Relative vs. Absolute Voltage

Absolute Voltage

If voltage at point A is quoted with reference to the earth point - the absolute potential - 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