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# Wiring Fundamentals: Voltage

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## Voltage

### 02.31

 00:00 - When we're dealing with electricity, it can be hard to develop an intuitive feeling about what is actually going on, and how the different concepts relate to one another. 00:08 After all we can't actually see electricity and we usually only feel it when something has gone very wrong. 00:14 To help us develop a sense of what is happening, we're going to use a household plumbing analogy. 00:19 We'll think about a simple water pipe with one end connected to the main supply pressure and a tap on the other. 00:25 This is something easy to visualise so we're all familiar with. 00:28 And for the purpose of automotive electrics, does a pretty good job of relating the basic principles to one another. 00:35 Voltage is equivalent to the water pressure applied to one end of the pipe by the water mains system. 00:40 If you live somewhere with decent water pressure, this might be 60 psi for example. 00:45 When you open the tap at the other end of the pipe all the way, you drop the pressure at this end of the pipe to the same as atmosphere. 00:51 As we now have 60 psi of pressure at one end of our pipe, and zero psi at the other end, there is a pressure difference of 60 psi across the length of the pipe and the water will flow along it out of the tap. 01:04 Heading back to the electrical word, this is like connecting one end of our wire to the positive terminal of a 12 volt car battery. 01:10 You're applying 12 volts of electrical pressure to one end of the wire. 01:14 If you then connect the other end of the wire to the negative terminal of the battery, it's like opening the tap. 01:19 You're applying zero volts of electrical pressure to the other end of the wire. 01:24 This means there's now a 12 volt difference in electrical pressure along the length of the wire and the electrons in the wire will start flowing. 01:31 This might seem pretty simple to follow initially, but there are some subtle points that require a little more explanation. 01:36 In our pipe analogy you can imagine that when you had your tap at the end of the pipe closed, you could fit a pressure gauge to the pipe anywhere along its length and you will measure the 60 psi of pressure being applied by the mains system. 01:49 This is because the water is not flowing. 01:51 It has nowhere to go because the tap is closed. 01:54 Similar to this, in the electrical world, when you connect one end of a wire to the positive terminal of a battery but leave the other end unconnected or floating, it's like the tap is closed. 02:04 The electrons have nowhere to go, so they won't be flowing, and you'll measure the 12 volts of applied electrical pressure at any point along the wire. 02:13 The key concept here is that flow, be it physical or electrical, only happens when there is a difference in pressure at either end of the pipe or wire. 02:22 When you're reading or writing documentation, voltage has a unit of volts, and the symbol of a capital v.

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