Voltage, p.d. and e.m.f.
Definition.
Voltage 
across two points is the change in electric potential energy 
per unit charge passing between the points.
The unit of voltage is volt (V). By comparing units, ![\displaystyle{[\mathrm{voltage}]=\frac{[\mathrm{potential\,energy}]}{[\mathrm{charge}]}}](https://physicspupil.com/wp-content/ql-cache/quicklatex.com-a11d0669157321e3385e3efae3133944_l3.svg "Rendered by QuickLaTeX.com")
, i.e., 
Remark. Equivalently, (i) voltage across two points is the change in electric potential between the points. In other words, (ii) voltage across two points is the potential difference 
(p.d.) between two points.
Definition.
Electromotive force (e.m.f.) 
of a power source is the electrical energy per unit charge supplied by the source, when there is charge passing through it.
Remark. The e.m.f. of a source is measured when it is in open circuit, i.e., the source is not in use such that no current is being drawn.
Voltage refers to e.m.f. when describing a power source, e.g., a cell.
Voltage refers to p.d. when describing an external circuit component, e.g., a load.
Measuring Voltage
By voltmeter, connected in parallel to a component in a circuit. An ideal voltmeter should have infinite resistance.
Voltage in Series and Parallel Circuits
In a series circuit, the sum of the potential differences across each load is equal to the e.m.f. of the power source.
In a parallel circuit, the potential difference across each load is the same as the e.m.f. of the power source.
Cells in Series and Parallel
In a series arrangement, the e.m.f. add up.
In a parallel arrangement, the currents add up.