Electromotive Force

What is Electromotive Force?

Electromotive Force is an electrical phenomenon which results in the generation of electric potential (voltage). Electrochemical cells or changing magnetic fields generally give rise to electromotive force.

It is regarded as the fundamental property of an energy source that permits the motion of charges within the circuit.

Sources of Electromotive Force

Basically, non-electrical energy sources give rise to electromotive force by changing one form of energy into another. More specifically, a source of emf is a device that provides electrical energy.

Some examples of sources of emf are as follows:

  • Electrochemical cells
  • Generators
  • Transformers
  • Solar or photovoltaic cells
  • Thermoelectric devices
  • Thermocouple

Symbol

Symbolically, the electromotive force is represented as E and it is abbreviated as emf.

Introduction

The involvement of the term force in emf makes it quite confusing but it must be clear to you that here we are not dealing with any kind of external force. The electromotive force implies the energy per unit charge that is used for separating the charges.

More specifically, it can be said in a way that, inside the cell there exist some internal forces due to which the charges inside it move.

This separation and movement of electrons result in electric current.

Hence electromotive force is not any kind of force but a potential difference that causes the flow of current in a circuit.

What is Electromotive Force?

You must be thinking that how a non-electrical energy source gives rise to the flow of electric current when connected in a circuit?

A battery when present as a single unit and not connected in any circuit despite holding the charge does not has any flow of current. But how its presence in a complete circuit causes the generation and maintenance of the flow of electric current.

So, to understand this consider the circuit shown below with a source of emf (battery) connected with a bulb.source of emf in electric circuit

Here we have taken a battery of 9V which is a two-terminal device whose one terminal is comparatively at a higher potential than the other.

When the source of emf is connected to the bulb then electrons start moving from the negative terminal of the battery through the bulb towards the positive terminal of the battery. The movement of charges generates an electric current (that flows in the opposite direction of the electron flow) and this leads to cause the lightening of the bulb connected in the circuit.

Further, the source of emf tries to maintain the difference in potential across the two ends and battery pumps the electrons from the positive terminal towards the negative terminal. This causes maintenance of the potential difference across the two ends.

You must note here that some amount of work must be done to move the negative charges from the positive terminal towards the negative terminal. So, the energy required to do the work is obtained from the chemical reactions within the battery.

Ideally, a battery possesses zero internal resistance thus the terminal voltage is equivalent to the emf supplied by the battery.

Unit of emf

We have recently discussed that emf is the work done on the unit charge to maintain the difference in potential between the two ends. Thus,electromotive force eq1

Since work done (energy) is expressed in Joules and the unit of charge is Coulomb. Therefore,electromotive force eq2

So, emf is expressed as J/C i.e., Volts.

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