how to use ohms law

Ohms Law Power Wheel: Understanding the Electrical Formula Wheel

Last updated on February 27th, 2024.

Understanding Ohm’s Law is crucial for electricians. It’s the key to mastering how voltage, current, and resistance interact, simplifying complex electrical concepts.

Luckily, grasping Ohm’s Law is straightforward. Basic math skills are all you need, and the Ohm’s Law Wheel makes it even simpler to comprehend.

How To Use The Ohm’s Law Formula Wheel

ohms law electrical formula rule

You might notice the diagram resembles a triangle, but focus on the goal: determining a circuit’s amperage without an ammeter. Here’s how it’s done.

Simple, divide the VOLTAGE by the RESISTANCE of the circuit. How do you know this? Because of the Ohm’s Law formula wheel.

In the formula wheel you’ll see three letters – each one represent a value.

E or V = VOLTAGE (volts)

I= CURRENT (amps)


using the ohm's law formula wheel

So, if you need to find voltage, current, or resistance, simply place your finger over what you’re trying to find and the formula wheel will do the rest.

The Ohm’s law formula wheel is represented mathematically in three simple equations.

I (current) x R (resistance) = E (voltage)

E (voltage) ÷ R (resistance) = I (current)

E (voltage) ÷ I (current) = R (resistance)

Ohm’s Law Example Problems
ohm's law circuit

Find the resistance of the circuit. By looking at this circuit we know the values of two components: voltage (12v) and resistance (3 ohms). How do we find current?

We plug our known values into our formula wheel and work the equation.

12 volts ÷ 3 ohms = 4 amps

It’s as simple as that. Let’s tackle another example to find a circuit’s resistance using the given values:

Voltage = 120v

Current = 17 amps

Now plug the known values into our formula wheel and work the equation.

120 volts ÷ 17 amps =  7.05 ohms

I told you it was simple. Easier than installing a whole house surge protector er?

Granted, it might not be quite that straightforward, but it’s certainly simpler than fitting a refrigerator surge protector – which is just plug and play.

Principle’s of Ohm’s Law – Proportional and Inversely Proportional

There is a rule about ohm’s law that you need to be familiar with.

that the electrical current (I) flowing in an circuit is proportional to the voltage (V) and inversely proportional to the resistance (R).

This means that if the voltage is increased then the current will increase as long as the resistance doesn’t change. If the resistance is increased and voltage remains the same, then the current will decrease.

Increasing Resistance

120 volts ÷ 5 ohms = 60 amps

120 volts ÷ 10 ohms = 12 amps

120 volts ÷ 20 ohms = 6 amps


Therefore, if the voltage is increased, the current will increase provided the resistance of the circuit does not change.

Increasing Voltage

120 volts ÷ 25 ohms = 4.8 amps

240 volts ÷ 25 ohms = 9.6 amps

480 volts ÷ 25 ohms = 19.2 amps

As you can see when we increase the voltage and leave the resistance the same, the current increased (directly proportional to the voltage).

Ohm’s Law PIE Formula Chart

ohm's law pie chart

The PIE chart is similar to the voltage, current, and resistance formula wheel. Power is measured in watts and is defined as:

the rate at which work is done when one ampere (A) of current flows through an electrical potential difference of one volt (V)

Formula Wheel with Ohm’s Law and PIE

Ohm’s Law Formula Wheel

Now we’re getting somewhere. This formula wheel is a combination of both Ohm’s Law and the PIE formula.

It looks more complicated but in reality, it’s easy to use (you may need a calculator), and it works the same way as the previous charts.

The formula wheel is divided into four sections, each section has three formulas. If you need to find volts then you would use the E section, current – the I section, resistance – the R section, and power – the P section.

When using the formula wheel you need to follow these steps:

  1. Know what you’re trying to solve for: current (I), voltage (E), resistance (R), or power (P).
  2. What values you already know (you need two): current (I), voltage (E), resistance (R), or power (P)
  3. Find the section of the formula wheel that your values plug into.
  4. Solve the equation

When making calculations you have to use compatible values. What I mean is kilohms should be converted to ohms, milliamperes should be converted to amperes.

Whether you’re an electrician apprenticeship or Journeyman electrician, learning Ohm’s law is an essential part of an electricians role.

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About Thomas Hawkins

I run Electrician Apprentice Headquarters, a one-stop-shop for learning how to become a licensed electrician in the USA. I'm a licensed Master Electrician with over 20 years experience working in the Mining & Construction industries. Why do I do it? Well, because even plumbers need heroes.

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