Current

There are two ways of considering current direction. The first is the truth: that electrons flow from low potential to high potential (a bit like rolling uphill) as they are negatively charged. This is not very intuitive.

Instead we assume that the charged particles in an electrical circuit are positive, flowing from the positive terminal of the battery to the negative terminal. Since current in a loop is the same throughout, this assumption does not affect our calculations. 


Key Concepts

If there is a potential difference between the ends of a resistor, charge will flow. The rate of flow of charge is current.

Essentials

Earthing

If two charged bodies are connected by a conductor, charge will flow until there is no potential difference between them. If one of those bodies is the Earth, charge will flow until the potential of the body is 0 V.

This is because the Earth is a vast repository of charges, and so it is able to provide or take any charge required in order to make a body neutral.

Current by another equation!

It is still the case that current is the rate of flow of charge (\(I={Q\over t}\)).

However, it is also possible to calculate current from the following quantities:

  • n, the number density of charge carriers
  • A, the cross-sectional area
  • v, the drift (net) velocity of charge carriers
  • e, the charge on each charge carrier (electron)

The resulting equation is: \(I = nAve\)

Current is due to the movement of free electrons. Since they are negative, they drift in the opposite direction to the current flow.

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