Physics 121 Lab 6A: Ohmic versus Non-Ohmic Devices
Resistors:
Plot the potential difference across the resistor as a function of current through the resistor.
Do your data fall on a straight line? If so, fit the data with a straight line and from the curve-fit determine the
resistance of your resistor. Calculate a percent difference between your measured (with the meter) and
experimental (from your graph) values of the resistance.
The attached graphs show two different scenarios, namely resistors wired in series and parallel. The first graph in
figure 1 shows data taken on a 470Ω resistor, the blue curve. From the slope of the line you can determine the
resistance and this is approximately 467Ω , which is 0.6% from the accepted value. Now for the red curve two
resistors were wired in series, one of which was a 470Ω resistor. On the attached graph, determine the value of
the second unknown resistance.
For the graph in figure 2, we have data taken on a single 470Ω resistor, the blue curve, and the 470Ω resistor wired
in parallel with a second unknown resistor, the red curve. On the attached graph, determine the value of the second
unknown resistance.
LightBulb
Plot the potential difference across the light bulb as a function of current through the bulb.
Do your data fall on a straight line? If so, fit the data with a straight line and from the curve-fit determine the
resistance of your light bulb. If your data do not fall on a straight line, what does the shape of the curve look like?
On your graph for the light bulb data, determine the approximate resistance of the filament when it cool (at low
current) and when it is glowing brightly (at maximum current).
Let R0 be the resistance of the light bulb at room temperature Let R be the resistance of the bulb when it is glowing
brightly. Calculate the ratio R/R0.
The temperature dependence of a resistance is given approximately by the formula R = R0 [1 + α (T − T0 )] , where R
is the resistance at temperature T and R0 is the resistance at temperature T0. α is the temperature coefficient of
resistance, some of which are tabulated for different materials in figure 3. In your case the filament is made of
tungsten.
Using T0 as room temperature and R0 as the resistance of the light bulb at room temperature, estimate the
temperature of the light bulb filament when the bulb is glowing brightly. Show your calculations.
Plotsofpotentialdifferenceacrossresistorsversus
thecurrentthroughtheresistors.Thebluedots
representacircuitwithoneresistorwhilethered
dotsrepresentacircuitwithtworesistorswiredin
series.
PotentialDifference(V)
12
10
y=793.46x+0.0087
8
y=466.68x+0.0067
6
4
2
0
0
0.005
0.01
0.015
0.02
0.025
Current(A)
Figure 1: Potential difference across resistors as a function of current through the resistors.
Plotsofpotentialdifferenceacrossresistorsversus
thecurrentthroughtheresistors.Thebluedots
representasingleresistorcircuitwhilethereddots
representacircuitwithtworesistorswiredin
parallel.
PotentialDifference(V)
12
10
8
y=466.68x+0.0067
6
y=131.04x+0.0307
4
2
0
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Current(A)
Figure 2: Potential difference across resistors as a function of current through the resistors.
Figure 3: Resistivities and temperature coefficients of resistance for various materials.