Electricity

Calculation of Voltages and Current in a Series Circuit.

If the supply voltage (from the cell) is 12 Volts,
what are the voltages across each resistor?

From the previous page,
the total resistance of the circuit below is 9 Ohms.

We can use V = I x R to find the current,
which in a series circuit is the same everywhere.

I = V ¸ R
   = 12 ¸ 9
             = 1·333 Amps.

Using the same equation V = I x R for each resistor in turn
(and rounding up numbers) gives

V₁ = 1·333 x 2
         = 2·667 Volts.

V₂ = 1·333 x 3
        = 4·000 Volts

V₃ = 1·333 x 4
        = 5·333 Volts

You must always say what the units are at the end of the calculation.
If you write V₃ = 5·333 without putting the word "Volts" afterwards,
you will lose a mark in the exam.

We can see that the largest resistor (4 Ohms) has the largest voltage (5·333 Volts)
and the smallest resistor (2 Ohms) has the smallest voltage (2·667 Volts) across it.
In energy terms, the largest amount of work is done by the charge
moving through the largest resistance.

Finally, we can check that the voltage for all of the components
adds up to the supply voltage.

V₁ + V₂ + V₃ = 2·667 + 4·000 + 5·333
 = 12 Volts.

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