IIT JEE , AIEEE Forum - Physics , Electricity

anantawasthy

sir, what is the difference between potential and potential difference.my book says that 2 conductors(plates) are uncharged initially then if a +ve charge is given to cond.2 and the charge is transferred from cond. 2 to cond 1 in very small installments dq each till cond.1 gets charged cy charge +Q and cond2 by -Q.then it says that at every stage of charging conductor 1 is at a higher potential than conductor 2.therefore work is done externally in transferring each installment of charge. i could not understand how it is at a higher potential.plz also tell the difference between potential difference and potential

edison

When you are talking about charges it implies electric potential.

Following concepts may improve your understanding regarding the same.

Potential Reference at Infinity

The general expression for the electric potential as a result of a point charge Q can be obtained by referencing to a zero of potential at infinity. The expression for the potential difference is:

Taking the limit as

gives simply

for any arbitrary value of r. The choice of potential equal to zero at infinity is an arbitrary one, but is logical in this case because the electric field and force approach zero there. The electric potential energy for a charge q at r is then

where k is Coulomb's constant.

Voltage

Voltage is electric potential energy per unit charge, measured in joules per coulomb ( = volts). It is often referred to as "electric potential", which then must be distinguished from electric potential energy by noting that the "potential" is a "per-unit-charge" quantity. Like mechanical potential energy, the zero of potential can be chosen at any point, so the difference in voltage is the quantity which is physically meaningful. The difference in voltage measured when moving from point A to point B is equal to the work which would have to be done, per unit charge, against the electric field to move the charge from A to B.


Used to calculate current in Ohm's law.	Used to express conservation of energy around a circuit in the voltage law.	Used to calculate the potential from a distribution of charges.	Is generated by moving a wire in a magnetic field.

Work and Voltage: Constant Electric Field

The case of a constant electric field, as between charged parallel plate conductors, is a good example of the relationship between work and voltage.

The electric field is by definition the force per unit charge, so that multiplying the field times the plate separation gives the work per unit charge, which is by definition the change in voltage.

This association is the reminder of many often-used relationships:

Voltage Difference and Electric Field

The change in voltage is defined as the work done per unit charge against the electric field. In the case of constant electric field when the movement is directly against the field, this can be written

If the distance moved, d, is not in the direction of the electric field, the work expression involves the scalar product:

In the more general case where the electric field and angle can be changing, the expression must be generalized to a line integral:

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