Electric Current
- Defined as the flow of electric charges. These could be in a conductor, or through
vacuum, or through a gas or liquid.
- We define electric current I in terms of the rate of flow of electric charge across some
area A, as in a wire:
∆Q
I ave =
∆t
Also as ∆t goes to zero , we can define the instantaneous current flowing as
dQ
I=
.
dt
Units for electric current are 1 C/s = 1 A (ampere, or amp)
- In most metal conductors, the “free” charges that are mobile and make up the electric
current are electrons (negative). In a proton accelerator, they are protons (positive).
In a salt solution of Na+ and Cl- ions immersed in an electric field, both types of
charges move. Whatever charge makes up the I is called the charge carrier. By
convention, we say that the direction of the current is that of the flow of positive
charge. If the actual charge carriers are electrons then the current flows in the
opposite direction to the electron flow.
Example:
How many electrons are flowing through a wire per second if the current is 1 mA (=10-3
A)?
∆q/∆t = 10-3 = N(1.6 x 10-19) C/s so N = 10-3/1.6 x 10-19 = 6.25 x 1015 electrons/s