The RJ Journal - Electronics

Current, or electric current, is the flow of free electrons in a conductive material due to a difference in electric potential, a voltage. Current is written with the letter I and is measured in amperes (A). Reference to current in a text or a schematic diagram can then be written as I3=20mA.

In order for current to flow in a conductor, the conductor has to be part of a closed circuit, that is, if the conductor is a wire it has to be connected at both ends to the same electric circuit. In analogy with the closed water pipe system, the current in an electric circuit is like the water flow in the pipe system. In contradiction to the closed water pipe system, the current will not flow out of a conductor that is disconnected at one end like water would flow out of a pipe that is disconnected at one end.

When voltage is measured, it is measured between two points. If you want to measure current you have to break the circuit and put the instrument in series with the current flow. There are other methods, mainly for measuring larger currents, without interrupting the circuit but I will not go into this here since it is not used for the low currents in electronic circuits.

If you could look inside a piece of wire in a closed circuit with current flowing through it you would see that when one electron is put in at one end of the wire, another electron pops out from the other end of the wire. The electrons themselves moves rather slowly through the wire (about a couple of tenths of a mm per second). The time difference between when one electron is put in at one end and when the other electron pops out from the other end is very fast though. You can say that the information about the input of the first electron is delivered by the output of the other electron and this is done at a speed little lower than the speed of light.

The current in a conductor is proportional to the voltage both in direction and magnitude. A DC voltage gives a DC current and an AC voltage gives an AC current. AC current is normally measured with its RMS value, just like AC voltages. As for voltage, the RMS value of an AC current is the equivalent DC current that would give the same power as the AC current in the same resistive load. The peak value of a sinusoidal AC current is also equal to about 1.41 times the RMS current value. To complicate things further, the AC current can be out of phase with the AC voltage. This happens if the current is flowing through reactive components, which are capacitors and inductors, and the amount and direction of the phase shift is depending on the net capacitance or inductance that the voltage source sees. This is mainly of concern for power electricity but should also be known to electricians since filter components can have the same effect on an AC signal, say an audio signal.

When working with electronics we will mostly have to do with currents in the mA (milli Ampere) range. An LED needs between 1 and 20 mA to shine with a bright light. A 3V flashlight with an ordinary bulb uses about 500mA. A smaller motor a couple of amperes, the starter motor in a car uses 30-60A.

Batteries are rated with a voltage and a current capacity in Ah (ampere hour) or mAh (milli ampere hour). If a battery is marked with 500mA it means that it can deliver 500mA for one hour or 1A for half an hour or 250mA for two hours before it runs out.

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