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The greatest cause of confusion when performing otherwise simple electrical calculations is knowing which voltage, current, resistance etc. should be used in the formula.
For instance, to calculate power, multiply volts by amps.
If you have a tram running from a 550 volt power station and taking 100 amps of current, the power available to the tram may not be 550v x 100A = 55,000 watts. This is because some power is lost in the wiring between the power station and the tram.
In this example, suppose the voltage lost in the overhead wire is 50 volts, the tram motors will only be getting 500 volts.
The equation becomes:
Power at the tram = Voltage across the tram motors x Current through the tram
P = 500v x 100 A= 50,000 watts
Likewise, the power lost in the overhead wire is
Power in the wire = Voltage across the ends of the wire x Current through the wire
P = 50v x 100A = 5,000 watts (Which explains where the rest of the power from the power station went)
The critical point is that the voltage used in the above calculation is NOT the voltage ON the wire but the voltage difference between its ends.