The very basics of electricity

Moving electrons and storing them for when you want them

Static

• Around 600 BCE, it was discovered that if amber (fossilized tree sap) is rubbed with cat’s fur (meow!) it can be made able to pick up light objects, such as feathers

• It was believed that friction turned the amber magnetic, similarly to the ore called magnetite

• In 1600, the term “electricus” was used to described objects that, like amber, could pick up things when rubbed with fur

• Electricus means “like amber”

Nature likes neutral

• Removing electrons from a substance makes it positive-charged and immediately become highly attractive to any neutrally charged substance

• The electrons will “want” to flow from the neutral object (or negative-charged object) to the positive object

• Electrons cannot travel through nothing! • If the objects are touching – or if a conductor is

available – the electrons will flow until the charge difference is evened out

• If they can’t – then you have a static charge

Static Electricity

• Static means staying in one place, not moving

• Static electricity can be generated easily by friction

• Rub a balloon on your head…

• Wear socks on a carpet and drag your feet…

• Rub a plastic rod with your cat…

Static build up

• Charge differences can accumulate until they release all at once – such as when they find a conductor (or some other way to equilibrate – or “ground”)

• A “static shock” is when something has had electrons stripped away so that is becomes highly attractive to anything with electrons

• The electrons then flow extremely rapidly to “ground” themselves and equal out the charge imbalance

Not actually my daughter

Convection current create charge differentials!

review

• Review – we’re talking about electrons moving, and that means we need to remember things terms like force and energy

• What are the units for these things and what do they mean?

Coulomb’s Law

• Charles Augustin de Coulomb discovered that the force any charged particle exerts on another charged particle is determined by an inverse square law

• The law details: – Any charged thing will

make any other charged thing move

– How much energy it moves with is based only on how charged they are, and how far apart they are

Force of interaction

Distance between charged

particles

Amount of charge of

thing 1

Amount of charge of thing 2

A constant

Newton’s gravitational laws are identical!!!

Gee that’s great, Mr. K – but where is this going?

If you are going to have electricity – you need to have a way to push – or pull –

electrons. This first part was to establish what electricity is and why electrons move!

Review and moving forward…

• The Coulomb Force is the force of “electrostatic interaction” – what does this mean?

• How does the inverse square law describe this force?

• The Coulomb is the Standard Unit of electrical charge (symbol is C)

Storing static charge…to use later

• If you have created a charge, you can store the “capacity” of that charge in a device called a capacitor

• An early version of this was called a Leyden Jar

• Capacitors are still used to start motors, provide power, regulate current flow in circuits, etc…

Leyden jars

• Benjamin Franklin put this to use, trying to literally catch “lightning in a bottle”

• By showing that Lightning could charge a Leyden jar, lightning was electricity

What is this thing on the ground, I wonder?

Leyden jars and batteries

• Leyden jars could store quite a lot of power • "I would not take a second shock for the kingdom of France.“

– An early experimenter with Leyden Jars, who felt the shock of a Leyden jar he’d made

• Benjamin Franklin noted that a number of Leyden jars together would be like a “battery of cannon”

The first battery

• In 1800, Alessandro Volta built the first chemical battery

• Discs of Copper and Zinc separated by paper soaked in saltwater, called a Voltaic Pile

• When the top and bottom contacts were connected by wire, electrical current would flow through that wire

Symbol for a battery in a circuit diagram

A Cu-Zn Voltaic “cell”

• Electrons begin at the anode

• Here, Zinc prefers to be positive, so it ditches its electrons, which are now free

• Copper loves electrons (more than Zinc), so it happily accepts Zinc’s

• So electrons flow from anode to cathode in a typical battery (they don’t all have to be Zn-Cu)

The potential energy of the battery is measured in Volts

The Volt

• An SI unit that describes the electrical potential energy per unit of charge (given in Joules per Coulomb)

1 V = 1 J/C

• It is the “push” on the electrical charge moving through an electrical current (such as along a wire in a circuit) – are they being pushed a lot? Pushed a little?

• Is the current a huge, rushing river or a little babbling brook?

Coppertop – duh!

Cathode

Anode

Batteries versus capacitors

• Both store charge

• Capacitors can be recharged, and discharge themselves extremely quickly

• Battery’s (the kind we’ve seen) cannot be recharged, their potential comes from chemical reactions.

• Once the chemical reagents run out, the battery is dead