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Explain the microscopic nature of Electric current?i.e What is is average current and Instantaneous current? A microscopic view what really happens?

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From a microscopic point of view you can image metal (conductors) in a lot of different ways. The easiest model is the Drude model in which atoms are fixed in the space and everyone have one or two (in a metal) free electrons. When you apply an external electric field this particles move as a consequence of Coulomb force. It's important to say that electric field permeated space with light velocity and so in everyday situation electrons along all the wire (for instance) start "immediately" to move. So the current is the charge variation in time in a fixed point along the wire. The concept of current is intrinsically connected to time. You can measure current average simply consider in a point the charge variation for the wanted time interval. In order to understand instantaneous current think of the difference between instantaneous and average velocity in classical mechanics. Put your time interval very small you reach in practise the instantaneous current value.

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Electrical current can be carried by conduction electrons, or by 'holes'.

For ordinary matter, there is roughly one electron per two daltons of matter, which is to say, $\frac 12 N_Ae$. This roughly works out at $480*10^6$ coulombs per kilogram. Many of these electrons are bound in the inner orbitals, but there are still plenty of conduction electrons per kilogram, that a 10-ampere current is not going to move in a day.

The bulk of electric current is then carried by random jumps of a very tiny amount of conduction electrons, which slush through the metal, or a similar process of an absence of electrons (holes), bubbling the other way through the metal, under the effect of $E$ along the conductor. The exact nature of the carrier is found by the Hall effect.

The field, on the other hand, travels very fast. But it is a field, not a flow of electrons. This mainly happens, from each segment of the wire, and is replenished by similar currents started in the previous segment of wire.

The energy is carried outside the conductor, in the poynting vector. The purpose of the conductor is to shape these fields.

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