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Understanding voltage by digging a hole

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John is 1.8 meters (6 feet) tall.

And in this email I'm going to put him into a hole to show you what negative voltage means.

But first, what does it mean that John is 1.8 m tall?

Could you find his height by only looking at his head? No. You have to compare the top of his head to the ground he is standing on to be able to find his height.

That John is 1.8 m tall really means that the top of his head is 1.8 m higher than the ground that he stands on.

​
It's the same thing with voltage.

You can't say anything about the voltage without comparing it to another point.

It's common to define a zero point, or "ground", in a circuit. In a simple battery circuit, the ground point is usually the minus terminal of the battery.

So if someone says that "this point is 5V", they usually mean it's 5V compared to ground.

​

What is a negative voltage?

Back to John. What if we dig a hole of 1.8 meters, then put John into the hole.

(Poor John)

His feet are now 1.8 meters below the ground.

Another way of saying this is that his feet are at MINUS 1.8 meters.

John is still the exact same person, he's just placed differently compared to the ground. Therefore the position of his feet becomes negative.

​
It's the same with voltage.

For example, imagine two 9V batteries. A battery being 9V means the plus terminal is 9V higher than the minus terminal.

Now we take one battery and say that its minus terminal is going to be the ground in our circuit.

What happens if we connect the plus of the second battery to the minus of the first one (i.e. we connect it to ground)?

​

​

Nothing really happens with the batteries. They're exactly the same as they were before connecting them.

And no current flows.

But what is the voltage on the minus terminal of the second battery?

Since the minus is 9V lower than the plus, and since the plus is connected to ground, the minus of the second battery must be minus 9V.

So, a minus voltage is nothing strange.

​

​

Keep On Soldering!
Oyvind @ build-electronic-circuits.com

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PS! If you find these lessons useful and want to learn more, check out Ohmify. It's my online academy for electronics where you'll learn everything from how voltage works to how you can design a microcontroller circuit.

You'll also learn practice skills such as soldering, printed circuit board design, and much more. And there are a bunch of project tutorials you can follow to build cool things like robots, amplifiers, synthesizers, and more.

​Click here to check out the details >>​

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