Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Hill diagram as a model for potential difference

Scientists often speak of a coulomb ‘falling through’ so many volts and transferring so much electrical energy. That is rather like a 1 kilogram rock falling through so many metres. The rock can transfer more gravitational potential energy to kinetic energy if it falls down the side of a 100 metre cliff than if it falls only 10 metres. 
In a similar way a 1 coulomb charge of electricity transfers more energy if it falls through an electrical potential difference of 100 volts than if it falls through 10 volts. 
The rock falls through a gravitational potential difference and the coulomb falls through an electrical potential difference.


You can draw a hill diagram showing how the battery pushes a coulomb of charge up to a high level of energy in joules, so that it can then spend that energy as it runs down the various hills to the bottom on its way round the circuit. The coulomb does not really have joules like bottles in a rucksack. The coulomb is pushed by electric forces generated by the battery. Those forces grip it wherever it is in the circuit and drive it on round, transferring electrical energy which comes ultimately from the battery. 
You might picture a 6 volt battery giving 6 joules to every coulomb with instructions ‘remember to spend all this energy on your way round the circuit and then you will get another load of 6 joules for the next round’. 
In a way the battery is like a moving ramp such as the machine used to raise gravel to the top of the tower for sorting it, or like an escalator for people. It raises electric charge, measured in coulombs, uphill to a higher level of electrical potential energy. Then as the electric charge travels round the rest of the circuit it is running downhill, transferring energy to warm up the filament of the lamp as it makes collisions of some kind in the filament. The connecting wires are assumed to have almost no resistance and so the coulomb just rolls along without transferring any energy.


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