# Relative vs. Absolute Voltage

## Electric circuits and the voltage at any point

### Absolute Voltage

If voltage at point *A* is quoted with reference to the *earth point* - the *absolute potentia*l - then the voltage is written *U _{A}*.

### Relative Voltage

If voltage at point *A* is quoted with reference to point *B* - then the voltage is written *U _{AB}*.

### Example - Relative and Absolute Voltage

The total resistance in the series circuit above can be calculated

*R = (33 ohm) + (47 ohm) + (33 ohm)*

* = 113 ohm*

The current in the circuit can be calculated

*I = (12 volts) / (113 ohm)*

* = 0.11 ampere*

The relative voltage between point *A* and *B* - *V _{AB}* - can be calculated

*U _{AB} = (33 ohm) (0.11 ampere)*

* = 3.6 volts*

The absolute voltage in point *A* relative to earth is

*U _{A} = 12 V*

The absolute voltage in point *B* relative to earth is

*U _{B} = U_{A} - U_{AB} *

* = (12 volt) - (3.6 volt)*

* = 8.4 volt*