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Electric current

What is Electric current?

There are randomly moving free electrons in a material. The number of free electrons in any material depends on whether it is conductor, insulator or semiconductor.  More number of free electrons is available in a conductor while few in insulators. At normal room temperature, free electrons move randomly in the material as illustrated in fig. below.

These random movements of electrons do not constitute electric current. But due to any external force or conditions, like battery,  electrons can be  directed  to flow in one particular direction.

The movement of electrons in a material in one particular direction, also known as drift , constitute electric current.

Definition: Electric current is the net amount of electron charges flowing through any given cross section of a conductor in a unit time.

Symbol    I  or I

SI Unit     Electric Current is Amperes (A) or Amps

 I = Q/t     coulomb/sec


We already know charge Q = 1.69×10-19 coulomb (col)

1 col  = 1/ 1.64×10-19 electrons

  1 col  = 6.24×1018  electrons


Now 1A = 1 col/sec

  Or 1A   = 6.24× 10 18 electrons/ sec


1A current means 6.24 × 10 18 electrons will be flowing within 1 sec time through unit cross section of the conductor.

For smaller amount of current represented as

mill Ampere (mA)    = 10-3 A

micro Ampere (µA)  = 10 -6A

nano Ampere (nA)    = 10 -9A


1 : If current of 100 µA flows for a time of 15s. Find the amount of charge that flows?

Solution    : Given I=100 µA=100 x 10-6A     , t=15s

I  = Q/t

Q = It

= 100 x 10-6  / 15

= 1.5 x 10-3 C

= 1.5 mC

2: The  charge of 30mC is transferred in time 20ms. Find the value of current?

Solution    :  Given  Q = 30mC = 30 x 10-3C   , t= 20ms = 20 x 10-3s

I  = Q/t

= 30 x 10-3 / 20 x 10-3

= 1.5 A

3: Calculate time required for current of 0.5 A to transfer charge of 60 C?

Solution    :  Given Q= 60C, I= 0.5A

I  = Q/t

t = Q/I

= 60 / 0.5

= 30 s


Exercise problems

1.       A current of 1A flows for 20 minutes. Calculate the amount of charge that flows? [ Ans: 1200 C]

2.       How much current would flow if 80 C of charge would flow for 150s time?  [Ans : 0.5A]

3.       Calculate the amount of time required for current of 120µA to transfer charge of 1mC.

[Ans:  8.33s]

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