Experiment 2

Ohmís Law




1) To learn the voltage-current relation (Ohmís law), and

2) to understand the concept of electric resistance and learn how to read color-coded ceramic resistors




A computer with the Internet connection, a calculator (The built-in calculator of the computer may be used.), paper, and pencil




Ohmís law simply states that the ratio of voltage across a resistor to the current through that resistor is a constant called the electric resistance of that resistor.

In SI units, V is in volts, I is in amperes, and R is in ohms.




Click on the following link:  http://www.walter-fendt.de/ph14e/combres.htm  A circuit will appear that can be made to look like the circuit in Fig. 5.   In Fig. 5,  the voltmeter reads the voltage across R, and the ammeter reads the current through R.  If you change the (variable resistor) rheostat 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 the current I through R causes the voltage across R to change, too.   You will; however, see that the ratio V/ I  will remain constant.


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 12.0-V battery in series with a 100.0-Ω resistor. 


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


2) If you click on the tab that says" Series Connection",  it places another 80.0-Ω resistor in series with your first 80.0-Ω 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 80.0-Ω resistor in parallel with the left one.


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


5) Click on the "Refresh" key to bring the applet to its original mode of a 12.0-V 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. 

Fig. 5


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 100.Ω resistor  If the 100-Ω resistor is the only resistor in the circuit, all of the battery voltage (electric pressure) drop across that 100.-Ω 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 100.-Ω resistor in series with the first one by clicking on the "Series Combination" key.


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


11) Place an ammeter in the circuit by selecting the "Amperage" square.  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 100.-Ω resistor.  Since the total resistance is now 200.Ω, 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 100.Ω 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. 5.   You are now familiar with the use of the applet and ready to do the experiment.  Keep the circuit as it is on the applet


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.  Let us change the resistance of the right resistor a few times and each time record the current through Rs as well as the voltage across Rs 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 record 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 say 80Ω to 200 Ω, 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


Current, I  through Rs


Voltage, V across Rs


Rs = V /


1 25.0      
2 55.0      
3 105      
4 115      
5 135      







Calculations:    Just show a sample calculation.


Comparison of the Results:

Calculate a percent errors on Rs values for in both cases.

Conclusion:   To be explained by students


Discussion:      To be explained by students