Electric Circuits: Potential Difference

Potential difference (V), also known as voltage, is the work done or energy transferred per unit charge.
It is measured in volts (V), where one volt equates to one joule of energy transferred per one coulomb of charge (1V = 1J/C).
The equation V=W/Q relates potential difference (V), work done or energy transferred (W), and charge (Q).

Electromotive Force (emf) is the amount of energy supplied by a cell or battery per coulomb of charge.
It is also measured in volts and can be determined by the formula ε = W/Q, where ε represents emf, W is the total energy provided by the cell or battery, and Q is the total charge passing through the circuit.
Understand that emf and potential difference differ in that emf is the total possible energy transfer, whereas potential difference is the actual energy transfer.

Energy transferred and power in a circuit can be determined using potential difference.
The formula W = QV can be used to calculate energy (W), where Q is charge and V is potential difference.
Power (P) can be calculated using the formula P = VI, where V is potential difference and I is current.

A potential divider is a simple circuit which takes advantage of the way voltages divide in series circuits.
A potential divider can be used to produce a lower voltage from a higher one, and is often used for signal processing.
The output voltage Vout in a potential divider can be calculated by Vout = Vin *(R2/R1+R2), where R1 and R2 are resistances in series, and Vin is the input voltage.

Any source of emf has an internal resistance which reduces the potential difference when a current flows.
When current is drawn from a source such as a battery, the actual (terminal) potential difference V is given by the equation V= ε - Ir, where ε is the emf, I is the current, and r is the internal resistance.
For a cell or battery with no current flowing (I = 0), the potential difference across its terminals equals the emf.