Engineering Principles V11

Clive W. Humphris

BASIC ELECTRONICS: Finding Circuit Voltages.  

When calculating a voltage, in this case a DC we are determining the potential difference (PD) developed across an electrical device due to the current flowing through it. This will be explained further when we come to explore Ohm's Law. In this instance we are finding the voltages developed across the resistor and the lamp.

The components shown are in what is called a series circuit, the resistor is in series with the lamp as the electron current flow is through one followed by the other. Remove either the resistor or the lamp and the circuit ceases to function, what's known as an open-circuit.

This simple circuit provides the opportunity to introduce some electronics mathematics which for the moment its sufficient to say that if we want to find how much current is being drawn from the battery when the lamp is lit, we can calculate it. Found by dividing the sum of the voltages across the resistor and lamp by the total circuit resistance. When the individual potential differences are added they will always equal the supply potential of the battery. This is known as Kirchhoff's Voltage Law.

Note: the polarity of the voltage. This is important when it comes to connecting your Voltage Test Meter where the black lead is applied to the more negative part of the circuit and the red lead to the more positive.

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