Difference Between EMF and Voltage

EMF and Voltage

The most common confusion in students is what’s the difference between EMF and voltage. To understand the difference between EMF and voltage, let us first understand what do we mean by the terms EMF and voltage.


What is  EMF?

  • EMF stands for electromotive force. EMF is the voltage at the terminals of the source in the absence of an electric current

  • The concept of EMF defines the amount of work required to separate the charge carriers in the source current, such that the force acting on the charges at the terminals of the source is not a direct consequence of the field. Emf is developed as a result of the internal resistance.

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  • The electromotive force (EMF) is defined as- The amount of work done in the energy transformation and the amount of electricity that passes through the electrical source or the generator.

  • EMF is measured in Volts and denoted by the symbol ε (or E). 


What is Voltage?

  • The voltage is defined as the amount of energy required to move a unit charge from one end to another end. Voltage is measured in Volts and denoted by the symbol V.

  • The voltage is mainly developed between the two poles of the electric circuit i.e. it developed between the anode and cathode of the battery. 

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  • The positive terminal of the battery is known as the cathode and the negative terminal of the battery is known as the anode. The potential at the cathode of the source will be higher than the potential at the anode.

  • When a potential difference or the voltage is developed across the passive elements is known as the voltage drop. (Passive elements-the electrical elements that do not generate power, such as resistors, capacitors, etc. which are used to dissipate, store charges)

  • The voltage developed is a result of the electric field.


Difference Between EMF And Terminal Voltage

Now the major emf and voltage difference is, voltage or terminal voltage is too small in comparison with the emf. It implies that the Intensity of Emf developed will be always greater than the voltage as the voltage exists in a loaded circuit. Due to external resistance, there is always voltage drop or energy loss which will lead to varying intensity. But, emf is always constant.

Let us look at other voltage and EMF difference as listed below:

Difference Between Voltage and EMF

S. no

EMF

Voltage


Emf is the voltage developed between two terminals of a battery or source, in the absence of electric current. 

Voltage is the potential difference developed between the two electrode potentials of a battery under any conditions.


It is the potential difference between the two terminals of a battery or cell in an open circuit. 

Emf is an open circuit voltage.

It is the potential difference between the two terminals of a battery or cell in a closed circuit.

Terminal voltage is a closed-circuit voltage.


Emf is independent of the resistance of the electrical circuit but is dependent upon the internal resistance of the circuit.

It is directly proportional to the resistance between the two terminals.


The formula used to calculate emf is given by:

ε = I(R+r)

Where,

R- External resistance of the electrical circuit.

r- Internal resistance of the given circuit

The voltage is calculated by using ohm’s law, given bt:

V = IR

Where,

I- Current flowing through the circuit

R- External resistance of the electrical circuit


EMF of any circuit can be measured by using a potentiometer.

The voltage developed in an electric circuit is measured by using a Voltmeter.


SI unit of emf is volt(V).

SI unit of the voltage is volt(V).


Emf is defined by non-coulomb force or non-electric force operation.

The voltage is defined by Coulomb force or electric force operation.


Work done by the emf will be the maximum work of the cell or battery.

Work done by voltage will not be the maximum work of the battery.


Emf is induced in an electric field, gravitational fields, or magnetic fields.

Voltage is induced only in an electric field.


Intensity is always constant.

Intensity will be varying due to voltage drop across the external resistance.


These are some major notable differences between the emf and terminal voltages. Though both are measured as potential differences, they are not the same.


Solved Examples

  1. Consider an electrical circuit with a potential difference of 5V, a current of 0.9A, and the internal resistance of the battery is 0.7ohms. Calculate the EMF of the battery.

Ans:

Given,

Potential difference = V = 5V

Current in the circuit = I =0.9A

Internal resistance of the battery =r = 0.7

Now, Emf of the circuit is given by:

=> E=I(R+r)

Where,

R- External resistance of the electrical circuit.

r- Internal resistance of the given circuit

I- Current flowing through the circuit

On rearranging the above expression,

=> E=IR+Ir

We know that the product of current in the circuit and the external resistance is the potential difference across the resistance. Thus,

=> E=V+Ir

Substituting given values in the equation,

=> E = 5 + (0.9 x 0.7) = 5.63 volts

Therefore, the EMF of the battery is given by 5.63V.


  1. A battery provides a current of 1A through a 3ohm coil and 0.8A through a 5ohm coil. Calculate emf and the internal resistance of the battery.

Ans:

Given,

Let the emf of the battery be E and the internal resistance of the battery be r.

Now,

Emf of battery is given by:

E = I(R+r)

Where,

R- External resistance of the electrical circuit.

r- Internal resistance of the given circuit

I- Current flowing through the circuit

For 3 Ω coil: E = 1(3+r)……..(1)

For 5 Ω coil: E = 0.8(5+r)……….(2)

On solving (1) and (2) we get the value of the internal resistance of the battery, r = 5 Ω

Now emf of the battery is, E= 8V

Therefore, the emf and internal resistance of the battery are 8volts and 5ohms respectively.


Did You Know?

  • Various types of batteries are available in the market and the emf of batteries will vary from each other. 12V emf batteries are the standard ones used for practical purposes.

  • The emf of batteries also determined by the type of chemical reaction involved. Lead-Acid batteries used in cars and other vehicles are the most common types.

  • Though Emf stands for electromotive force, it is still the voltage developed in the circuit. Here force means energy per unit charge.

FAQs (Frequently Asked Questions)

1. What is the relation between the terminal voltage and the EMF?

Terminal voltage is given by,

=> V = IR……(1)

We know that emf of the battery is given by:

=> E = I(R+r)

=> I = E/(R+r)……(2)

Substituting the value of I in the equation (1),

= > V = ER/(R+r)....(3)

Equation (3) gives the relation between terminal voltage, emf, and external resistance.

2. Why is emf not a force?

Though Emf stands for electromotive force, it is still the voltage developed in the circuit in the absence of current, we can say emf is the open circuit voltage. Here force means energy per unit charge. Hence, emf is not a force.