1. The Discovery of Current Electricity

Galvani and his frogs

Luigi Galvani (1737-1798)Anatomy ProfLooking for ‘Life Force’Found that electricity made

frog legs twitch(but that wasn’t the important discovery!)

Galvani and his frogs

Galvani and his frogsShowed that contact

between different metals and the muscles also made the legs twitch.

So were the two metals producing electricity somehow?

Alessandro Volta

Took up Galvani’s discoveryShowed that two different

metals were producing the same effect as ‘electricity’

Alessandro Volta

Wondered if static electricity and ‘metal’ electricity were really the same thing.

Invented the ‘Voltaic Pile’First battery

Alessandro Volta

He showed that the ‘Voltaic pile’ had the same effect as static

by collecting both in a ‘Leyden jar’.

Alessandro Volta

Leyden jarCapacitors

J J Thomson showed that ‘cathode rays’ appeared to be negative particles that moved through metals.

Now called ‘electrons’

Electric current: – rate of transfer of positive charge

Batteries push lots of chargesBut not very hardHigh current – Low VoltageLots of charges, but low

concentration

Two ways to get current flowing – 2Chemical action:

To flow, a current needs a ‘closed circuit’

The switch completes a path from one end of the battery to the other

Current is flowing charges

A simple ‘closed circuit’.

Path of current:Ammeter Ammeter

measures flow of measures flow of charges (current)charges (current)

Voltmeter Voltmeter measures measures concentration of concentration of charge (voltage)charge (voltage)

Current is flowing charges

AmmeteAmmeterr

VoltmeteVoltmeterr

2.4 volts

0.25 amps

++--

When charges reach a thin filament they give up energy – as heat and light.

How do they ‘carry’ this energy?

Charges carry energy!

High voltsHigh volts

Low Low voltsvolts

EN

ERG

Y

The energy is carried as potential energy due to the concentration of concentration of the chargesthe charges.

Lower concentration lower potential energy

Charges carry energy!

Charges have to go faster when they reach the filament...

hit atoms harder … and so lose more energy.

Charges carry energy!

Low negative concentration (–1 V) Higher negative concentration (–12 V)

A little like water flowing over a waterfall

Big drop (high ‘voltage’)

Charges carry energy!

High gravitational potential energy

Low gravitational potential energy

ENERGY

Less drop, more water

Charges carry energy!

Less potential energy per kilogram ... but more kilograms

ENERGY

Less ‘volts’ but more ‘current’

SummaryHistorical introduction sets scene as a ‘human

adventure’. Follows formation of ideas.Battery illustrate concepts of voltage and current

well.Voltage as ‘charge concentration’ can be imagined

– and is correct physics. (Sum of kq/r terms)Water flow as analogy for current has problems

due to lack of ‘negative water’ and direction of flow.

Water cycle as driven by Sun’s energy is a reasonable analogy for flow of energy around a circuit – compare the gravitational potential energy with the electrical potential energy.

Sets scene for Power = Volts x Current = E/C x C/t

Electricity Basics

The AtomProtonsNeutrons

electrons