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
$Δt$
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,
$ΔQ$
is constant
Therefore, in this situation the current
$I=ΔtΔQ $
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
$Q$
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
$ΔI$
current is flowing through an area
$ΔA$
. 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
$θ$
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