2.18 understand that: voltage is the energy transferred per unit charge passed & the volt is a joule per coulomb.

If energy = voltage × charge, it follows that:
voltage = energy / charge or V =E / Q

In words, voltage is ‘the energy transferred per unit charge passed’ and it therefore follows that one volt is equal to one joule per coulomb.

A 12V battery transforms 12J of chemical energy into electrical energy for every coulomb of charge flowing.

A 6V lamp converts 6J of electrical energy into heat and light for every coulomb of charge that flows.