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difference between emf and potential difference

What is emf source?

A source of electromotive force is a device in which non electrical energy is changed into electrical energy.Examples of e.m.f sources are electrical cells,thermocouples and generators.

The e.m.f of the source is able to maintain its upper terminal positive and its lower terminal negative charge.

When the switch is closed,the cell drives a clock wise current i round the circuit external to it.As a positive charge enters the source of e.m.f at its lower potential point,the e,m.f of the source does an amount of work on positive charge to enable it to go to a point of higher potential.

A point of higher potential is a region where there is a large number of positive charge than there are elsewhere.A point of low potential is aa region where a lesser number of positive charges then negative charges.

What is an emf ?

The emf of the cell is defined as “The energy converted from non electrical forms to electrical form,where one coulomb of positive charge passes through the cell.”

How to measure emf ?

We can measure emf  approximately by voltmeter connected directly across the terminals of the source.The formula used to find emf is given as:

E = W/Q


E= e.m.f

W=energy converted from non electrical form to electrical form

Q= positive charge

What unit is used to measure emf ?

The unit of e.m.f is the joule per coulomb which is equal to the volt.and is defined as: “If one joule of work in bringing a unit positive charge of one coulomb  from infinity to a point,then the potential at that point will be one volt.”

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What is potential difference in a circuit?

The potential difference between two points is defined as:”The energy converted from electrical form to other forms when one coulomb of positive charge passes between the two points.”

In symbols it is represented as:

V= W/Q


V = Potential difference

W= energy converted from electrical form to other form

Q= charge

What is the SI unit of electric potential difference?

The SI unit of potential difference is the same as the e.m.f which is volt.The potential difference between two points in a conductor is one volt if 1 joule of energy is converted from electrical form to another form when one coulomb of positive charge flows through it.”

How to measure potential difference?

To measure the potential difference in volts between two points separated by a load,the voltmeter is always connected in parallel to the load.

The ideal voltmeter must have resistance very much larger than the load which is connected across. This is due to avoid of drawing a large current from circuit it is connected.

The gravitational analogue of simple electric circuit:

In case of a source of the e.m.f doing work on positive charge to move it from a point of lower potential to a point of higher potential,the person does work in lifting up the balls from ground to the top of chute.

The balls then roll downwards into the region of viscous oil.Just as electrical energy is converted to heat and light in case of current flowing through bulb,the gravitational potential energy of the balls is converted into thermal energy in viscous oil when balls descend through the viscous oil.

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