3.1 - Static Electricity - Charging

Charging an insulator by friction

• Atoms are built from positive protons and negative electrons. Most objects have equal numbers of each, so they have zero net charge - they're neutral.
• When two insulating materials are rubbed together, negatively charged electrons are transferred from one to the other.
• Insulators don't let charge flow, so transferred electrons stay trapped where they land. This trapped charge is static electricity.

Which material gains which charge

• The material that gains electrons is left with a negative static charge.
• The material that loses electrons is left with an equal positive static charge.
• Only negative electrons move - positive charges don't. An object becomes positively charged only because electrons have moved away from it.

Polythene and acetate rods

• Polythene rod - Rubbing transfers electrons from the cloth onto the rod, leaving the rod negatively charged.
• Acetate rod - Rubbing transfers electrons from the rod onto the cloth, leaving the rod positively charged.

Like charges repel, opposite charges attract

• When two charged objects are brought close, they exert a non-contact electrostatic force on each other.
• Opposite charges (one positive, one negative) attract each other.
• Like charges (both positive or both negative) repel each other.
• The force gets weaker the further apart the objects are.

Testing charge with a suspended rod

• Suspend a rod of known charge from a thread so it can swing freely.
• Bringing up an object with the same charge repels the rod (pushes it away).
• Bringing up an object with the opposite charge attracts the rod (pulls it towards it).

Charged objects attract uncharged objects by induction

• A charged object can attract an object that has no overall charge through induction.
• For example, a balloon rubbed on hair gains electrons and becomes negatively charged:
  1. When placed against a wall, the balloon repels electrons near the surface, pushing them slightly deeper in.
  2. This leaves the wall's surface positively charged.
  3. The positive surface is attracted to the negative balloon, so the balloon sticks.

Everyday examples

• Comb and paper - A charged comb induces an opposite charge on small scraps of paper, attracting them.
• Bending water - A charged rod held near a thin stream of water induces a charge in the water, bending the stream towards the rod.

Detecting charge with a gold-leaf electroscope

• A gold-leaf electroscope detects whether an object is charged.
• It has a metal disc connected by a metal stem to two thin pieces of gold leaf that hang side by side inside an insulated box with windows.

Bringing a charged rod near

1. The rod induces charge through the disc, down the stem and into the leaves.
2. Both leaves gain the same type of charge, so they repel each other and rise.
3. Taking the charged rod away allows charge to redistribute, and the leaves fall back together.

Touching a charged insulator onto the disc

• Charge transfers directly onto the electroscope, leaving the stem and leaves with the same charge so the leaves rise.
• If the leaves don't rise when an object is brought near or touched to the disc, the object is uncharged.

Too much static causes a spark

• As charge builds up on an isolated object, the potential difference between the object and the earth (at 0 V) gets larger.
• If the potential difference becomes large enough, electrons can jump across the gap to the earth or a nearby earthed conductor as a spark.
• Lightning is a very large spark between a charged cloud and the ground.

Discharging safely to earth

• A charged conductor can be discharged safely (earthed) by connecting it to earth with a metal strap or wire.
• If negatively charged, electrons flow down the strap to the ground.
• If positively charged, electrons flow up the strap from the ground to neutralise it.