.
Process of charging by induction (for conductors of electricity)
- charging a single insulated metal sphere to obtain positive charges
- 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
- 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.
- Finger is then removed from the conductor
- 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
Name 3 electrostatic hazards
Lightning, fires due to sparks caused by sudden electrical discharge, damage to electronic equipment
How is lightning produced
- 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
How to overcome lightning
Lightning conductors/rods on buildings can provide a conducting path for electrons in the air to discharge safely onto the ground
How to overcome the electrostatic hazard of fires due to sparks caused by sudden electrostatic discharge
- 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
How to overcome damage to electronic equipment
- Electrical components are stored in antistatic packaging (metallic film)
2 applications of electrostatics
Electrostatic paint spraying and photocopier
How does electrostatic paint spraying work
- Paint droplets from an aerosol become charged from rubbing against the nozzle of the spray
- Droplets are attached to the metal body of the car which is earthed
- Since droplets have the same charge, they repel each other in the spray and spread out evenly, resulting in an even coat of paint
How does a photocopier work
- Photocopiers make use of the metal selenium which conducts when it is exposed to light and is an insulator when not exposed to light
- The whole surface of the drum is charged positively by rotating it near a highly charged metal wire
- When a printed page is photocopied, light is reflected off the page and projected onto the drum
- 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.
- Fine particles of the carbon powder (toner) are attracted to the charged areas of the drum and presses against the copy paper
- Toner is transferred onto the photocopy as the drum rotates and presses against the copy paper
- Heat is then supplied to melt the toner powder and fix it onto the paper surface.
Purpose of an electrostatic precipitator
It is used to remove fine ash and dust from chimneys
How does an electrostatic precipitator work
- It consists of metal plates that are positively charged and a number of metal rods running through them which are negatively charged.
- 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
- Negatively charged particulates are attracted to the metal plates where they are collected.
