Steady Current, Instantaneous Current and Current Density

In this digital era, we can't imagine our world without electricity. All the devices we use run on it

And when we switch on the devices, then electric charges flow through them and the circuit this device is connected to

This flow of charges will produce current in the circuit. And we can calculate the current using the above formula

So, to calculate the current, all we have to know is the amount of charge flowing for a given time through a given area

If n number of electrons passing through the area in time then the expression for current becomes

With this idea, we can discuss steady current and instantaneous current. and current density

Steady Current is when an equal amount of charge is flowing through a given area for a given time

Suppose the number of charges flowing through an area is not changing with time

So in this case, is constant

Therefore, in this situation the current is constant

We call this current steady current

Now let us discuss instantaneous current

In real life, we see that current is not always steady. Different amounts of current flow at different time intervals

Suppose the number of charges flowing is changing with time. Thus is a function of time

In this case, we do not get any fixed value of current. Rather at different time different values of current will appear

So, If we find the current or rate of flow of charges at a particular instant then that is called the instantaneous current

Mathematically, expression of instantaneous current is written as above

And in a reverse case, if we already know current, then we can find the amount of charge flowing in that given area

So here, We have learnt to relate charge flowing with time

Similarly, Current can also be related to the area through which it is flowing

This leads us to the concept of current density

Amount of current flowing per unit cross-section area is called current density

Though current is scalar quantity, current density is a vector quantity and its direction is same as the direction of the area

Suppose current is flowing through an area . Then the expression of current density is as above

If current density is known, current flowing through that area can be calculated by as above

Suppose the current is flowing at an angle with the direction of the area

Hence, current density can be written as above

Current density is a measurable quantity and is a required unit for measurement

S.I. unit of current is Ampere denoted by A

Since current density is current per unit cross-section area, we can say

Revision

When an equal amount of charge flows through an area in an equal interval of time, it is called steady current

Electric current flowing through an area per time instant is called instantaneous current

We can find total amount of charge for a time interval if current is known by

Amount of current passing perpendicularly per unit cross-section area is called current density

If current is at some angle θ with cross-section area, then current density is given by

If current density is known, current flowing through that area can be calculated by

The End