The flow of electrons around a circuit is known as the electrical current. This is given the symbol I and is measured in Amperes, or Amps (A) for short. The "pressure" that drives the current is known as most commonly the Voltage (V), but also as the Potential Difference (PD) or Electromotive Force (emf). This is measured in Volts (V).
The amount of current that will flow for a given circuit is dependent upon the resistance of that circuit. This resistance is measured in Ohms and is given the symbol Ω.
The relationship between Voltage, Current and Resistance is given by Ohms Law that states that "the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points". Represented mathematically this is:
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| Volts = Amps x Resistance
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| V = I ⋅ R
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Voltage is the pressure that pushes current through a circuit and it can be in two forms, namely Alternating, or Direct. This produces a corresponding Alternating Current (AC) or Direct Current (DC).
In AC circuits the rate at which the voltage is alternating is known as the frequency (f) and this is measured in Hertz (Hz.) In Europe and much of the world the electricity supply is generated at 50Hz, which means the voltage changes 50 times a second, or once every 20 milliseconds. In the USA and a few other places this frequency is 60Hz.
Again, with AC waveforms, since as much of the waveform is positive as is negative, the average value of the waveform is zero. Obviously this isn't a
good way of specifying how much Voltage (or current) is in the waveform. One way round this is to quote the peak values, however, since it is possible
to have a distorted waveform, the amount of energy conveyed may be different for two waveshapes of the same peak value. A way round this is to use a mechanism
called ROOT MEAN SQUARED, or RMS for short. RMS is calculated by squaring the waveform (multiplying it by itself) which makes everything positive (A -ve x -ve makes a +ve remember), taking the Mean (or Average)
of this value, and then Square Rooting the result to obtain a value. Don't worry, you don't need to do this in the real world. All you need to know is that
for a sinewave system the peak voltage is equivalent to √2 ⋅ Vrms and that RMS voltages and currents can (and should) be measured with true rms multimeters.
Throughout Europe, the Single Phase utility voltage is 230Vrms and three phase is 400Vrms.
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