| | Essential questions | | How is resistance measured? | |
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Ohm’s law, I = V/R, is the fundamental relationship among current, voltage, and resistance in a circuit. Devices that measure resistance are based on Ohm’s law. These devices apply a known voltage and/or current, then determine the resistance. In this investigation you will use a similar experimental technique to measure the resistance of a lamp. 
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Part 1: Current through different resistors
- Find a 100 Ω resistor. You will have to read the resistor values using their colored bands.
- Construct the circuit on the breadboard, and power it using a +3 V power supply (or two D-cell batteries).
- Connect an ammeter in series with the resistor and measure the current through the resistor.
- Repeat the experiment for at least four other resistors between 50 Ω and 2 kΩ (2,000 Ω). Tabulate your results for R and I. Calculate 1/R in each case.
- Graph I on the vertical axis and R on the horizontal axis.
- Make a second graph with I on the vertical axis and 1/R on the horizontal axis and measure the slope of the graph.
- Describe the relationship between current and resistance for a fixed voltage.
- What is the value of the slope of your second graph?
- What does the slope of this graph represent? Why?
How to do Part 1 using a myDAQ
You can conduct a similar investigation using the myDAQ. In this case, you will use a fixed voltage of +5 V, vary the resistance (using resistors from 50 Ω to 2 kΩ), and measure the current for each case.
- Gather a supply of resistors with rated values from 50 Ω to 2 kΩ. You will have to read the resistor values using their colored bands.
- Construct the circuit on the breadboard using a single resistor, and power it using the myDAQ power supply set to 5 V.
- Measure the current through the resistor.
- Repeat the experiment for at least four additional resistors. Tabulate your results.
- Graph your data with I on the vertical axis and R on the horizontal axis.
- Create a second graph with I on the vertical axis and 1/R on the horizontal axis. Fit a straight line to your data and measure the slope of the line.
- Describe the relationship between current and resistance for a fixed voltage.
- What is the value of the slope of your second graph?
- What does the slope of this graph represent? Why?
Part 2: Resistance of a light bulb
- Replace the resistor with the lamp.
- Measure the current through the illuminated lamp.
- Calculate the resistance of the lamp using Ohm’s law, Rmeas = V/I.
- What is the resistance of the lamp?
- Imagine you have a complicated circuit containing many resistors. Describe in words how you can use Ohm’s law to find the effective resistance of the entire circuit.
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How to do Part 2 using a myDAQ
You can use the myDAQ to measure the current through the lamp and thus calculate the resistance through the lamp.
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