Ohm’s Law

Ohm’s law is a mainstay in calculations related to circuits.  It doesn’t matter whether the circuit is simple or complex, nor does it matter whether the device is small and battery powered, or large and part of the national power grid. Engineers and technicians often end up using Ohm’s law equations to understand what is happening—or what is supposed to happen—in the circuit.

The Ohm’s law equations relate voltage, current and resistance.  It’s really just a single equation that can be rearranged one of three ways depending on what unknown you need to solve for:

   
In this activity, you will:

• Calculate and write scripts for all three versions of the Ohm’s law equations.  
• Save time and effort by solving for one quantity with others that are known or measured.
• Observe it is used to design features into products/projects in the Calculate Resistance to Get the Most Current section.

In this activity, you won't:

• need to build or probe a circuit!

Calculate Current from Measured Voltage and Resistance

One application of Ohm’s law is calculating current through a resistor from measured voltage.  That way, you won’t have to worry about changing the circuit to measure current.  Just probe a resistor's leads for the voltage across it, and then use I = V / R to calculate the current through it.  

Take a look at the resistor probed in the circuit below. From previous activities, you know the resistance is 220 Ω and the voltage is 1.20 V.

Instead of connecting the circuit to an ammeter, you can just use I = V / R to calculate the current through the circuit since you know that the resistance is 220 Ω and the voltage is 1.20 V.  Here’s the calculation:

I = V / R
  = 1.20 V / 220 Ω
  = 0.0054545... A
  ≈ 5.46 mA

You can also use Ohm’s law to calculate the smallest resistor to safely use with an LED circuit.  Why do that?  To get maximum brightness!  The LEDs in your kit are rated for up to 20 mA.