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Making Predictions About a Resistor's Properties: Physics Lab

Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

In this physics lab, you'll explore how the total resistance of a circuit changes based on the configuration you put the resistors in. You'll see how this change matches the mathematical formulas used to calculate the total resistance of a circuit.

Introduction: Resistors

Goal: To see how the configuration of a circuit changes how a resistor behaves.
Age: High School and up.
Time to complete: Approximately 1 hour.
Safety concerns: Be careful and stay protected so nobody gets electrocuted when working with a live power source. Use proper safety procedures when working around electricity.

Resistors are simple elements made out of materials that make it harder for an electrical current to pass through it. Resistors are made in a variety of resistance values. The higher the resistance value, the harder it is for an electrical current to pass through.

A resistor
resistor properties

You'll find that most electrical products have resistors inside them. Open up your computer and you'll find tiny resistors on your motherboard. Open up your television and you'll find resistors on the circuit board that runs your television.

Interestingly, depending on how you configure your circuit, your resistors will behave differently. In this physics lab, you'll build two different circuits and test the total resistance of each circuit to see how it compares with the properties of your resistors. Your measured result should be the same as your calculated result.

Materials

To do this physics lab, you'll need the following materials:

  • A power source
  • 3 100 Ohm resistors
  • 1 250 Ohm resistor
  • 4 connecting wires
  • 8 wire connectors
  • A digital multi-meter (DMM)
  • Paper and pencil

Steps

Safety Tip! Make sure your power source is disconnected and turned off before using it to build your circuit.

1. Build this circuit:


resistor properties


This type of resistor configuration is called a series configuration where the resistors are connected with each other. As you can see, the electrical current must flow through each resistor to return to the power source.

2. Calculate the total resistance using this formula for resistors in series:

  • Rtotal = R1 + 2 + … + n

Write down your calculation in your notebook. This is what you expect your measured value to be.

Safety Tip! Before turning on your power source, make sure that you are not touching any of the wires in your circuit and make sure that none of your tools are touching the circuit either.

3. Turn on your power source. Take your DMM and measure from the top of the first resistor R1 to the bottom of the second resistor R2. Record this measurement in your notebook. This is the total resistance of your circuit.

4. Calculate the total resistance using this formula for resistors in series.

  • Rtotal = R1 + 2 + … + n

Write down your calculation in your notebook.

5. Rearrange your resistors to make this circuit:


resistor properties


The resistors in this circuit are in a parallel configuration. In this type of configuration, your current can flow through either the first or the second resistor.

6. Calculate the total resistance using this formula for calculating the total resistance of resistors in parallel:


resistor properties


Record your calculations in your notebook. This is your expected value.

7. Turn on your power source. Use your DMM and measure the total resistance from one common side of all the resistors to the other common side of all the resistors. So, for a group of parallel resistors, simply pick one resistor and measure the total resistance across this resistor. Record this measurement in your notebook.

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