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Electronics 102: Voltage

Watch on YouTube: Electronics 102: Voltage

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What does voltage do?

To simplify this as much as possible, just think of voltage as a pushing force that helps electrons flow around a circuit. I highly recommend watching the video above to gain a better understand of this.

Voltage helps "push" electrons around a circuit.

Voltage vs Current

Here’s a graph from the experiment in the video above. I’ve plotted the voltage against the current. You can see very clearly that as you increase voltage, more current flows.

 

According to Ohms law (which will be a future lesson), current is directly proportional to voltage. This means that one will increase in relation to the other. So in theory, this would be a perfectly straight line through the origin of the graph.

 

In reality, it's never a perfectly straight line, as shown by my graph below. There are many other factors involved such as tolerances, motor characteristics, etc.

Now that we know the simplified version of voltage, lets take a look at the real definition of voltage…

What is voltage?

There are actually two types of voltage, AC, which is 'Alternating Current' and DC, which is 'Direct Current'. But for this lesson, we're just going to generalize voltage as one thing.

"Voltage is the difference in electric potential energy, per unit charge, between two points."

Terminology

I want to cover is the terminology involved. This is where a lot of people get very confused. This is understandable because people are constantly using different names for all of these things:

Electric Potential Energy

Electric potential energy is the capacity to do work (Measured in Joules).

Electric potential is the amount of work required to move a charge from point A to point B, per unit charge. (Measured in Joules per Coulomb).

Electric Potential

Electric Potential Difference

Electric potential difference is the difference in electric potential between two points. (Measured in Joules per Coulomb)

Voltage / Potential Difference / Electric Potential

These all mean the exact same thing! Voltage is an electric potential difference expressed in Volts.

This all probably sounds a bit confusing, but lets break it down…

What is Potential Energy?

Potential energy is the capacity to do work. Work is any activity involving a force.

 

There are different types of potential energy. For example, stretching an elastic band creates elastic potentially energy, which has a potential to do work when you let it go.

 

The 9V battery below has electric potential energy. While disconnected from a circuit, it doesn’t do anything, but because of the chemical reactions happening inside the battery, it has the potential to do work.

What is Electric Potential?

Electric potential is simply the amount energy required to transport one unit of charge between two points.

For example, in a 9V battery, an electric potential of 9 volts is required to transfer 1 unit of charge from point A to point B.

 

It’s very important to know that electric potential is not the same as electric potential energy. We measure electric potential in Volts (or Joules per Coulomb), Whereas electric potential energy is measured in Joules, this is why they're not the same thing.

 

Electric current flows from a point of high electric potential, to a point of lower potential. This is why current flows when you connect a circuit to a power source that has a difference in electric potential. We call this conventional current flow.

Energy

There are many different forms of energy. We measure energy in Joules. All we need to know for this lesson is how energy and voltage are related.

This rule is nice and easy to remember. For example, 5V would be 5 Joules per Coulomb. 9V would be 9 Joule’s per Coulomb, you get the idea.

Electric Charge

The unit of electric charge is the coulomb. When we have current flowing, rather that saying we have 6.24 x 10^18 electrons flowing at a given point per second, we just say that we have 1A flowing. This makes everything so much easier.

 

Example 1

Lets go back to our little experiment. You can see that at 5V, the current draw was roughly 0.24 Amps.

 

At 5V, we have an electric potential, or an electric potential difference of 5V. We know that 5V is equivalent to 5 Joules per Coulomb. We also know that 0.24A is equivalent to 0.24 Coulombs per second.

The product of the two gives us the energy flowing through the motor per second. As shown below:

 

Example 2

In this example, you can see that at 10V, the current draw was roughly 0.42 Amps.

 

At 10V, we have an electric potential, or an electric potential difference of 10V. We know that 10V is equivalent to 10 Joules per Coulomb. We also know that 0.42A is equivalent to 0.42 Coulombs per second.

The product of the two gives us the energy flowing through the motor per second. As shown below:

 

Bringing it all Together...

So lets bring everything we just learned together. This is what most people call a 9V battery…

But now we know that what this actually means, is that this 9V battery has an electric potential difference of 9V between the positive and negative terminal. We also know that this 9V battery can deliver 9 Joules of energy per Coulomb of charge.

 

So now we know what the voltage of this battery actually represents. The voltage is the difference in electric potential energy, per coulomb, between the positive and negative terminal. That’s it... not so confusing after all. How awesome is that!

How is voltage measured?

Typically we measure voltage with a voltmeter or an oscilloscope, these both measure voltage in volts. As you saw above, power supplies measure their own output voltage, which makes things a lot easier for us engineers! It allows us to see what the output voltage is instantly, as opposed to having to measure it every single time.

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