*The following information is a single lesson in a larger project. Find more great projects *here*.*

**Return to Previous Lesson: **Build a Parallel Circuit

**Lesson Overview:**

Now we'll learn a fundamental law of electricity!

## Step 1: Ohm's Law

You have already learned a bit about current, voltage, and resistance. Did you know that in electrical circuits, all three are related to each other?

When you change one of these in a circuit, it affects the others. The relationship between them is known as Ohm's Law, named for Georg Simon Ohm, who discovered it.

VOLTAGE (V) = CURRENT (I) x RESISTANCE (R)

Take a look at the diagram below. This is a helpful way to visualize Ohm's Law.

You can use the image to help remember the relationships between voltage current and resistance. Put your finger over any of the three, and you see how it relates to the other two!

- Continue to the next step.

## Step 2: Applying Ohm's Law

Take a look at the circuit in the Workplane. We are using a device called a multimeter to show us how much current is flowing through a resistor and LED. A multimeter is a tool that can verify the amount of resistance, current, or voltage in your circuit.

In this step, you will see how the measurement on the multimeter window changes as you increase and decrease the value of the resistor.

When measuring amperage (i.e. current) in your circuits, values will be in the milliamp range. 1 mA = .001 A, which is one thousandth of an amp.

- Simulate the circuit. Make sure the multimeter tool is in Current Mode by clicking on the "A" or "Amps" icon, as in the picture above.
- While the simulation is running, change the value of the resistor by highlighting it and using the drop down menu. We suggest trying a range from 100 ohms to 100 k-ohms.
- How does the brightness of the LED change with the amount of current flowing through it?
- How does the current change with resistance? How low of a resistance value can you set before the LED burns out (represented by a starburst symbol)? At this point, the multimeter will tell you the maximum current that the LED can handle.
- Continue to the next step.

## Step 3: Calculating Current Flow

In the previous step, you saw that resistance is inversely proportional to current flow. When performing Ohm's Law calculations, you can apply the equation to individual components. Let's take a closer look at what happens to a 220 ohm resistor in this circuit.

Referring to the circuit diagram below, the 5 supplied by the battery is shared by the LED and resistor -- the LED uses 2V, so there is a 3V drop left over the resistor.

Apply the equation V = I x R to the resistor to figure out how much current is running through the circuit.

- Simulate the circuit again, and change the resistor value back to 220 ohms.
- To find the amperage running through the circuit, replace the values in the equation, focusing on what happens to the resistor: You should have 3 = I x 220 You’ll find that I = 0.0136. That’s about 13.5 thousandths of an amp, or 13.5 milliamps (13.5 mA).
- 13.5 mA is a safe current value for these LEDs, which is why we always use a 220-ohm resistor to protect LEDs (limit current) in the Arduino projects.
- Continue to the next step.

## Step 4: Review

Congratulations, you now understand some basic electrical theory!

You've learned about the properties of voltage, current, and resistance while building a circuit on a breadboard. You also used buttons to demonstrate the concept of components in series and parallel. Finally, with some components like LEDs, resistors and buttons, you created the simplest interactive system: a user presses a button and a light turns on.

The fundamentals of working with electronics will be expanded upon in upcoming projects. In those projects, you should think about how Ohm's Law applies to your circuits.

Don't forget the image below!

- While it's not necessary to use one for the Arduino Basic Kit projects, a multimeter can be a useful part of any engineer's toolbox. There's a good description of how to use on online at arduino.cc/multimeter. You can also check out the 123D Circuits multimeter tutorial.
- Congratulations on completing this project!

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