Current and Potential Calculations and Formulas

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Current and Potential Calculations and Formulas

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Current:

Electric current is defined as the rate of flow of charge.

The current I flowing through a conductor is the amount of charge Q flowing through a cross-section of the conductor in time t.

Unit of Current: The SI unit of charge is coulomb (C) therefore current is measured in coulomb per second which is called ampere (A).

One ampere of current is the current which flows when one coulomb of charge passes in one second.

The smaller units of current to measure a weak current are milli-ampere and micro-ampere which can be related to ampere as below:

If n electrons pass through the cross-section of a conductor in time t, then the total charge passing through the conductor is given as:

Potential A force is said to do work if, when acting on a body, there is a displacement of the point of application in the direction of the force. For example, when a ball is held above the ground and then dropped, the work done on the ball as it falls is equal to the weight of the ball (a force) multiplied by the distance to the ground (a displacement).

Like charges repel each other and unlike charges attract each other. Thus it can be assumed that if a charge is to be moved towards a like charge, some force will have to be applied to overcome the force of repulsion and therefore some work would have to be done.

Using this approach, the potential at a point in a region of charge is measured in terms of the work done in moving a test charge (a known charge of a very small value) from a point of zero potential to that point. Force between the two charges at infinite separation is zero, therefore we consider that the potential to be zero when the charges are at infinite separation.

Therefore, the potential at a point is defined as the amount of work done in bringing a unit positive charge from infinity to that point.

Unit of Potential: The SI unit of work is joule (J) and that of charge is coulomb (C) therefore potential is measured in joule per coulomb which is called volt (V).

The potential at a point is said to be one volt when one joule of work is done in bringing one coulomb of charge from infinity to that point.

If W is the work done in bringing a test charge Q from infinity to a point, then the electric potential at that point is given as the first formula below, from which it can be inferred that the work required to move a charge Q from infinity to a point with potential V will be as per the second formula.

Potential Difference In practice when considering the flow of current through a conductor, we need to consider the flow of charge between two points of the conductor. Therefore it is not necessary to know the exact potential at each point but it is sufficient to know the potential difference between the two points.

The potential difference between two points is defined as the amount of work done in moving a unit positive charge from one point to the other.