Electric Current
Electric current is the result of the relative motion of net electric charge. In metals, the charges in motion are electrons. The magnitude of an electric current depends upon the quantity of charge that passes a chosen reference point during a specified time interval. Electric current is measured in amperes, with one ampere equal to a charge-flow of one coulomb per second.
A current as small as a picoampere (one-trillionth of an ampere) can be significant. Likewise, artificial currents in the millions of amperes can be created for special purposes. Currents between a few milliamperes to a few amperes are common in radio and television circuits. An automobile starter motor may require several hundred amperes.
The total charge transferred by an unvarying electrical current equals the product of current in amperes and the time in seconds that the current flows. If one ampere flows for one second, one coulomb will have moved in the conductor. If a changing current is graphed against time, the area between the graph's curve and the time axis will be proportional to the total charge transferred.
Electrical currents move through wires at a speed only slightly less than the speed of light. The electrons, however, move from atom to atom more slowly. Their motion is more aptly described as a drift. Extra electrons added at one end of a wire will cause extra electrons to appear at the other end of the wire almost instantly. Individual electrons will not have moved along the length of the wire but the electric field that pushes the charge against charge along the conductor will be felt at the distant end almost immediately. To visualize this, imagine a cardboard mailing tube filled with Ping-Pong balls. When you insert an extra ball in one end of the tube, an identical ball will emerge from the distant end almost immediately. The original ball will not have traveled the length of the tube, but since all the balls are identical it will seem as if this has happened. This mechanical analogy suggests the way that charge seems to travel through a wire very quickly.
Heat results when current flows through an ordinary electrical conductor. Common materials exhibit an electrical property called resistance. Electrical resistance is analogous to friction in a mechanical system. Resistance results from imperfections in the conductor. When the moving electrons collide with these imperfections, they transfer kinetic energy, resulting in heat. The quantity of heat energy produced increases as the square of the current passing through the conductor.
A magnetic field is created in space whenever a current flows through a conductor. This magnetic field will exert a force on the magnetic field of other nearby current-carrying conductors. This is the principle behind the design of an electric motor.
An electrical generator operates on a principle similar to an electric motor. In a generator, mechanical energy forces a conductor to move through a magnetic field. The magnetic field forces the electrons in the conductor to move, which causes an electric current.
A current in one direction only is called a direct current, or DC. A steady current is called pure DC. If DC varies with time it is called pulsating DC.
If a current changes direction repeatedly it is called an alternating current, or AC. Commercial electrical power is transported using alternating current because AC makes it possible to change the ratio of voltage to current with transformers. Using a higher voltage to transport electrical power across country means that the same power can be transferred using less current. For example, if transformers step up the voltage by a factor of 100, the current will be lower by a factor of 1/100. The higher voltage in this example would reduce the energy loss caused by the resistance of the wires to 0.01% of what it would be without the use of AC and transformers.
When alternating current flows in a circuit the charge drifts back and forth repeatedly. There is a transfer of energy with each current pulse. Simple electric motors deliver their mechanical energy in pulses related to the power line frequency.
Power lines in North America are based on AC having a frequency of 60 Hertz (Hz). In much of the rest of the world the power line frequency is 50 Hz. Alternating current generated aboard aircraft often has a frequency of 400 Hz because motors and generators can work efficiently with less iron, and therefore less weight, when this frequency is used.
Alternating current may also be the result of a combination of signals with many frequencies. The AC powering a loudspeaker playing music consists of a combination of many superimposed alternating currents with different frequencies and amplitudes.
We cannot directly observe the electrically charged particles that produce current. It is usually not important to know whether the current results from the motion of positive or negative charges. Early scientists made an unfortunate choice when they assigned a positive polarity to the charge that moves through ordinary wires. It seemed logical that current was the result of positive charge in motion. Later it was confirmed that it is the negatively charged electron that moves within wires.
The action of some devices can be explained more easily when the motion of electrons is assumed. When it is simpler to describe an action in terms of the motion of electrons, the charge motion is called electron flow. Current flow, conventional current, or Franklin convention current are terms used when the moving charge is assumed to be positive.
Conventional current flow is used in science almost exclusively. In electronics, either conventional current or electron flow is used, depending on which flow is most convenient to explain the operation of a particular electronic component. The need for competing conduction models could have been avoided had the original charge-polarity assignment been reversed.
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