Magnetism and the Earth Lightning and Electrostatics The Discovery of Current Electricity Volts, Amps and Ohms Electricity and Magnetism Electromagnetism This is a bit of a mix of things. Need to sort it out better!

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

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? 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 Big drop (high ‘voltage’) High gravitational potential energy ENERGY Low gravitational potential energy

Charges carry energy! Less drop, more water Less potential energy per kilogram ... but more kilograms ENERGY 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