Intro

So, you need to learn ohm’s law, but you’re lost, or have no idea where to start. Don’t panic, you’ve come to the right place. In this video we are going to simplify basic ohm’s law for electricians, look a demonstration, and finally put the tires to the pavement and do some real calculations using what we’ve learned. We will cover series, parallel, and combination circuits in future installments.

Meet Dustyn

Hey my name is Dustyn; I’m a licensed master electrician with over 12 years of experience in the electrical industry. I am also a test prep and apprenticeship instructor.

What is Ohm’s Law

So, what exactly is Ohm’s law? To answer that question we have to acknowledge a problem with electricity. We can’t see it, we can’t hear it, smell it, taste it, or feel it. Sure, we can see lightning, we can hear thunder, and we can get zapped by it, but those are all observable effects from electricity, not the electricity itself. So, the problem with electricity is that is very difficult to observe, but we can measure it.

The things we can measure are voltage, current, and resistance. These are the key players in the battle that is ohm’s law! Let’s take a look at the lineup.

The values

Voltage is kind of like water pressure. Think of low pressure like low voltage, and high pressure like high voltage.  It’s how much “force”, or more technically “electromotive force”, which is the technical definition of voltage, is available. Voltage is symbolized by the letter “E”. I know that’s weird, why isn’t it the letter “V”? It’s because we use the letter “V” as our units – volts, so that leaves us needing a different letter to symbolize the unknown value. Therefore, we use “E,” from “electromotive force”. For example, E = 12V.

Current is similar to flow rate, or gallons-per-hour. A garden hose is a relatively low gallons-per-minute, so we could think of that as low current, whereas a river is moving lots of gallons-per-minute, and would be thought of as a higher current. Current is symbolized by the letter “I” for similar reasons as voltage. Current is measured in amperes, or amps, so we use the letter “A” for the units, and symbolize it with the letter “I”, which stands for “intensity.” For example, I = 4A.

Resistance is the final contender, which can be thought of as a restriction in a water line. The harder you squeeze a hose, the more the flow is restricted. Same for resistance. Resistance is measured in Ohms. It is symbolized by the letter “R”, which stands for resistance, and the unit is symbolized by the uppercase omega symbol. For example, R = 8Ω. Resistance is all around us, all wires have a natural resistance to them. Bad connections act like a resistance, and we have to treat them as such. We even have a component called a “resistor” that is an on-purpose resistance that can be added to a circuit.

Ohm’s law is what relates all three of these items together. Ohm’s law states that voltage is equal to current times resistance. This gives us the first formula: E = I * R. We can rearrange this formula, and derive two additional formulas; giving us a total of three ohm’s law formulas. Here’s the deal, we don’t have to memorize all three, we can just remember the first one: E = I * R. By filling in this triangle, we can cover up the value we want to solve for, and get the resulting formula. For example, we can cover up “E”, which leaves I next to R. When the leftover values are next to each other, that signifies multiplication. So, E = I * R. The formula we started with. If we wanted to solve for current, we simply cover up “I”, which leaves E over R. That gives us I = E / R, the second of our formulas. Finally, we can solve for resistance by covering up “R”, which leaves E over I. That gives us R = E / I. This trick is great for remembering ohm’s law, as you only need to memorize one formula. You can simplify the triangle by drawing a “T” bracket, and filling in the values.

Cartoon

I’m sure you’ve all see this little cartoon before, or something very similar. It is a great graphical representation of how ohm’s law works. Voltage (or pressure) is trying to push current (amps) through a wire (pipe), but resistance is restricting how much can actually move. Let’s visualize this concept using the water pipe analogy we’ve been using.

Demonstration

Here we’ve got an aquarium pump and a length of tubing. This hose clamp is going to act as our resistance. With the clamp fully open and the pump running, we have our maximum current flow. As the clamp is tightened down (increasing resistance), we can see the current flow decreases; less water (current) is making it through the pipe. If we increase the voltage, by adding a second pump, we can see the pressure (voltage) increases, and we can push more current through that restriction (resistance).

Practical Calculations

Let’s do a few practical ohm’s law calculations, one for each of the three ohm’s law equations. Consider this circuit. How much current would flow if we had a resistance of 6 ohms and a voltage of 12 volts? Looks like we need to solve for I, which is our placeholder for current. So, let’s grab our triangle, and cover up the letter “I”. That leaves “E” over “R”, meaning I = E / R. Now it’s just a matter of plugging our values in: I = 12V / 6Ω. 12/6 = 2 amperes. Easy peasy!

