Difference Between EMF and Voltage

Dhristi JEE 2022-24

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 we mean by the terms EMF and voltage.

 

Electromotive force and voltage are often mistaken to be the same but there are differences between them. They are sometimes confused with electromagnetic fields as well. One can say that they are all related, but are not the same. Electromotive force and voltage are directly related to the generation of electromagnetic fields (EMF). Electromotive force is an invisible form of energy produced by the interaction of electric and magnetic fields which results in the movement of electrons from one point to another.


Electromotive Force (EMF) 

Electromotive force or also known as EMF is an energy input for charging through a battery cell. In other words, it creates and maintains a voltage in the active cell, by supplying energy in Joules to each unit of Coulomb charge. This is represented by "ε" and the unit of measurement is the same as voltage, which is Volt. 


EMF is the maximum potential difference between two points of the battery when no current flows from the source in the case of an open circuit. That is, it is caused by EMF and is affected by voltage or potential difference. A generator or battery is used to convert one energy into another. In these devices, one terminal is positively charged and the other is negatively charged. Therefore, an electromotive force is work done on a unit electric charge.


Voltage 

Voltage is the force that causes an electric charge to flow. This is the potential difference between two connections where one connection collects more electrons than the other. Voltage is defined as potential energy per charge. 

 

Voltage is measured in volt (V), which is the derivation unit of potential. A voltage drop is a drop in potential along the current path through a circuit. The higher the resistance of a component, the greater the voltage drop between connections. When electricity encounters resistance,  potential energy is lost because it is converted into another form of energy to do the work. For example,  electric potential energy is converted into thermal energy by a resistor.

 

Difference between EMF and Voltage 

Electromotive force 

Voltage

Electromotive force is the specification of the potential difference that occurs inside the electric source

Voltage is a term for the  potential difference between any two points in a circuit

The potential difference measured between the armature of a generator, solar cells, and chemical cells is sometimes referred to as EMF.

The potential difference measured across the load, circuit component, is referred to as voltage.

Electromotive force follows the coulomb force operation.

Voltage follows a non-coulomb force operation.

E = I * (R + r)

V = I * R


Points to Remember 

  1. EMF or electromotive force is the potential difference generated by one or more cells or a changing magnetic field in a solar cell, and voltage is the potential difference measured at any two points in the magnetic field. 

  2. The SI unit and voltage of EMF  are the same (volt). 

  3.  The magnitude of the EMF depends on the change in the magnetic field, and the voltage depends on the magnitude and resistance of the current. 

  4. Voltage can be thought of as the difference between two electrical states in an electric field, but EMF is the force that causes the difference in electrical states.


What is  EMF?

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

  2. 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 internal resistance.

  3. 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.

  4. 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. 

  • 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.


The SI unit of emf is volt(V).

The 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.

 

2. 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 is 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 on Difference Between EMF and Voltage

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.

3. What happens to the current when the voltage is increased? 

According to Ohm's law, the current I is proportional to the voltage V and inversely proportional to the resistance R, according to the law. Therefore, if the resistance in the circuit is maintained, the current will increase as the voltage increases.

4. Can electromotive force be negative?

Yes, the electromotive force can be negative. Consider an example of generating an EMF in such a way that the inductor cancels the input power. The generated EMF is considered negative because the direction of flow is opposite to the actual power. Therefore, it is possible that the electromotive force can also be negative. 

5. How do you explain the difference between terminal voltage and EMF?

Terminal voltage is defined as the potential difference past the terminals of a load when the circuit is switched on but whereas EMF is defined as the maximum potential difference that is delivered by the battery if no flow of current is provided. 

6. How do you define Electromotive Force? 

Electromotive force is defined as the electric potential generated by either an electrochemical cell or a change in a magnetic field. EMF is a commonly used abbreviation for electromotive force. 

7. What is the formula to calculate voltage?

Voltage present is equal to the number of resistance times the current. The formula is: Voltage (E) = Current (I) x Resistance (R), or E = IR.
Or Voltage can even be represented by V = IR. 


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