Electricity

Static

Source of Electric Charge

The Greek Thales noted that when rubbed by a cloth, amber (small a) would attract small particles.

The Greek word for amber (small a) is elektron,

The only charge we’re concerned with in discussions of basic electricity.

Positive and Negative

Positive charges do exist, but most are locked in the nucleus of the atom.

Positive in electricity means less negative. Abdul’s principle: opposites attract (likes

repel). Coulomb’s Law: like Newton’s Law of

Gravity with charge replacing mass. Also an inverse square relation.

Electric Field Lines

Conduction

Moving charges by touching them (more later).

Conductors and Insulators

Very similar to the idea in thermodynamics No real insulators, just poor conductors. Most good conductors are metal: first gold,

silver, then copper being the top three. Rubber and glass are good insulators, BUT ANYTHING will conduct electricity with

enough push.

Electrostatic devices

Whimhurst generator (the spinning disks with the small sparks)

van der Graaf generator (the big sphere with the big sparks)

Electroscope: gold foil strips separated by excess charges.

Induction

Moving charges w/o touching (using Abdul’s principle.)

Moves the charges in the van der Graaf generator and the electroscope.

Lightning

Negative charges collect on the bottom of a storm cloud in a process similar to a van der Graaf generator.

Positive charges collect on the highest point on the ground.

Intervening air breaks down and becomes ionized, capable of carrying electricity.

Strike starts from the ground and joins the stroke from the cloud at about 30 feet.

Voltage

This is the “push” behind moving charges. Not the whole story when it comes to

dangerous shocks.

Current Electricity

AC (alternating current) or DC (direct)– Westinghouse and Tesla

DC first used in industry– Edison

Sources of DC

Solar cells (also known as photovoltaic cells).

Piezoelectric crystals. DC generator (discussed later) Batteries

Voltaic Pile

Batteries

First known as the Voltaic Pile– As in Alessandro Volta

Alternating layers of two different metals and wet cardboard.

Modern batteries made from two different metals and an acid or alkaline paste.

Even a citrus fruit will do!

Batteries in Series and Parallel

Parallel: + to + and – to – Like for jump-starting a car. More turning ability in a motor. Series: + to – Like in a flashlight More speed in a motor.

Current

A flow of charge over time More charge, or in less time, more current. Measured in Amperes A small current with enough voltage to

push it IS dangerous!

Ohm’s Law

George Simon Ohm says that the ratio of voltage to current is resistance.

Resistance, measured in Ohms, restricts the flow of electricity in a circuit.

Any electrical device—motor, light, clock—has resistance. Even people!

Series and Parallel Circuits

Think of a water circuit with a pump, hoses, and someone to step on the hose.

Only one path for the series circuit; therefore the resistance builds up.

Many paths in a parallel circuit, therefore more current can flow.

Fuses

Small strips of metal in series with a circuit If the current gets to be too much, the fuse

melts, breaking the circuit.

Short and Open Circuits

A short circuit is where a conductor connects the power source (battery) back onto itself without going into the circuit.

An open circuit is where a major wire is broken and current cannot flow (like when a fuse blows).