Transcript Electricity & Magnetism

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Magnetism and the Earth
Lightning and Electrostatics
The Discovery of Current Electricity
Volts, Amps and Ohms
Electricity and Magnetism
Electromagnetism
Static Electricity
(a little more)
Otto von Guericke - Magdeburg, Germany
Otto von Guericke - Magdeburg, Germany
Otto von Guericke - Magdeburg, Germany
Otto von Guericke - Magdeburg, Germany
Vacuum pump
Otto von Guericke - Magdeburg, Germany
Otto von Guericke - Magdeburg, Germany
Magdeburg
Germany
Otto von Guericke
Magdeburg
Germany
Galvani and his frogs
 Luigi Galvani (1737-1798)
 Anatomy Prof
 Looking for ‘Life Force’
 Found that electricity made frog
legs twitch
(but that wasn’t the important
discovery!)
Galvani and his frogs
Galvani and his frogs
 Showed 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 discovery
 Showed 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 jar
 Capacitors
 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
Two ways to get current flowing – 1
Mechanically push charges on to dome.
Two ways to get current flowing – 1
 Van de Graaff pushes
charges up onto the
dome until they are so
concentrated that
they jump off as
sparks.
 HIGH VOLTAGE
 (high concentration)
 but low current
Two ways to get current flowing – 2
Chemical action:
 Batteries push lots of charges
 But not very hard
 High current – Low Voltage
 Lots of charges, but low concentration
Current is flowing charges
 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
2.4 volts
 A simple ‘closed
circuit’.
 Path of current:
 Ammeter measures
flow of charges
(current)
 Voltmeter measures
concentration of
charge (voltage)
Voltmeter
Ammeter
-
0.25 amps
+
Charges carry energy!
 When charges reach a
thin filament they give
up energy – as heat and
light.
 How do they ‘carry’ this
energy?
High volts
Low volts
Charges carry energy!
 The energy is carried
as potential energy
due to the
concentration of
the 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.
Low negative concentration (–1 V)
Higher negative concentration (–12 V)
Charges carry energy!
 A little like water flowing
over a waterfall
High gravitational
potential energy
 Big drop (high ‘voltage’)
Low gravitational
potential energy
Charges carry energy!
 Less drop, more water
Less potential
energy per
kilogram ... but
more kilograms
Less ‘volts’ but
more ‘current’
Summary
 Historical introduction sets scene as a ‘human
adventure’. Follows formation of ideas.
 Van de Graaff and 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