We all know that every object has a tendency to move from a position of higher potential to a position of lower potential. Even winds flow from an area of higher pressure to an area of lower pressure.
Similarly, even electrical current follows the same convention. But first we need to understand how-
Current and Electric Potential:
Potential is the electrical state of a conductor which determines the direction of flow of charge when two conductors are either kept in contact or joined by a metallic wire.
Current is defined as the rate of flow of charge. Its unit is ampere (A). Its symbol is I.
Let there be a conductor A having a negative charge i.e. having excess free electrons and another conductor B having a positive charge i.e. having a deficit of electrons. When these two conductors are kept in contact or joined by a metallic wire, the positive ions in B are not free to move but the free electrons in A are. Thus the electrons move from A to B i.e. from a region of higher concentration of negative charge to lower concentration of negative charge. This transfer of electrons will continue to occur till both A and B acquire the same charge.
The conductor A is said to be at a lower potential (negative) and B is said to be at a higher potential (positive). As electrons flow from A to B, to keep the convention of flow from higher to lower level we say that the electric current flows in a direction opposite to the flow of electrons i.e. from A to B.
Thus, while electrons flow from A to B, the electric current flows from B to A.
As we know, work is done when we move an object by using force. Work is thus also done when we move the charge from one point to the other.
Thus, the potential difference between two points is defined as the work done in moving a unit positive charge from one point to the other. Its unit is Volt (V). Its symbol is V.
When current flows through a conductor, the wire offers some obstruction to the flow of current. This obstruction offered to the flow of current by the conductor or wire is called its resistance.
Resistance is important in order to regulate the amount of current that flows through a conductor.
Since every conductor has free electrons as well as an equal number of positive ions, the positive ions do not move while the free electrons move and in doing so, collide with the positive ions. This slows down the speed of flow of electrons. This slowing down of the flow of electrons is called the Resistance offered by the metal wire to the electrons flowing through it.
Thus, the electrical resistance of a circuit component or device is defined as the ratio of the voltage applied, to the electric current which flows through it:
Resistance is denoted by R and its unit is Ohm (?).
We know that current flows in a conductor when potential difference is maintained across its ends. The relationship between current and potential difference is denoted by a law called the Ohm’s Law.
According to Ohm’s Law, the current flowing in a conductor is directly proportional to the potential difference across its ends provided that the temperature and other physical conditions of the conductor remain constant.
It means that when the potential difference applied across a circuit is doubled, the current flowing in it also gets doubled.