Difference between EMF and Voltage

Differences Digital Electronics Electronics & Electrical

EMF and Voltage are the two fundamental aspects of electrical and electronics engineering. They may appear as same, but they are different electrical quantities. In order to understand the difference between EMF and voltage, we first need to understand their meaning.

What is EMF?

EMF stands for Electromotive Force. The measure of energy the power source (ex. Battery) gives to each Coulomb of charge is known as EMF of the power source or battery. The EMF is denoted by letter ‘E’. The SI unit of EMF is the same as the voltage, which is Volts.

The EMF is the factor which creates and maintains the potential difference or voltage between two points in an electric circuit. In a practical electric circuit, the EMF is the maximum potential difference between two terminals of the power source (say battery) when no current flows from the source, i.e. the source is connected in an open circuit. The power sources such as generator, battery, etc. are used to produce EMF.

Refer the electric circuit, the EMF can be given by,

$$\mathrm{\mathit{E\, \mathrm{=}\, I\left ( R\mathrm{\, +\, }r \right )}}$$

$$\mathrm{\mathit{\Rightarrow E\, \mathrm{=}\, V\mathrm{\, +\, }Ir}}$$

What is Voltage?

The difference in the electric potential between two points in an electric circuit is known as voltage. The voltage is also known as potential difference or electric pressure or electric tension. If the work done in moving a unit positive charge (Q) from one point to another in the electric field is W joules, then the voltage between two points is given by,

$$\mathrm{Voltage,\mathit{V}\, \mathrm{=}\,\frac{Work\: done\left ( \mathit{W} \right )}{Charge\left ( \mathit{Q} \right )}}$$

The voltage or potential difference between two points in an electric circuit is caused either by an electromotive force (EMF) or by build-up of electric charge. Basically, the voltage can be considered as the electric pressure that forces the electric charge to flow in an electric circuit. The voltage is denoted by letters "V or v" and is measured in volts (V). The electric voltage can be of two types viz. alternating voltage or direct voltage.

From the above circuit, the voltage or potential difference (V) can be given by,

$$\mathrm{\mathit{V\mathrm{\, \mathrm{=}\, }IR\mathrm{\, \mathrm{=}\, }E-Ir}}$$

Difference between EMF and Voltage

The following table shows the key differences between electromotive force or EMF and potential difference or voltage −

Basis of Difference	EMF	Voltage
Definition	EMF or electromotive force is the measure of energy given by a source of power (say battery) to each coulomb of charge.	The difference in the electric potential between two points of an electric circuit is known as voltage.
Alternate name	No alternate name for EMF.	Voltage is also called potential difference or electric pressure or electric tension.
Work performed	EMF is the work performed in moving a charge from one point to another in a source.	Voltage is the work performed in moving a charge from one point to another through a conducting wire.
Function	EMF creates and maintains the potential difference or voltage between two points of an electric circuit, i.e. if there is no EMF, then no voltage will be there.	The voltage causes the electric current to flow between two points of the circuit. If there is no voltage, no current flow would be there.
Representation	The EMF is usually represented by ‘E’ or ‘e’ or ‘ε’.	The voltage is denoted by the letter ‘V’ or ‘v’.
Expression	The EMF of a power source is given by,$\mathrm{\mathit{E\, \mathrm{=}\, I\left ( R\mathrm{\, +\, }r \right )}}$ Where, I is the circuit current, R is the resistance across the load terminals and r is the internal resistance of the source. The EMF of a source is also given in terms of work done and charge as − $\mathrm{\mathit{E\, \mathrm{=}\, W/Q}}$	The voltage between two points is given by the product of current flowing and the resistance between the two points, i.e. $\mathrm{\mathit{V\, \mathrm{=}\, IR}}$
Magnitude	In an electric circuit, the magnitude of the EMF is always greater than voltage.	Voltage is always less than EMF.
Effect of circuit resistance	The EMF of a source is independent of resistance of the circuit.	The voltage depends upon the value of resistance of the circuit. Thus the change in the circuit resistance, changes the voltage between the two points.
Measuring device	The potentiometer or EMF meter is used to measure the measure the EMF of a power source.	The voltage between two points in an electric circuit is measured by using a voltmeter.
Constancy	The EMF of a source is always constant.	The voltage between two points varies according to the resistance between the points.
Operation followed	EMF follows the coulomb force operation.	Voltage follows the non-coulomb force operation.
Dependency	EMF depends upon change in magnetic field (in generator), chemical energy (in battery), light intensity (in PV cell), etc.	Voltage between two points depends upon the current and resistance between the two points.
Cause and Effect	EMF is the cause of voltage between two points.	Voltage is the effect of EMF.
Existence	The EMF exists in an electric circuit even when the electric current does not flow in the circuit, i.e. circuit is open.	Voltage does not exist in a circuit when the current through the circuit is zero.
Energy gain & loss	The EMF gains the energy in an electric circuit.	The voltage losses the energy in an electric circuit.
Term used	The term EMF is used only for the source of energy.	Voltage can be used to represent the difference in potential of any two points of an electric circuit.

Conclusion

From the above discussion, we can conclude that both EMF and voltage are measured in volts, but they are not the same. One of the major differences between voltage and EMF is that the EMF is the energy given to each Coulomb of charge, whereas the voltage is the amount of energy required to move a unit charge from one point to another.