CHARGE AND CURRENT

The concept of electric charge is the underlying principle for explaining all electrical phenomena. The most basic quantity in an electric circuit is the electric charge. Important characteristics of electric charge are :-

1. The charge is bipolar, meaning the electrical effects are described in terms of positive and negative charges.
2. According to experimental observation, the only charges that occurs in nature are integral multiple of electronic charge e=-1.602*10-19 coulomb.
3. Electrical effects are attributed to both the separation of charge and charges in motion.
4. The law of conservation of charges states that charge can neither be created nor destroyed, only transferred. Thus, the algebraic sum of electric charges in a system does not change.

Effect of electric charge can be experienced when we try to remove our woolen sweater and have it stick to our body or walk across a carpet and receive shock.

Charge is an electric property of the atomic particles and is measured in coulombs(C).

Now, consider the flow of electric charges. A unique feature of electric charge is that it can be transferred from one place to another, that means it is mobile, where it can be converted to another form of energy. The motion of charge creates an electric fluid(current).

We know that a conducting wire consists of several atoms and a battery is a source of electromotive force. When a conducting wire is connected to a battery, the charges are compelled to move. Positive charges move in one direction while negative charges move in opposite direction.

This motion of charges creates electric current. Convention is to take the current flow as the movement of positive charges, that is, opposite to the flow of negative charges as shown in figure below: -

The electric effects caused by charges in motion depends on the rate of charge flow. The rate of charge flow is known as the electric current.

Mathematically, the relationship between current, charge and time is,

                                        
$\; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \;$

where, 
i=the current in Ampere

q=the charge in Coulomb

t=the time Second

The charge transferred between time t₀ and t is obtained by integrating both sides of equation (1.1) we get;

Equation (1.1) suggests that current need not be a constant valued function.

When a current is constant with time, we say that we have direct current (dc). Thus, a direct current (dc) is a current that remains constant with time.

On the other hand, a current that varies with time, reversing direction periodically is called alternating current (ac). Thus, an alternating current is a current that varies with time periodically. Alternating current is used in our household, to run the refrigerator, toaster, air conditioner and other electrical appliances.

 As mentioned earlier, direction of current flow is conveniently taken as the direction of positive charge movement. Based on this convention, a current of 4Amp may be represented positively or negativity. This is shown in figure below where a lamp is connected in series with a battery.

In above figure i₁₂ =4Amp, this means the current through the lamp with its reference direction pointing from 1 to 2. Similarly, i₂₁ is the current with its reference directed from 2 to 1. Of course, i₁₂ and i₂₁, are the same in magnitude and opposite in sign, because they denote the same current but with opposite direction. Thus, we have,