Difference between EMF and Voltage

The difference between EMF and voltage is that the EMF is the potential difference measured across a power source without load connected to it whereas voltage is the potential difference measured between any two points in a circuit. We use the term EMF when we speak about batteries, generators, transformers and other power sources whereas the term voltage is used when we speak about loads, circuits and circuit components.

Difference between EMF and Voltage

EMF is the abbreviation of Electromotive Force while voltage is the measure of potential difference between two points. It can be a little difficult to differentiate them. To understand the difference between EMF and voltage better, let’s compare them side by side.

EMF vs Voltage

EMFVoltage
DefinitionEMF is the term used for the potential difference generated within a power sourceVoltage is the term used for potential difference between any two points in a circuit.
GeneralThe potential difference measured across the armature of a generator, photovoltaic cells and chemical cells can be referred to as EMF.The potential difference measured across a load, circuit components can be referred to as Voltage.
ExampleDifference Between EMF and VoltageDifference Between EMF and Voltage
OperationElectromotive force follows the coulomb force operation.Voltage follows a non-coulomb force operation.
NotationElectromotive force is denoted by .Voltage is denoted by the letter V.
UnitThe SI unit of EMF is volts.The SI unit of voltage is volts.
FormulaEMF, = -N.(dϕ/dt)
(or) for DC sources
EMF, = V = I.(R+r)
Where,
N is the number of turns of the coil.
dϕ/dt is the change in magnetic flux.
I is the current.
R is the resistance.
r is the internal resistance.
Voltage, V = IR
Where,
I is the current.
R is the resistance.
MagnitudeEMF measured across a source is always higher than the load voltage.Lesser than EMF.

Important points to remember

  1. EMF or electromotive force is an electric potential difference generated by a cell or a changing magnetic field or cells in a solar panel whereas voltage is the potential difference measured across any two points in a magnetic field.
  2. The SI unit of EMF and voltage are the same (volts).
  3. The amount of EMF depends on the change in the magnetic field whereas the voltage depends on the magnitude of current and resistance.
  4. Voltage can be considered as the difference in electrical states of two points in the electric field whereas EMF is the force that causes the difference in electric states.

Experiment explaining Electromotive force

The following video shows how emf is induced in a coil, how to measure the voltage across a battery terminal and how emf differs from voltage.

Summary: Electromotive force vs Voltage

It is very important to understand the difference between EMF and voltage. The biggest difference between electromotive force and voltage is that the potential difference measured across the armature of a generator, photovoltaic cells and chemical cells can be referred to as EMF whereas that measured across a load, circuit components can be referred to as Voltage.

References

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