Voltages and Currents

Voltages and Currents in Resistor Networks

Finding the Unknown

In addition to working out the resistance, we can use Ohms law to work out voltages and currents in resistor networks. Before you try this it would be a good idea to look at some basic facts about resistor networks.

In a SERIES CIRCUIT the same current flows through all components. Each component however, will have a different VOLTAGE (p.d.) across it. The sum of these individual voltages (V_R1+V_R2+V_R3 etc) in a series circuit is equal to the supply voltage (EMF).

In a PARALLEL CIRCUIT the same voltage is present across all components but a different CURRENT can flow through each component. The sum of these individual component currents in a parallel circuit is equal to the supply current. (I_S = I_R1+ I_R2+ I_R3 etc.)

The Potential Divider Rule

If two or more resistors are connected in series across a potential (e.g. A supply voltage), the voltage across each resistor will be proportional to the resistance of that resistor. V_R1 ∝ R₁ and V_R2 ∝ R₂ etc.

To calculate the voltage across any resistor in the potential divider, we multiply the supply voltage (E) by the proportion of that resistor to the total resistance of all the resistors.

For example if R₂ is double the value of R₁ there will be twice the voltage across R₂ than across R₁. It follows therefore, that the voltage across R₁ will be one third of the supply voltage (E) and the voltage across R₂ will be two thirds of the supply voltage (E). So, if we know the supply voltage and the resistor values we can work out the voltage across each resistor and therfore we could work out by PROPORTION, what the voltage should be at any point in the circuit.

Using these few facts it is possible to work out an amazing amount of information about the currents and voltages in a circuit once we know what resistances are present. Try it out for yourself with our "Find the Missing Value" Quiz.