1. the discovery of current electricity. galvani and his frogs luigi galvani (1737-1798) anatomy...
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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