PHYSICS
STATIC ELECTRICITY
Laws of Electrostatic
Electric Charges
➔ Symbol for electric charges: Q
➔ SI unit: coulomb (C)
*Only electrons can be added or removed from an atom
First Law of Electrostatics
➔ Like charges repel, unlike charges attract
➔ Repulsion is the only test for presence of electric charges
➔ For neutrally charged objects, when placed near negatively charged object:
- Like charges repel (Reason)
- Electrons move away from the negatively charged object to (the side of the
neutrally charged object furthest away from the negatively charged object)
- (Side of the neutrally charged object closest to the negatively charged object)
contains more positive charges than negative charges, and thus positively
charged by induction
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- Force of attraction between positive charges in the neutrally charged object
is greater than the force of repulsion between electrons to the negatively
charged object as they are closer
- Generates a net force that causes neutrally charged object to be attracted by
the negatively charged object
*Negative charges → electrons in questions
Electrical Insulator & Conductors
*Electricity insulators transferred due to charging by friction
*Electrical conductors transferred due to charging by induction
Principles of Electrostatics
Charging by Friction
➔ For electrical insulators
➔ An insulator with higher tendency to lose electrons
will lose electrons to the insulator with lower
tendency to lose electrons
➔ Only surface electrons are transferred and remain at
the surface where the material has been rubbed
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*Insulators can only be charged through rubbing with another insulator
*The higher the tendency to lose electrons, the larger the atomic radius (additional
knowledge)
Charging by Induction
➔ For electrical conductors
Charging Two Metal Spheres by Induction
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1) Place a charged rod close to one of the conducting spheres that are in contact
2) Positive charges in the rod in the rod attract free electrons to the left, leaving the
right side positively charged
3) Keeping the rod in position, separating the two spheres to isolate the two groups of
opposite charges
4) Remove the charged rod. The two spheres now have equal but opposite charges
Charing One Metal Sphere by Induction
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1) Place a negatively/positively charged rod close to the conducting sphere. As unlike
charges attract, the positive/negative charges accumulate on the side nearer to the
charged rod while the negative/positive accumulate on the other side
2) Touch the sphere on the right to allow the electrons to be repelled further away
from the person’s body
3) Remove the hand from the sphere
4) Remove the charged rod
Neutralizing Charged Insulators & Conductors
Neutralizing Charged Insulators
Neutralizing Charged Conductors
➔ Earthing = To provide a path for excess electrons to flow away from (or to ) an
electrical conductor
➔ A charged conductor can be discharged by earthing, because electrons are free to
move about in conductors
➔ Earthing thus makes the number of positive and negative charges in a conductor
equal – the conductor becomes neutral or is neutralized
Electric Field
➔ A region of space where an electric charge experiences an electric force
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➔ Electric force = Attractive/repulsive force that electric charges exert on one another
➔ Direction of electric field at a point = DIrection of force that would act on a positive
charge placed at that point in the field
*Electric field lines travel away from positive charge
*Electric field lines travel towards the negative charge
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*Electric lines that are further apart indicate a weaker electric field in the region
*Electric lines that are closer indicate a stronger electric field in the region
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1) When a positively-charged glass rod is placed near the brass cap, negative charges
are induced on the brass cap due to flow of electrons from the gold leaves to the
brass cap
2) Gold leaves are positively charged and the positively-charged glass rod is placed
near the brass cap, the gold leaves diverge
Hazards of Electrostatics
Lightning
➔ Caused by large quantity of electric charge built up in the heavy thunderclouds
➔ Thunderclouds charged by friction between the water molecules and air molecules
➔ Charge can ionize the air when sufficiently large → provides conducting path for
large quantity of charge to be discharged to nearest or sharpest object on ground
➔ Lightning conductors
- Prevent damage to small buildings
- Provides a steady discharge path for the large number of electrons in the air
to flow from the top to the building of the Earth → chance of lightning
bolt(due to sudden discharge) is minimized
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Fires & Explosions due to Electrostatic Discharge
➔ May occur due to excessive accumulation of electric charges produced by friction
➔ Electric charges accumulate on an aircraft during flight and on a petrol tanker
transporting flammable liquids
- Tyres are made of slightly conductive rubber → large amount of charge
build-up on the body of the aircraft during flight can be discharged
harmlessly during touchdown
Electrostatic Discharge & Electronic Shielding
➔ Electrostatic discharge can damage electronic equipment
(e.g circuit boards, hard drives) – usually packed in
anti-static packaging(thin layer of metalized film – acts as an
electrostatic shield for the equipment placed inside)
Applications of Electrostatics
Electrostatic Precipitator
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1) Incoming gas with unwanted particles enter the electrostatic precipitator
2) The unwanted particles become negatively charged by gaining free moving electrons
and negative ions when they come into contact with the negatively charged wires
and become negatively charged by friction
3) As unlike charges attract, the negatively charged particles are attracted to the
positive plates
4) Dust and ash are the scrapped and removed from the plates
5) The clean gas where unwanted particles are removed exit the precipitator
Spray Painting
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- Spray droplets have the same positive charge so they repel one another as like
charges repel which spreads the paint more evenly
- Spray droplets stick more to the panel as they are oppositely charged, which lowers
the cost
Van De Graff Generator
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