I like to compare the flow of electric current to cars
on a road or a queue of people.
Take the cars on the road – if a dual carriage way
narrows to a single lane there is always a slowing of the cars on the dual carriageway
section. You must assume that the cars fill the road in both sections. Although drivers have
not reached the single section they 'know' it is there because of the reduced speed of their
cars on the dual section. The same is true of the queue of people if they are trying to get
through a narrow door – as in the lunch queue at the school where I last taught. The number
of narrow sections in a ring road round a city will determine the rate of flow of cars in the
whole road.
Now to the electrical version.
In a series circuit the current must be
the same in each section of the wire. This current is determined by the total source of
potential within the circuit. The loss of energy through any component depends on its
resistance and if we think of a resistor as simply a piece of wire then the loss of energy
increases steadily down the wire.
Since Resistance = resistivity x length]/cross
sectional area for a wire of constant thickness the resistance is directly proportional to
length
The current is constant throughout the circuit and so as the electrons move
through each component their loss of energy per unit length depends on the resistivity of that
component.
So – the current is determined by the total resistance of the circuit (like
the number of narrow road sections), the loss of energy in each component depends on the
resistivity of each component and the size of the component. The electrons 'know' that there
are other components in the circuit by the rate at which they can flow (the current). Take
other components out of the circuit and the electrons' flow rate will immediately increase.
The energy loss in a particular specimen through which they are moving depends
on the material of the specimen.