17.1 - 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.

Conductors and insulators carry charge differently

• Electrical conductors - Let charge flow through them easily (usually metals, such as copper and silver). If electrons are rubbed on, they flow straight off, so conductors don't normally hold a static charge.
• Electrical insulators - Do not let charge flow (such as plastic and rubber). Electrons rubbed on stay trapped where they land, allowing static charge to build up.

Testing a material

• Put the material in a circuit in series with a cell and a bulb.
• If the bulb lights, charge is flowing - the material is a conductor.
• If the bulb stays off, no charge can flow - the material is an insulator.

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).

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.