Let’s explain what voltage is.

Volts push current around an electric circuit. Voltage behaves like a pushing force, forcing electrons to start moving around, which creates an electric current. You can use different voltages as electrical signals that represent data. Electrons push other electrons, current flows, and the electrons on the top side get attracted to the positive side of the battery, and everything flows in a complete circuit.

Voltage is a difference in electrical potential energy, per unit of charge, between two points.

what does this unit of charge mean? Well, do you remember how I was saying that electrons are negatively charged particles, and 1 amp is 6.24 x 10^18 electrons flowing per second? That’s a really awkward number, and engineers hate using it. Instead, we use a standard unit of charge, called the coulomb. The total charge on 6.24 x 10^18 electrons is equal to 1 coulomb of charge. And since electrons have a negative charge, this charge would be negative. You can see now that it’s much easier to just say 1 ampere is equal to 1 coulomb of charge flowing per second. And 2 amps is 2 coulombs flowing per second. Now let’s tie these two concepts together. When we talk about electrical potential energy per unit of charge, we mean that a certain number of joules of energy are being transferred for every unit of charge that flows. For example, let’s say this is a 1.5 volt battery. That means that for every coulomb of charge that flows from the battery, 1.5 joules of energy are being transferred. 1.5 joules of chemical energy are being converted into electrical potential energy. Then this electrical potential energy or “voltage” pushes electrons around the circuit, and for every coulomb’s worth of electrons that flow, 1.5 joules of energy are getting delivered to the light bulb, and converted into light and heat. Now let’s go back to my example with the motor. With the power supply set to zero volts, no current can flow. But with a flick of a switch, now the power supply delivers 1 volt, or 1 joule per coulomb. And over here, the power supply is measuring the amount of current flowing through the motor. It’s roughly 1.8 amps. 1.8 amps means that 1.8 coulombs are flowing from the power supply every second, and for every coulomb, 1 joule of energy gets transferred. So 1 volt times 1.8 amps means that 1.8 joules of energy are flowing through this motor every second. If we increase the power supply’s voltage to 2 volts, the higher voltage pushes more current, and now we have 2 coulombs per second flowing. 2 volts multiplied by 2 amps means that 4 joules of energy are flowing through this motor every second. And of course with more energy flowing through the motor every second, obviously the motor is going to do more work, and spin faster. Ok, now that you understand energy per coulomb, let’s go back to our definition of voltage and talk about this part. Voltage is the difference in electrical potential energy between two points. In other words, voltage is always relative. We all say this is a 9 volt battery, but that’s not 100% correct. What we are supposed to say, is that there is an electrical potential difference of 9 volts between this negative terminal, and this positive terminal. There’s a difference of 9 joules for every coulomb that flows out of the battery. And that is what makes this a 9 volt battery. Over here, in casual conversation we might say this is a 5 volt USB port. But what we really mean to say is that there is 5 volts between this positive power pin, and this negative power pin. These two pins are used for sending digital messages, and there’s a rapidly changing 3.3 volts between them which carries the information. So voltage is always measured between two points, and this is why voltage is also sometimes called potential difference. That’s right, voltage can sometimes be called potential difference, tension and a lot of other names.