Experiment 2

Ohm's Law




 The objective of this experiment is to verify Ohm's law.




A computer with Internet connection, a calculator, paper, and pencil




Ohm's law states that the ratio of the voltage across a resistor to the current through that resistor is a constant called the "electric resistance, R of that resistor.

V is in volts and I in amps.  That makes unit of R to be "volts per amp." that is called "Ohms."  The symbol for "Ohm" is "Ω" pronounced "omega."




Click on  http://www.walter-fendt.de/ph14e/combres.htm .   A circuit will appear that can be made look like the circuit in Fig. 1.  The voltmeter reads the voltage across R, and the ammeter reads the current through R.  If you change the rheostat (variable resistor) setting, the total resistance will change, and with a fixed voltage (supplied by the battery), the current I will change as well.  The change in I through R causes the voltage across R to change, too.   The V/I ratio however remains constant according to Ohm's law.


First, you need to practice with the applet to get familiar with the way it works.   Although the applet reads "Combination of Resistors" as its title, it is very suitable for this experiment.  To begin with, the applet shows a 12.0V battery in series with a 100.0Ω resistor. 


1) Let's first change it to a 10.0V battery connected to an 80.0Ω resistor by typing in these values in the applet.  Go ahead and make the changes.  After each single change, you should hit the "Enter" key.  


2) If you click on the tab that says" Series Connection",  it places another 80.0Ω resistor in series with your first 80.0Ω resistor.   Go ahead and try it. Whichever resistor that you click on now, it gets highlighted that means it is selected.  Go ahead and select the left resistor.


3) With the left resistor selected, click on the "Parallel Connection" tab.  This places an 80.0Ω resistor in parallel with that left one.


4) Change the top left 80Ω to 190Ω and the lower left 80Ω to 75Ω.   Also change the right 80.0Ω to 35.0Ω.  Now, you have a module (a 190 & a 75 in parallel) that is in series with a 35Ω resistor.


5) Click on the "Refresh" key to bring the applet to its original mode of a 12.0V battery in series with a 100.-Ω resistor.


6) Place an ammeter in the circuit by clicking on the "Amperage" button on the applet.  Note that an ammeter must always be in series with the element that it is supposed to measure the current through it. 

Figure 1


7) Place a voltmeter across the resistor by first having it highlighted and then click on the "Voltage" button.   A voltmeter will appear across the 100Ω resistor.  If the 100Ω resistor is the only resistor in the circuit, all of the battery voltage (electric potential) drop across that 100Ω resistor and the voltmeter you just placed across it will read the same voltage as the battery voltage.


8) Remove the voltmeter and the ammeter by deselecting them on the applet.


9) Place another 100Ω resistor in series with the first one by clicking on the "Series Combination" key.


10) Place a voltmeter across the left 100Ω resistor by first selecting it and then clicking the "Voltage" tab. 


11) Place an ammeter in the circuit by selecting the "Amperage" tab.  Now that there are two similar resistors, each resistor drops the voltage by 6.00 volts.  The voltmeter shows a voltage of 6.00 volts across the left 100Ω resistor.  Since the total resistance is now 200Ω, the current in the circuit is V/R = 12V/200Ω = 0.0600Amps as read by the ammeter.  Check the calculation for correctness.  Of course, if you multiply the 0.0600A current by the 100Ω resistance, it will give you a voltage of 6.00volts across the left resistor as is also experimentally read by the blue voltmeter.


At this point you should have a circuit on the applet that has the same structure as the circuit in Fig. 1.   You are now familiar with the use of the applet and ready to do the experiment.  Keep the circuit as it is in Fig. 1.


Performing the Experiment:


1) Name the left resistor Rs, and change it to 125Ω .  Let Rs be the resistor under study.  Also, change the battery voltage to 10.0 volts.  Use the right resistor as your rheostat, the variable one, or the one we want to change.  The procedure is to change the resistance of the right resistor a few times and each time record the current through Rs as well as the voltage across it to see if the V/I ratio for Rs remains constant. 

You may use the values given in Table 1, under the "Rheostat Resistance."  For example, as Trial 1 suggests, change the resistance of the right resistor to 25.0Ω on the applet.  Immediately you can read the current and voltage for Rs from the applet's ammeter and voltmeter and record them in the appropriate columns of Table1.  As a check, quickly calculate the V/I ratio and verify that it is very close or equal to 125Ω.  Record all results (both measured and calculated) to 3 decimal places.  This could go to 6 significant figures.


2) Repeat this procedure for each of the remaining 4 rows of Table 1.   For each row, after changing the rheostat resistance on the applet, record the I and V read by the meters, calculate Rs from the V/I ratio and write it in Table 1.


3) Calculates the mean value of Rs in the last column of Table 1 and record it in the bottom right corner.  This is the measured value for Rs.  The accepted value of Rs is of course 125Ω, the value we knew from the beginning.


4) Calculate a %error on Rs using the following % error formula:


5) Repeat steps 1 through 4 for an example of your own.  Select another value for Rs between 80Ω to 200Ω, and another 5 rheostat values ranging from 95Ω to 160Ω.  Make a similar Table and name it Table 2.   Your report must include both Tables 1 and 2.



Given and Measured:


The accepted value for Rs is 125Ω.



Rheostat Resistance


Voltage V across Rs


Current I through Rs


Rs = V/


1 25      
2 55      
3 105      
4 115      
5 135      

Mean Rs:




Calculations:  Just show a sample calculation.


Comparison of the Results:

Calculate a % error on the mean value of Rs.

Conclusion:  To be explained by students.


Discussion:   To be explained by students.