Essential Ideas in Electronics: Voltage (Part 1)

VOLTAGE IN CIRCUITS

Fig 5 shows a possible circuit to provide the system blocks of figs 2 and 3. In this circuit the wires that carry the signals between blocks are shown in red. If you want to measure the ‘level of the voltage on’ these signal wires (you’ll see later why I’ve put this expression in quote marks), you need to use the arrangement shown in fig 6 where the highlighted symbol with a ‘V’ represents the voltmeter: The important thing to notice about the diagram in fig 6 is that the voltmeter, like the one in fig 4, is connected to both the signal line and the wire attached to the negative side of the battery. This is because a voltmeter measures the difference in something called the electrical potential between the two places it is attached to. This difference is expressed in volts. Electrical potential is also measured in volts. You’ll find out what electrical potential actually is in part 2 of this article; for now you can start to use the proper phrase. So, voltage is a measurement of difference; in fig 6 you are not measuring the voltage ‘on a wire’, rather you are measuring the difference, expressed in volts, between the electrical potential on the signal wire and the electrical potential on the zero volt side of the circuit (that’s why quote marks were used earlier). In fig 6 this difference is shown as 9.00V – which means the electrical potential of the signal wire is 9 volts greater than the electrical potential on the other wire which, following a common convention, we will agree to call zero volts or 0V.

 

There is one other thing you should notice about the voltmeters in figs 4 and 6. The positive side of the voltmeter (the red wire in fig 4 and the ‘+’ symbol in fig 6) is always connected to the signal line. The other side of the voltmeter (called the negative side) is connected to 0V. You can, if you wish, measure voltage between any two wires in a circuit. However, as a general rule, it is only useful to take measurements that can be compared sensibly to other voltage measurements. Because of this, the negative side of the voltmeter is always attached to the 0V side of the circuit and voltage measurements are always relative to this point of the circuit. This is a good place to bring in a useful analogy for voltage: Voltage is quite a lot like height (see table overleaf). You’ll see more use of this analogy in Part 2 of this article. To finish this first part of the article, there is one more important thing you should know. The word ‘voltage’ is not a proper scientific term. It’s a bit like using the word ‘metreage’ instead of ‘height’, as in “what metreage are you?”. The correct term is potential difference because it is the measurement of the difference in electrical potentials (in the same way you might ask the question ‘what’s the height difference?’ about two mountains). Because the word ‘voltage’ is in such common use, these articles will continue to use it – but you have been warned; don’t let a physicist catch you using the wrong term! Part 2 of this article will explore what an electric potential actually is and how a voltage (potential difference) can be created.

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