Friday, July 31, 2009

Ohm's Law Without the Tears

The words “Ohm’s Law” triggers the fight or flight survival response in many air conditioning students. The thought of manipulating algebraic formulas with arcane symbols can cause enough anxiety that it is difficult for the simplicity of the underlying concept to break through. In truth Ohm’s Law is relatively simple.

I start by explaining why the letters E, I, and R are used. They stand for electrical properties, not the units we use to measure those properties. E stands for electromotive force, the force that causes electrons to move. We measure electromotive force in volts, that is why people often say that E stands for volts. Students often logically ask, why not use V? Explaining that E stands for Electromotive force will help them to remember E. I stands for the Intensity of the current flow. We measure the current intensity in amps. Using I to represent current is probably the most baffling of the three letters. I must confess that I personally had been working and teaching Ohm’s Law for several years before learning the I stands for current Intensity. When students asked why I was used to represent amps I simply said that is just the way it is. However, knowing what the I stands for helps people remember it. Finally, R represents resistance. Most people don’t have a problem with this. We measure resistance in ohms.

Of course Ohm’s Law is a mathematical relationship, but you don’t have to use mathematics to discuss the relationship of volts, amps, and ohms. For example, if you think about resistance as the opposition to current flow, it is easy to see that resistance and current will generally move opposite of each other. I like to use the analogy of a balance with voltage E being the pivot and resistance R and current I on opposite sides of the balance. This helps students to visualize that if resistance decreases, current must increase. Similarly, if resistance increases, current must decrease. This simple relationship is far easier for most students to understand than a mathematical equation.

The balance analogy assumes that voltage stays the same. Similar analogies can be used to discuss what happens if resistance stays the same and voltage changes. Draw the same balance but add an anchor or pivot on the end with resistance. Then it can be seen that if voltage moves up, current will have to move up with it. Likewise, if voltage moves down, current will have to move down with it. The same type of relationship can be shown by anchoring current. Then resistance must move up and down with voltage.

After graphically demonstrating these relationships, you can distill it all down to 3 simple rules:

  • If voltage stays the same, resistance and current move opposite each other.
  • If resistance stays the same, voltage and current move up and down together.
  • If current stays the same, voltage and resistance move up and down together.

Do a few class “shout outs” just to drill in the concepts and build confidence. After the class understands the relationship of voltage, resistance, and current, then you can introduce the formula and explain that it is simply a mathematical way of showing the same thing. Work a few simple problems that they can easily solve in their head for reinforcement and confidence building. I would wait until the next lecture to dive into calculations for loads in series and parallel. Let them enjoy their technical competence for a day.

Take a look at Unit 28 Basic Electricity in Fundamentals of HVAC/R to see a full treatment of circuits and Ohm’s Law using the balance analogy. A relatively simple treatment of Ohm’s Law series and parallel calculations is also shown, including detailed examples that are worked out.

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