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Electric Current DefinedThe Flow of Charge through a Conductor
Jul 17, 2010 © Harry P. Schlanger
Just as a current in a stream involves the flow of water, electric current is the flow of electric charge - the amount of charge moving in unit time.
It does not matter if
electric charge
consists of electrons moving through the atomic structure of a metal, or protons travelling through
space from the sun - any moving charge constitutes an electric current. The magnitude of current
is defined simply as the rate of transfer of charge.
Charges Need a Pushing Force
Current flow is the result of charges that are pushed along the conducting path due to a force and this
is supplied by a potential difference along the conductor. Just like a stream cascades down a slope,
charges flow along the conductor from high potential energy to low potential energy.
Another way to view this
is that negatively charged electrons are attracted to a positive potential, and similarly, positively
charged particles are attracted to a negative potential.
Definition of Current Flow
We can think of current as the amount of
positive charge that flows past any point in a conductor in 1 second. When 1 coulomb of
charge flows past a point in 1 second, the current is 1 ampere. This ampere unit was named after the
French physicist and mathematician,
André-Marie Ampère (1775 - 1836).
Electric current is given the symbol I, and is
defined to be:
Electric current, I = Q / t
where Q is the amount of charge transferred in coulomb and t is time taken in seconds.
Some approximate values of typical currents are:
- Car starter motor 300 - 400A
- Toaster 2 - 5A
- Torch 0.25A
- Transistor radio 1mA - 1mA
Example of Using the Current Flow Formula
If 15 Coulomb of electricity passes a given point in 3 seconds, what is the current flowing in the conductor?
Data: Q = 15C and t = 3s
Substituting this data into the current formula, one obtains, I = Q/t = 15/3 = 5A.
Measuring Current Flow - Conventional Current
In an electrical circuit, an ammeter is used to measure current. The instrument is connected in series and is
calibrated such that the direction of current is in the direction of transfer of positive charge.
Because
the electrons move in the opposite direction to the current, the terms electron current and
conventional current are sometimes used to distinguish between them. However, it is important to
remember that the definition of current is the transfer of positive charge.
References:
- "Heinemann Physics 11 - Units 1 & 2". Carmel Fry et al. Harcourt Education, Port Melbourne,
Australia. 2003.
The copyright of the article Electric Current Defined: The Flow of Charge through a Conductor is owned by Harry P. Schlanger. Permission to republish in print or online must be granted by the author in writing.
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