Electromotive Force (EMF)

In Physics, electromotive force, abbreviated emf, is the electrical activity created by a non-electrical source. Several devices can produce emf by converting energy of one form to electrical energy. These devices can be either photodiodes, solar cells, batteries, thermoelectric devices or transformers, to name a few. When magnetic field variations pass through a surface, emf is produced. For instance, during a geomagnetic storm, the movement of the magnetic field of the Earth generates currents in a power grid. It is because the magnetic field lines are pushed about and pass across the conductors.

Electromotive Force Definition 

The highest electric potential difference created by any electrical energy source, such as a battery, generator or solar cell, is known as electromotive force.

Voltage can come from a variety of devices. When connected to a circuit, these devices create a potential difference, thereby generating current. Electromotive force is a specific sort of potential difference (emf). Alessandro Volta coined the term when he designed the very first battery, also termed the voltaic pile, in the 1800s. The symbol of electromotive force is E. 

Explanation of EMF

Consider a simple circuit including a lamp and a battery of 12 volts. The battery can be thought of as a two-terminal device. Here, one terminal is maintained at a greater electrical potential than the other. The positive terminal, which has a + sign, is sometimes referred to as the higher electric potential. The negative terminal, which is designated with a minus sign, is frequently referred to as the lower-potential terminal. This is the source of electromotive force.

An electric charge doesn’t flow inside the battery when it is not attached to the bulb. However, when the bulb is attached, charges pass from one end of the cell to the other. This net flow of charge crosses the bulb in between and illuminates it. If we consider a traditional positive flow of current, charges leave the positive terminal, cross the bulb and enter the negative end of the cell. But, in the case of resistors and metallic wires, electrons contribute the maximum to generate current. Therefore, considering the mobility of electrons for circuit analysis is more realistic. So, the electrons flow from the negative terminal to the positive terminal of the battery after passing through the bulb.

Electrons must be transported from the positive terminal to the negative end of the battery. It will allow the emf source to retain the potential between both terminals. To keep the potential difference, the source of electromotive force serves as a charge pump, transporting charges from the negative to the positive terminal. This raises the charge’s potential energy and as a result, the charge’s electric potential.

Work needs to be done to transport the negative charges to the negative terminal. This necessitates energy use, which is obtained from the battery’s (emf source) chemical reactions. This energy or the electromotive force is responsible for the flow of electrons from one terminal to the other, generating current. To retain the potential difference between the battery terminals, the positive terminal’s potential is kept high and the negative terminal’s potential is kept low.

Unit of Electromotive Force

Volt is the SI unit of electromotive force. It is expressed as the work done on a unit charge to generate electric potential or Voltage. Since, the unit of work done is Joules (J) and the unit of charge is Coulomb (C), the electromotive force (E) is given as Joules/ Coulomb, i.e. Volt.

E= J/C.

Electromotive Force Formula

The Electromotive force formula is given as

EMF= Voltage of the battery + (internal resistance of the battery X current across the circuit)

i.e. E = V+ Ir

Where, E = electromotive force

V= voltage of the battery

I = current across the circuit

r= battery’s internal resistance.

Electromotive Force Dimension

Electromotive force, E= Work done/ charge.

The dimension of work done is represented as M1 L2 T-2

The dimension of charge is given as I1 T1, 

where, M=Mass, L= Length, T= Time,and I= current.

Therefore, the dimension of electromotive force is given as M1 L2 T -2 / I1 T1 

= M1 L2 T-3 I-1

Conclusion

Electromotive force is an important topic in Physics and Electronics. The electric potential created by an electrochemical cell or a changing magnetic field is known as electromotive force. Several devices can act as the source of electromotive force. These devices can be solar cells, photodiodes, batteries or thermocouples. The above article unpacks the electromotive force definition along with its unit, formula and dimension.