Energy supplied by a battery or a cell per coulomb Q of charge passing through it is known as EMF or electromotive force. The magnitude of emf is provided by V (potential difference) across the cell terminals, when no current flows through the circuit.

e = E/Q

Where,

• e = emf or electromotive force (V). 

• W = Energy (Joules).

• Q = charge (Coulombs). 

Both emf (electromotive force) and pd (potential difference) are measured in V (Volts)

Electromotive force (emf) formula can also be given as,

e = IR + Ir or, e = V + Ir

Where,

• e = the electromotive force (Volts),

• I = current (A),

• R = Load resistance,

• r = the internal resistance of cell measured in ohms.

Difference between EMF and Potential Difference?

EMF is that amount of energy (in any form) changed into electrical energy per coulomb of charge and potential difference is that amount of electrical energy which changes into other forms of energy per coulomb of charge. Cell, solar cell, battery, generator, thermocouple, dynamo, etc are some of the sources of emf.

Solved Example

Example: Find the terminal potential difference of a cell when it is connected to a 9-ohm load with cell emf = 2 Volts and resistance (internal) 1 ohm?

Solution:

Given:

emf =2

External resistance = 9 ohm

Internal resistance = 1 ohm

Since I = V/R

And R = External resistance + Internal resistance = 9 + 1 = 10 Ohm

Now, I = 2/10 = 0.2 Ampere

e = V + Ir

2=V+ (0.2)1

V =2-0.2

Hence , the external resistor gets, V = 1.8 Volts

Conclusion

Understanding EMF theory and its calculation is believed to be fundamental for any physical and electrical engineering student. This paper shall guide through the basic formulae, definition and usage.

 

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