If a resistor is connected across an emf source, the accumulation of excess positive charges and negative charges will extend from the emf source’s terminals to the resistor’s terminals. So the same voltage that is the emf of the source is transferred across the terminals of the resistor. This causes an electric field to be established in the resistor.
It is this electric field which produces the electrical force FE that pushes the mobile charge carriers to move through the resistor. A positive charge therefore loses EPE when it makes its way from the positive to the negative terminal of the resistor. The potential difference (PD) (measured in volts) across the resistor thus represents the EPE lost per unit charge in the resistor.
For example, if the PD across the filament of a light bulb is 3 V, it means that 3 J of EPE is converted into 3 J of heat and light energy, for every C of charge passing through the filament.