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Electromotive Force

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What is Electromotive Force?

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The electromotive force is abbreviated as EMF. The electromotive force is the voltage at the terminals of the source in the absence of an electric current. The concept of electromotive force refers to 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.

What is meant by electromotive force? The electromotive force (EMF) is defined as- The amount of work done in the energy transformation (or conversion) and the amount of electricity that passes through the electrical source or the generator. The electromotive force (EMF) is measured in Volts and denoted by the symbol ε (or E). In this article, we will discuss mainly what is electromotive force, what is emf in physics, etc.


What is EMF in Physics?

Now we will understand what is EMF in physics and what is meant by emf in physics. So, the electromotive force is the maximum potential difference between two electrodes of the cell when no current is drawn from the cell. The electromotive force is denoted by E or sometimes it is also denoted by the symbol ε.

We know that the charges move in the electric circuit, for the motion of the charges in a given electric circuit we need to apply an external force to it. We say that the battery or an external electric source such as a battery applies such force which will give acceleration to the charges and it is known as the electromotive force. Despite its name, it’s not a form of force but a potential difference.

If the electromotive force is not a form of force at all, then why is it termed as the electromotive force, what are emf and potential difference, and what is the source of emf? To answer these doubts, consider a simple circuit of a lamp connected to a battery, as shown in the Figure below.


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The battery (any electro-voltaic cell) can be represented as a two-terminal device that keeps one terminal at a higher potential than the second terminal. The higher electric potential is sometimes known as the positive terminal and it is generally labelled with a plus sign. The lower-potential terminal is known as the negative terminal and labelled with a minus sign. This is known as the source of the emf.

When the source of the electromotive force is disconnected from the lamp, there is no net flow of charges within the emf source. Once the battery is reconnected to the lamp, charges will flow from one terminal of the battery, through the lamp (causing the lamp to glow), and back to the other terminal of the battery. If we consider positive current flow which is also known as the conventional current flow, positive charges leave the positive terminal, travel through the lamp, and enter the negative terminal of the emf source. This is how an emf source is configured. Now, what is the electromotive force of a cell? The electromotive force of a cell is the potential difference developed at both end of the given battery.


What is the Unit of Electromotive Force?

What is the electromotive force measured in? Let us have a look at what is the unit of electromotive force, the formula for electromotive force is given by,

⇒ ε = V + Ir

Where,

V- The applied potential difference.

I- The current flowing through the circuit.

r- The internal resistance of the circuit.

Therefore, the unit of electromotive force in volts. The electromotive force (EMF) is expressed as the number of Joules of energy given supplied by the source divided by each Coulomb to enable a unit electric charge to move across the circuit. Mathematically it is given by:

⇒ ε = Joules/Coulomb

Therefore the dimensions of the electromotive force M1L2T-3I-1.The answer to the question what is the si unit of electromotive force, from the expression of emf we can say it is Joules/coulomb.


Example:

1. Consider an electrical circuit with a potential difference of 7V, a current of 1A, and the internal resistance of the battery is 0.7ohms. Calculate the EMF of the battery.

Ans:

Given,

Potential difference = V = 7V

Current in the circuit = I = 1A

Internal resistance of the battery = r = 0.7Ω

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 = 7 + (1 × 0.7) = 7.7 volts

Therefore, the EMF of the battery is given by 7.7V.

FAQ (Frequently Asked Questions)

1. What is Meant by the EMF of a Battery and What is EMF and Voltage?

Ans: Electromotive force is defined as the amount of work done in the energy transformation (or conversion) and the amount of electricity that passes through the electrical source or the generator. The electromotive force (EMF) is measured in Volts. The voltage of the source is different from that of the emf of the source. Voltage is the potential difference developed between the two electrode potentials of a battery under any conditions.

2. What is the Measure of Electromotive Force?

Ans: The electromotive force is measured in volts, denoted by V.