Electric Current Definition
In metals, there are a large number of free electrons which moves randomly from atom to atom.
When the electric potential difference is applied across a wire of metal, free electrons which are negatively charged, attracted by positive terminal of the cell. Due to this force of attraction, free electrons start moving towards the positive terminal of the cell.
This moving stream of electrons is called the electric current.
The flow of electrons in the wire is from Y to X i.e. from the negative terminal to the positive terminal of the cell through the external circuit. But, the direction of flow of electric current is considered from X to Y. It is a convention.
The charge flowing per second in an electric circuit is the measure of electric current in that circuit.
Electric current, I = Q/t
The charge is measured in coulombs and time in seconds, therefore, the unit of electric current is coulombs/second or ampere.
When, Q = 1 coulomb, t = 1 sec then, electric current, I = 1 A.
Therefore, one-ampere current is that amount of electric current when one-coulomb charge flows through any section of wire in one second.
One electron carries 1.6 x 10-19 C of charge. Hence, for 1 C of charge, 1/(1.6 x 10-19) = 6.25 x 1018 electrons should flow in one second.
Thus, 1 ampere = 6.25 x 1018 electrons per second.
This, however, does not mean that a wire carrying a current of 1 ampere, 6.25 x 1018 electrons per second from one end of the wire to the other. Instead, it means that in one second, 6.25 x 1018 electrons enter at one end and the equal number of (other) electrons leaves at the other end.
The electric current is denoted in DC circuits by ‘I’ and in AC circuits by ‘i’. It is a scalar quantity although in diagrams we represent current in a wire by an arrow, it does not mean that the current is a vector quantity. The arrow simply indicates the direction of flow of positive charge in the wire.
Electric Potential Definition
When a body is charged, either electrons are supplied to it or electrons are removed from it, thus work is done in charging it. This work done is stored in the body in the form of potential energy.
Now, the charged body has the capacity to do work by moving another charge either by a force of attraction or repulsion. This ability of a charged body is called electric potential.
The capacity of a charged body to do work is called electric potential.
The greater the capacity of a charged body to do work, the greater is its electric potential. Obviously, the measure of electric potential is the work done to charge a body to one coulomb i.e.
Electric potential = work done/charge
V = W/Q
The work done is measured in joules and charge in coulombs, therefore the unit of electric potential is joules/coulomb or volts.
If W = 1 joule; Q = 1 coulomb; then V = 1/1 = 1 volt.
Hence, a body is said to have an electric potential of 1 volt if one joule of work is done to charge the body to 1 coulomb.
Electric Potential Difference Definition
When the electric potential of one charged body differs from the other charged body, there exists a potential difference between them. Both the bodies are under stress and strain and try to attain minimum potential.
Thus, the difference in the electric potential of two charged bodies is called potential difference.
The unit of potential difference is volt.
If one joule of work is done in transferring one coulomb of charge from one point to the other, the points are said to have a potential difference of one volt.
- Consider two charged bodies A and B. There is the excess of electrons on body A, therefore, it has an electric potential of 5 V (negative).
- Similarly, on a body B, there is the excess of electrons and it has an electric potential of 3 V (negative).
- There exist stress and strain between the two bodies which is called electric potential difference.
- If the two bodies are connected through a wire, the electrons will start flowing from body A to B till the two bodies attain the same potential. The electrons flow from one body to the other body only when potential difference exists.
- The flow of electrons stops when the two bodies attain the same potential and the potential difference is zero.
Potential difference = 5 – 3 = 2 V.