There are two types of charges: positive charge and negative charge. Like charges repel each other while opposite charges attract each other.
The force of attraction (or repulsion) between two charged particle is given by Coulomb’s law, according to which the interaction force between two charged particles is directly proportional to the product of charges and inversely proportional to the square of the distance separating them.
If we try to bring a small charge q close to an already exiting similar charge, Q, called source charge, it experiences a force of repulsion. Hence, work is to be done to bring it to any point at a distance from the source charge which is stored up as electrostatic potential energy.
Potential energy per Coulomb of charge is called potential. Chemical cell, such as dry cell is a source of potential difference and voltmeter is a device which is used for measuring potential difference between two points.
Electrically, materials are of two types: conductors which can conduct charges through them due to presence of large number of free electrons and insulators which do not conduct charge due to lack of sufficient number of free electrons.
When a cell is connected across a conductor the free electrons move from lower to higher potential in external circuit. The flow of electrons from lower to higher potential in the conductor constitutes electric current. Electric current is defined as the rate of flow of charge.
I = Q/t
Conventionally, the direction of current is taken as the direction of flow of positive charge, that is, opposite to the direction of flow of electrons. Thus, conventional current flows from positive to negative terminal of the cell in external circuit.
Current is measured in amperes (A) with the help of a device called ammeter. When potential difference across a conductor is increased, the current through the conductor also increases such that the ratio V/I remains constant.
The relation V/I = R is called Ohm’s law and the constant R is called resistance of the conductor. SI unit of resistance is ohm (Ω) and it is measured with the help of a device called Ohmmeter. Resistance of a given conductor is constant at a constant temperature.
Resistance may be combined in two different ways: in series and in parallel. In series combination resistors are connected end to end in such a way that same current flows through all the resistors. In parallel combination one end of all the resistors is connected to the positive terminal of the battery and the other end to its negative terminal.
There are three effects of electric current: heating effect, magnetic effect and chemical effect. When current is passed through a conductor, work is done in over coming the resistance, which manifests itself in the form of heat. This effect is called heating effect of electric current and is the basic principle behind electrical appliances like electric stove, electric iron, electric geyser, room heater, soldering iron and electric bulb.