10.1.4—E.m.f. and potential difference (p.d.) in terms of energy considerations

E.m.f. is the source energy rise per coulomb; potential difference is the energy transferred per coulomb across a component.

In a complete loop, source energy is shared among component transfers and internal losses. Use the direction of energy transfer to distinguish rises from drops.

A cell with ε=1.5 V supplies 1.5 J per coulomb; a lamp may receive less terminal p.d. when the cell is delivering current.

E.m.f. is not automatically the same as every measured voltage in a working circuit.