Experiment 22

Ohmic and Non-Ohmic Resistance

Objectives

This experiment is designed to teach the use of voltmeters and ammeters, the construction of elementary electrical circuits and the use of resister color codes.  The experiment should verify Ohm's law of resistance and the equivalent resistance formulas for resistors in parallel and in series.  It should clarify the distinction between Ohmic and non-Ohmic devices and demonstrate how the idea of resistance can be extended to non-Ohmic devices.

Procedure 1

Read and record the values of the three resistors you have been given.  The band nearest the end is read first and then you read towards the middle from there.  The color code is:

Black                     0                               Green                     5                              Silver                                     10%

Brown                   1                               Blue                       6                              Gold                                        5%

Red                        2                               Violet                     7                              Gold in third band = -1

Orange                  3                               Gray                       8                              (e.g., brown black gold = 1.0)

Yellow                   4                              White                     9

The first two bands give the two digits of the value, the third of the power of 10 and the fourth gives the % error of the value.  For example, a resistor reading orange blue orange gold is 36 x 10³ ± 5%.

Procedure 2

Set up the circuit in Figure 1 with the largest of your resistors and the voltmeter disconnected during current measurements.  Note that the voltmeter is in parallel and the ammeter is in series with the resistor.  These meters are always to be wired this way.  Record 10 values of voltage (don't go above 10 volts) and their corresponding currents.  Record a single value I and V on the other two resistors.  Make sure voltage and current are large enough to give accurate meter readings.

Procedure 3

Set up the circuit in Figure 2.  The resistors are now in parallel.  Record a single value of I and V volts.

Procedure 4

Set up the circuit in figure 3.  The resistors are now in series.  Record a single value of I and V.

Procedure 5

Using the circuit of Figure 1.  Place the light bulb in the position of the resistor.   Begin at very low voltages and work up until the bulb is bright.  Do not exceed 6 volts.  Failure to follow this instruction will make it necessary to wait until the bulb has cooled to resume the experiment.

Calculations

Compare the values of the resistors from Procedures 1 and 2.  Using the values of the resistors from Procedure 2, calculate the equivalent resistance in parallel and in series and compare with the values calculated from the data of Procedures 3 and 4.

Graphs

Graph the V vs. I values from Procedure 2.  What is the slope of the straight line and what is its meaning?  Is this an Ohmic device?  Why?

Graph V vs. I for the bulb.  What resistance does the filament have at a current of 0.02 amperes?  (The resistance is the slope of the curve at any particular point.)  Is this an Ohmic device?  Why?