# Ohm’s Law

#### Purpose

To verify Ohm’s Law for resistors.

#### Objectives

• To learn how to use multimeters as ammeters and voltmeters.
• To learn how to read the color codes for resistance.
• To become proficient in using spreadsheets for data recording and analysis.
• To learn how to straighten non-linear graphs by using non-linear axes.
• To learn how to use linear regression to determine best line through experimental data.

#### Pre-lab Activity

If you have not worked with multimeters before, go to the Instrumentation page and become familiar with how they work.

Assignment: Take the following safety quiz.  You have unlimited attempts, but you must get perfect score.

#### Equipment

• Power supply, wires
• Two multimeters

#### Theory

Ohm’s Law gives the amount of electric current through a resistor with resistance R that is connected across electric potential difference V as: $I = \frac{V}{R}$

#### Experimental Settings

Connect a 1-kΩ resistor and an ammeter to the power supply.  (Note, the ammeter must be in series with the resistor). Connect the voltmeter across the resistor.  For a picture of the setting, check the Physics Lab Site  DC Circuits Setting.

#### Activity 1. Voltage-Current Relation

Measure the voltage across the 1-kΩ resistor for ten different values of the current through it.

 I (mA) V (V) 1 2 3 4 5 6 7 8 9 10

Lab Assignment.

1. Plot the voltage V  versus the electric current I
2. Using linear regression, determine the slope of the graph
3. Compare the slope of the graph with the resistance value in your circuit. How well do the two agree/disagree?

#### Activity 2. Current vs. Resistance Relationship

Set the power supply to 10 V. Connect different resistors and measure the electric current through for each one of them. Use all the resistors available in the clear plastic cup.

 R (Ω) I (mA) 1000

Lab Assignment.

1. Plot the electric current I  versus the resistance R
2. Plot the electric current I versus the inverse of the resistance 1/R. Why is this graph better representation of your data than the simple I vs. R graph?
3. Using linear regression, determine the slope of I vs. 1/R graph
4. Compare the value for the slope with that for the voltage from the power supply. How well do they agree/disagree

Note.  To determine the extent to which two physical measurements, M1 and M2,  agree, we compare their difference to their average value: $\text{percent difference} = \left|\frac{M1 - M2}{\left(M1+M2\right)/2}\right| \times 100\%$