📖 In-Depth Theory
Building Up Static Charge
STATIC ELECTRICITY arises when electric charge builds up on an insulating material.
HOW STATIC CHARGE BUILDS UP:
When two insulating materials are RUBBED together, electrons transfer from one to the other.
Material gaining electrons → becomes NEGATIVELY charged.
Material losing electrons → becomes POSITIVELY charged.
CHARGE IS CONSERVED: total charge before = total charge after. The charges are equal and opposite.
EXAMPLES:
Rubbing a plastic rod with a cloth: rod becomes negatively charged, cloth becomes positively charged.
Walking on carpet: body builds up charge (electrons transfer to feet or from carpet).
Nylon clothing rubbed against skin.
ONLY ELECTRONS MOVE — protons are fixed in the nucleus. Positive charge is created by REMOVING electrons, not by adding protons.
INSULATORS vs CONDUCTORS:
Insulators: charge stays where it is built up — cannot flow away.
Conductors: charge spreads across the surface immediately — cannot build up static.
Forces Between Charges
LIKE CHARGES REPEL — two positive charges or two negative charges push each other apart.
OPPOSITE CHARGES ATTRACT — positive and negative charges pull each other together.
This is a NON-CONTACT FORCE — charges exert forces across empty space.
DEMONSTRATIONS:
Charged balloon sticks to a wall — the charged balloon induces an opposite charge on the wall surface.
Two charged rods of same material repel each other.
Hair standing up after rubbing — all hairs gain the same charge and repel each other.
INDUCTION:
A charged object brought near a neutral conductor causes charge SEPARATION (induction).
Free electrons in conductor move towards or away from the charged object.
Nearest end becomes opposite charge to the object → attraction.
ELECTROSTATIC HAZARDS:
Fuel tankers: static builds up from fuel flowing through pipes → spark → fire/explosion risk.
Electronic components: sensitive to electrostatic discharge (ESD) — wear antistatic wristbands.
Lightning: massive electrostatic discharge between cloud and earth.
Applications of Static Electricity
INKJET PRINTERS:
Tiny ink droplets given different charges → deflected by charged plates → precise positioning.
Computer controls charge on each droplet → forms text and images.
LASER PRINTERS / PHOTOCOPIERS:
Drum given static charge.
Laser removes charge from non-printing areas.
Toner (charged powder) sticks to charged areas → transferred to paper → heated to fuse.
ELECTROSTATIC PRECIPITATORS (smoke filters):
Electrodes create strong electric field in chimney.
Soot particles gain charge → attracted to oppositely charged plates → fall into collection bin.
Reduces particulate pollution from power stations and factories.
SPRAY PAINTING:
Spray gun charges paint droplets.
Object to be painted is given opposite charge.
Paint attracted to surface evenly — even coverage, less waste.
Used in car manufacturing.
DEFIBRILLATORS:
Deliver controlled electric shock to restart heart.
Capacitors store charge then discharge through patient's chest.
⚠️ Common Mistake
Only ELECTRONS move when static charge builds up — positive charges do not move. Rubbing makes one object negative (gains electrons) and one positive (loses electrons). Like charges repel, opposite charges attract — the same rule as for magnets but remember that electrical charges and magnetic poles are completely different things.