What is voltage?
If you live in the U.S., you know that a typical power outlet puts out 120 V.
You may know that the battery in your car is 12 V.
If you’re an Arduino enthusiast, you know probably know that the Uno has 5 V and 3.3 V outputs.
But what does all this mean?
What are volts, anyway?
This post is going to get back to the basics and explore that question.
If you’re an electronics veteran, you likely already know the answer. If you’re new you may not really understand what voltage is. Either way, read on. If you’re experienced, you’ll get a good review. If you’re new, you’ll get some answers.
On we march…
What is Voltage?
Electric Potential and Potential energy
First, let’s talk a bit about physics and some theory before we dive into a more practical idea of what voltage is.
Physics tells us that potential energy is the energy associated with the position or configuration of an object.
For example, a compressed spring has potential energy – it has the potential to do some kind of work when it unwinds or releases. But, when the spring fully uncompresses and comes to rest, it has no more potential energy and can’t do any more work.
Here’s another example everyone is familiar with — gravity. Let’s say you’re standing at the edge of the roof of your house 20 feet above ground. Because of gravity, your body has potential energy.
Let’s also say that you decide to test the structural integrity of your skeleton and step off the roof.
At the very instant you step off, your body has maximum potential energy. After that instant, the potential energy turns to kinetic energy as you fall. Then, the energy transfers to the ground as your body imparts a force on it (unfortunately for you, the ground also imparts an equal but opposite force on your body). Once it’s all over and you’re lying on the ground, you have zero potential energy and perhaps zero pulse (unless your skeleton is made from carbon fiber or titanium).
Figure 1 illustrates this concept in a much safer way, using a rock.
Figure 1: Right now, the boulder has maximum potential energy. When it falls, it will have kinetic energy and the potential energy will reduce as it gets closer to the ground (as the kinetic energy increases). Once it hits the ground and comes to rest, all the energy is used up and it has no potential energy.
So, what’s all this have to do with voltage and electricity?
Another term for voltage is electric potential.
This concept is a bit harder to visualize than gravity, falling objects, and springs.
For example, the positive terminal on your car battery is at a higher potential (more potential energy) than the negative terminal. For clarification on electric potential, consider figure 2.
Figure 2: electric potential a.k.a. voltage.
Above we see two oppositely charged plates, just like a capacitor. The positive side has a higher potential than the negative side. Perhaps the negative side is at a lower voltage, or ground (0 V), so it has less potential.
Now let’s introduce an electron. When the electron (in blue) is at the bottom near the negative plate, it has high potential energy because like charges repel and opposites attract. The particle will gain kinetic energy as it travels towards the positive plate. Once it reaches the positive plate, the electron has zero potential energy.
Note that the potential energy of the electron is independent of the potential energy of the plates. The plates still have a potential difference between them before the electron comes into play and after it comes to a rest at the positive plate.
The electron scenario is similar to the rock and your body. They both had zero potential energy when they finally reached the ground. Note however, that gravity is still there, just as the plates still had a charge on them after the electron came to rest.
One more thing to remember: only differences in potential energy are meaningful. So, electric potential is the potential difference between two points. We’ll touch more on this a bit later.
What is Voltage? The Practical Stuff
Now that our theory is out of the way, let’s talk about a more practical way to think about voltage. Consider figure 3.
Figure 3: a series circuit.
Here we have a series circuit with a battery, an LED and a resistor. As long as the battery isn’t dead the LED lights up.
The battery provides the voltage, or the electrical pressure. This pressure pushes current, which is simply moving electrons, through the wire, resistor, and LED causing it to light. The resistor limits this flow of current. If we left it out of the circuit, the LED would burn bright for a little while and then burn out due to excess current.
As you may have already witnessed on your own, electronic components can be destroyed by excessive current. And since current flow generates heat, it can also be a fire hazard.
To firmly grasp the concept of what voltage is, let’s go to an analogy we’re all familiar with — running water.
Voltage / Water Analogy
The circuit in figure 3 is kind of like a water pump with a valve attached to it.
With our water analogy, the pump produces the pressure we need to push the water through the pipe. We can think of this pump as the battery in our actual circuit as they both produce a pressure — one is electrical pressure and the other is fluid pressure.
Feast your eyes on figure 4.
Figure 4: water pump voltage analogy.
Next, we see the valve. The valve is like a variable resistor. When it’s almost all the way closed, very little water flows. When open all the way, maximum water will flow. So, the valve is analogous to the resistor in our circuit from figure 3.
Finally, we have the water itself. The water flowing through the pipe is like the current flowing through the circuit. Just as the current in the electrical circuit does work by lighting the LED, the water can do work if, say, we set up a water wheel at the end of the faucet. In fact, utilities often use water in hydroelectric dams to generate electricity.
Notice that like excessive current, excessive water pressure can be dangerous. And the more pressure the pump applies, the more water will flow.
If we allow too much water to flow at once, the pipes may burst, or the excess pressure may damage the faucet. Similarly, excessive current can fry the LED and make the wires hot, therefore burning them up. A higher voltage causes this excessive current to flow just as higher pressure from the pump causes more water to flow.
So now we know that voltage pushes current through a circuit to do useful work. The higher the voltage the higher the pressure. This means that current and voltage are proportional. If we increase the pressure in our water example, say by getting a bigger pump, more water would be forced through the pipe and it would come out of the faucet at a higher speed and pressure.
Ohm’s Law tells us this, but intuitively we know that applying a higher voltage to the circuit in figure 1 will make the LED shine brighter, assuming the resistor remains the same value. It’s not the voltage that makes the LED brighter, it’s the current. More voltage means more current flow.
One more important point to note is that when we measure voltage, or someone says something like the voltage at point A is 5V is that we are measuring voltage with respect to something. We hinted at this earlier, but here it is again: only differences in potential energy are meaningful.
For example, when measuring voltage across a resistor, we’re just measuring the voltage from point A with respect to point B, or one end of the resistor with respect to the other end. The resistor causes a drop in the “pressure” or voltage.
Figure 5: measuring voltage across a resistor is the same as measuring electric potential difference from point A to point B.
Often, you’ll measure voltage with respect to ground, which is the lowest voltage in a circuit, usually 0 V.
So, for example, if I wanted to measure the voltage at point A in the circuit from figure 6, I’d put one of my meter probes at point A and the other at ground since I’m measuring the voltage at A with respect to ground.
For more information on the definition of “ground” and what it means check out Getting Grounded: Types of Electrical Grounding and What They Mean.
Figure 6: measuring voltage with respect to ground is same as measuring the potential difference between ground and some other point on the circuit.
Now We Know What Voltage is
Now we know what volts are, at least from a bird’s eye view. For those who just want to scratch the surface, the concept of voltage as electrical pressure fits the bill perfectly.
On a low level, there’s more to the story of voltage. This story involves electric fields and calculus — stuff that’s beyond the scope of what most hobbyists need to know. Because of that, we’ll leave the low-level scientific-mathematical definition of voltage and electric potential to the universities and engineering text books, at least for now.
Perhaps writing such a post in future would give me a good excuse to blow the dust off my physics and electromagnetics text books. Such a post would be an advanced one no doubt, but would be a good review for engineers, advanced techs, uber nerds, and even myself.
Until then, drop a comment and tell us about your level of experience with electronics. Are you a noob? Engineer? Veteran? Weekend warrior? I’d love to know!