..Electric Current

Electric current is described as the flow of an electric charge travelling through a medium. The electric charge is usually carried by electrons through conductors, for example, a wire. The standard unit for current is the ampere (A), which is named after the French physicist André Ampere (1775-1836). He is the physicist who demonstrated the relationship between electricity and magnetism. He also invented the right-hand rule. A current of one ampere signifies that there is one coulomb (C) of charge passing through a cross section of a conducting wire every one second.

Current = I (ampere - A) = q (coulombs - C) / t (seconds - s)

Example of a situation where one could use this equation:

20 C of charge is is passing through a 2 mm long cross section of wire in 40 s. Solve for current.

external image u9l2c4.gif I = Q / t = (20 C) / (40 s) = 0.50 A
external image u9l2c8.gif

Direction of Electric Current:
The electric field within a wire moves in the same direction that a positive test charge would move. Since the electric charge is carried by electrons in the wire, the current current would move in the direction that the electrons travel in. Therefore, current proceeds in a direction that opposes the electric field. This model of electric current is known as the electron flow current. However this is not the convention used when studying current. Ben Franklin performed substantial studies regarding electric current and imagined that positive test charges carried the current. This is not incorrect because while current is carried by electrons in a metal wire, the charge carriers in other conductors can be positive, negative, or both. Regardless, the early convention was established to be in the direction that a positive test charge and electric field would move. This convention is still used to this day in some places.


Current Carrying Wires and Magnetic Field:
A current carrying conductor - for instance, a metal wire - will produce a magnetic field around it because of the motion of charge within the wire. This motion produces a magnetic field around the wire in the form of concentric circles.

external image current%20carrying%20conductor.jpg
And the magnetic field that is set up by an electrical current will produce a magnetic force. The combined equation of these variables is:
F (magnetic force - N) = I (current - A) x L (length of wire - m) x B (magnetic field - T)
For an applet of the magnetic field of a straight current-carrying wire, click here: http://www.walter-fendt.de/ph14e/mfwire.htm

Uses of Current and Magnetism:
The wire can be wound into a coil whereby you end up with an electromagnet with both a North and South poles. Electromagnets are commonly used because of their strength and ease of use. Electromagnets are used as components of some electrical devices: motors, generators, loudspeakers, MRI machines, as well as many other scientific instruments.








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