What about this circuit? Now we know the voltage and the current, but we don’t know the resistance. Let’s grab our triangle and cover up “R.” That leaves us with “E” over “I”, meaning R = E / I. Simply plug our values from the diagram into the formula, which gives us R = 24V / 6A, and we get a resistance of 4 ohms.

Finally, we have this example. Now we know the current and the resistance, but don’t know what the applied voltage is. Grabbing our triangle, we can cover up “E” – remember “E” is voltage – which gives us “I” next to “R”, meaning E = I * R. Plugging our values in we get E = 5A * 8Ω. That calculates out to E = 40 volts.

Midroll Note

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Wattage

We’ve just talked about the three items that can be measured for ohm’s law – voltage, current, and resistance. There’s actually a fourth item as well. That is power. Power is a unit of energy. We symbolize it using the “P”, and is measured in watts (“W”). Basically, it is the rate at which electrical work is performed. For example, a 60W light bulb does less work than a 100W light bulb. It puts out less light, and less heat. The 100W light bulb puts out more light, and more heat. There is another law that concerns Watt’s law. Watt’s Law states that power = current * volts (or amps times volts). This formula is easy to remember, because the variables spell the word “PIE” – check it out: P = I * E. Everybody like’s pie.

Let’s grab one of our previous example circuits. We calculated this first example out to find that the current was two amps. We can further calculate the power of this circuit by taking the current and multiplying it by the voltage. So, the wattage of the load is going to be P = 2A * 12V, or 24 watts.

Just like ohm’s law, watt’s law has three different arrangements of the same formula. And just like ohm’s law, watt’s law can be put into a triangle to help us remember all of these formulas. Just remember “PIE”, and use that to fill in the triangle. “P” goes on top, “I” and “E” go on the bottom. This helps us find the remaining two formulas: “I” = “P” / “E” and “E” = “P” / “I”.

The Wheel

No doubt you have seen a wheel like this before, and it certainly looks intimidating. The wheel is called ohm’s wheel, or watt’s wheel, and is actually pretty simple. The way this wheel works is we have four quadrants. Each quadrant calculates a different one of the four items we can calculate. Voltage, Current, Resistance, and Power. Each of the quadrants has three formulas within it. So, taking “E” for example, “E” equals I * R, “E” equals P / I, and E = sqrt(P * R). It works that way for the other three quadrants as well.

This wheel actually has Ohm’s law built within it. Here is E = I * R, like we just saw. Here is I = E / R, and here is R = E / I. Watt’s law is also in there. Here is PIE, or P = I * E (although it is written as E * I in the wheel, mathematically the same thing). and the other two formulas are there as well, here is I = P / E, and E = P / I.

That leaves us with six remaining formulas, where do those come from? These formulas are derived formulas. Basically, they are an algebraic mashup of both ohm’s law and watt’s law. I’ll show you one quick example, but don’t worry about knowing this – this is only for the curious. For example, let’s take this formula here: E = sqrt(P * R). It is derived by taking the two formulas I = P / E, and I = E / R, and combining them. I is equal to I, therefore P / E has to be equal to E / R. If we cross multiply and divide, we get E²/P = R. Multiply both sides by P and we get E² = P * R. Take the square root of both sides to undo the square on the E, and we get E = sqrt(P * R). All of these other formulas are calculated using the same algebraic concept: combine ohm’s law and watt’s law.

Make sure to watch the upcoming ohm’s law videos on series, parallel, and combinations circuits, and I’ll show you a way to solve them without really needing these derived formulas. Links for these will be in the description below when they become available.

Conclusion

Ohm’s law doesn’t have to be confusing. In a simple circuit there is only one voltage, one current, one resistance, and one power. When we get into series, parallel, and combination circuits, we will have multiples of each value, but I’ll show you to handle that when we get that point. Just remember, we really only need memorize two formulas: E = I * R, and PIE -> P = I * E.

If you want more practice with basic ohm’s law, I have a free worksheet and answer key you can download. Links in the description.

That wraps up basic ohm’s law. Don’t forget to feed the algorithm with all the things the algorithm craves; like, comment, and a subscription if I have earned it. See you on the next one!