13.2.2 Net Flow of Charges

Consider an isolated tungsten filament in a light bulb before it was connected to a battery.

As a conductor, the filament is buzzing with free electrons moving at very high speeds of the order of 106 m s-1. The electrons are always colliding with the comparatively massive ions of the material, bouncing off at random directions. Since everything is random, at the end of the day, as many electrons moved forward as backward. There is no net flow of electrons. In other words, there is no electric current.

Now let’s connect the filament to a battery so that a constant leftward electric field E is developed inside the filament. The electrons will now experience a constant electric force, goading them rightward as they continue to be bounced in all directions. What results is a very slow drift of the electrons (amidst the random collisions) as a group in the rightward direction. This net flow of electrons, constitutes an electric current.

Electric current is measured in amperes. 1 A corresponds to 1 C s-1. For example, if an ammeter reports a current of 3.0 A, it means that there is a net flow of +3.0 C of charges through the ammeter every second. Shouldn’t it be −3.0 C? You may ask. Well, it’s time to talk about the conventional current.

Concept Test

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