Solar Installation Class – Ohm’s Law

July 18, 2008 · Filed under class, experiment, solar power · Tagged circuits, education, electronics, experiment, ohm's law, solar power

Okay, for all of you waiting for notes from the solar panel installation class, here’s the scoop. The first class was cool, there’s a big machine shop there, and as a bonus, we’re also going to learn about machine controls for wind systems. Nice! There promises to be lots of applied action working with actual solar panels, so I should be able to share some real knowledge of what works and what doesn’t. For those of you who’ve been with me since the beginning, we’re going to repair my solar panel (the inspiration for this blog!), too. Finally!

But of course, as with most things in life (and especially with electronics!), it makes sense to understand the science behind things before getting your hands dirty, or shocked in this case! So We spent the first class learning the basics of voltage, current, power, and resistance. Ever heard of Ohm’s Law? (Those of you having evil science class flashbacks, don’t worry, I wouldn’t share it unless it was quite necessary for your safety.) Here is Ohm’s Law for DC circuits.

P = I * E and E = I * R (“pie” and “ear” phonetically, to help you remember)

See? Simple! Only four letters! What does it all mean? Well, in a circuit, you have four things at work. First, Power. Power (the P) is measured in terms of watts. It’s the work being done using the energy in the circuit. So if you baked six pies, and your clown friend used all of them to cream the faces of his fellow friends, the power would be those six pies you made available for him to throw. When you hear talk about kilowatt hours, this is a measurement of how much power is produced and available to do work by a system. Or how much you are using, as reflected on your power bill. So, once again, that’s POWER (P).

Next up, Current. Current is a measurement of the electrons flowing through the circuit. You see, in order for the energy to get from one atom to the next in the wire, it pushes electrons down the line, carrying a charge (since electrons are negatively charged particles). This is what we think of as “flow”. Current is measured in amperes and is represented by the letter I (that’s “i”). CURRENT (I).

Third, you have resistance, which is measured in Ohms (it is Ohm’s Law, after all!). Usually, you will see resistance represented by the logical symbol R, but Ohms also have their own symbol, Ω. For the purposes of this discussion, I’ll stick to R. Resistance is what it sounds like, a measure of how hard it is for the electrons to jump from one atom to the next. It’s a measurement of volts per ampere (or how hard it is to push one electron from one atom to the next). It’s not a 100% rule, but usually, you will find that resistance stays pretty much the same in your circuit calculations. It is most affected by distance traveled, the width of your wire, and by temperature.  RESISTANCE (R).

And, lastly, there’s voltage, tying it all together. Voltage is also known as ElectroMotive Force (EMF), giving rise to the symbol for voltage, E. Sometimes you’ll see V for voltage, but here I’ll be using E. Voltage is measured in Volts (whew, an easy to remember one!). Solar panels, wind turbines, etc, are generally hooked up to 12 Volt batteries, either in series or parallel (we’ll get to that in a second), allowing you to store the energy produced by the panel/turbine. Common setups will run at either 12V or 24V, depending on application. So, rounding it all up, we have VOLTAGE (E).

POWER (P) – CURRENT (I) – RESISTANCE (R) – VOLTAGE (E)

Got it? Good. Now let’s start putting it all together. Using the formulas above, you will see that Voltage = Current * Resistance. So, when the voltage goes up, the current goes up also… they are directly proportionate. This is a VERY IMPORTANT CONCEPT in circuits. It’s the equation E = I * R, or “ear”.

The other equation, P = I * E or “pie”, is for figuring out how much power is produced by a circuit, and is equally important. Written out, it’s Power = Current * Voltage. In any given situation you’re probably going to know two out of four variables and you’re trying to figure one of the other two. With these two equations, you can always solve for the ones you don’t have already. Here’s an example:

If you know that you have a 12V battery and you need to get 72 watts of power to run your iPod, how much current do you need? By the way, current is usually adjusted by using bigger or smaller wires, because the size of the wire affects the number of electrons that can easily flow through at any one time. This is due to resistance, but I’m getting quite ahead of myself.

So using our equations, E = I * R and P = I * E, you know that E = 12 volts and P = 72 watts. So, the second equation can easily be solved: 72 = I * 12, or I = 72/12. This works out to I = 6. Since I is measured in amperes, the answer would be I = 6 amperes. If you then wanted to figure out the resistance in the circuit, you can now plug these numbers into the first equation: 12 = 6 * R, or R = 12/6, which works out to R = 2 ohms. Using two variables, you’re able to figure out everything happening along those wires!

Now let’s say you wanted to use a battery system supplying 24 volts instead. Simply substitute and do the above calculations in the same way. What did you get? Here’s the breakdown. Since E = 24 volts now, and P = 72 watts, then you should arrive at I = 3 amperes via 72 = I * 24, or I = 72/24, or 3! From there you can figure out the resistance of this circuit: 24 = 3 * R, and therefore R = 8 ohms. There is more resistance along this circuit than along the circuit running on 12 Volts, and less current.

Okay, so that’s Ohm’s Law (of DC circuits) for you! I’m a student too, so this is about as deep as I can go into the subject right now, but expect more detailed explanations and the reasons for knowing all this junk soon. Stay tuned! In the meantime, if you want to read more about Ohm’s Law and why it’s important, read the Wikipedia article here.