Current Electricity
It is the branch of electricity which deals with the study of cause and effect of moving charges.
Conductors And Insulators
Materials which allow an electric charge to flow through them are called conductors or good conductors of electricity. All metals in solid form and aqueous solutions of salts are examples of good conductors of electricity. Gases in general are not conductors of electricity. They conduct electricity only under certain special conditions of temperature and pressure.
Materials which do not allow an electric charge to flow through them are called bad conductors of electricity or insulators. All plastic materials, glass, wood, rubber, ebonite, etc.,are examples of insulators in solid state. Pure water is an example is an example of insulator in liquid state. All gases in general are examples of insulators in gaseous state.
Electric Current
The rate at which electric charge flows through a conductor is known as electric current. It is denoted by 'i' and its SI unit is 'ampere'(A). Its CGS(Centimetre Gram Second) unit is 'biot'.
Electric current can be calculated mathematically by dividing the total amount of charge flowing through a conductor by the total time for which the current flows. Hence,
i = Q(Total Charge)
t(Total Time)
Unit of electric Current = Unit of Electric Charge
Unit of a Time
Ampere = Coulumb
Second
Hence, if 1 Coulumb of charge flows through a conductor every second, then electric current flowing through the circuit is said to be 1 Ampere.
Cause of Electric Current
Electric current through a conductor can be understood easily by an analogy of flow of water. Consider two cylindrical tanks as shown in the fig. The level of water in the first tank is greater than the level of water in the second tank. We will notice that the flow of water is from the first tank to the second tank. Hence, the level of water decides the direction of water flow. Also, the water flow ceases when the level of water in both the tanks becomes equal.
Similarly, in an electric conductor, the electric charge flows from one end to another end of conductor , only when an electric potential difference exists between the two ends of the conductor. Thus, an electric potential difference is the condition required for a conductor to have an electric charge through it from one end to the other and is measured in volts. If the potential difference across the two ends is zero, there exists no electric current through the the conductor.
Positively charged bodies are considered to be at a higher potential and the negatively charged bodies
are considered to be at a higher potential. So, when a positively charged body and a negatively charged are connected by means of a conductor, there exists s potential difference across the ends of the conductor and so an electric charge flows through the conductor from a positively charged body to a negatively charged body. Initially, it was considered that the flow of positive charges constituted an electric current. Later, it was discovered that it is not the movement of positive charges that cause electric current, but the flow of electrons that causes electric current in a conductor.
When it was discovered that the flow of electrons constituted electric current in a conductor, many theories and laws regarding electricity were proposed. So, these laws were not changed again and electric current was classified into two types. One type of electric current is the flow of charge from a body at high potential(positively charged body), to a body at a low potential(negatively charged body) as assumed before and is called 'conventional electric current'. The second type of electric current is the actual current which is, due to movement of negative charges from a negatively charged body to a positively charged body,called 'electronic current'.
