LAB 6
Resistance & Current
OBJECTIVES
1. Observe the behavior of ohmic and non-ohmic conductors.
2. Determine the effect of temperature on the resistivity of metals.
EQUIPMENT
DC power supply, 2 DMMs, breadboard, leads, 1kresistor, Capstone, current sensor,
6.3 V lamp, 30 Gauge Copper Wire, Thermometer, Beaker, Water, hot plate.
THEORY
Ohm discovered that that the voltage (potential difference) across metals and many
other solid materials is directly proportional to the current flowing through them. Passive
circuit elements made of these materials are called resistors and are said to obey
Ohm's Law: V = Ri, where i is current, V is voltage (potential difference), and R is the
resistance (constant for an ohmic conductor). A graph of current versus voltage for an
ideal resistor is a straight line with slope 1/R. A circuit element is non-ohmic if the graph
of current versus voltage is nonlinear. An incandescent lamp is an example of a nonohmic conductor, since the resistivity of its lamp filament, like most materials, varies with
temperature. As the filament gets hot, the resistance increases dramatically.
The resistance R of a conducting wire of length L is
L
A
where A is the cross-sectional area and  is the resistivity of a material. The resistivity 
for most materials changes with temperature. The relation between  and temperature
change T = T – T0 is approximated by the equation
R
  0 1  T 
Here T0 is a reference temperature, 0 is the resistivity at T0 and  is the temperature
coefficient of resistivity for the material.
PROCEDURE
Part 1: 1k Resistor
a. Draw a circuit diagram of a resistor R = 1k connected across a power supply, with
a voltmeter to measure the resistor voltage VR and an ammeter to measure the
resistor current i. Now build the circuit on a breadboard using two DMM. Use manual
ranging as needed to reduce the digits displayed on the DMM.
b. Adjust the power supply until VR = 0.5 V. Record VR and i in a table. Continue
recording data in 0.5 V increments up to VR = 5.0 V. Turn off the power supply when
not actively taking data. Record all of your data into a table.
c. Reverse the polarity of the voltage applied to the resistor by switching the leads at
the power supply. Measure i at 3 different values of VR and compare with your
previous results. Does the polarity of the voltage across the resistor make any
difference in its behavior?
d. Use Excel to plot current vs. voltage. Is the resistor ohmic or non-ohmic? How do you
know? Fit a straight line to the data and determine the resistance Rthy from the slope
of the line? Use a percent difference to compare Rthy to Rexpt where Rexpt is the
resistance measured by a DMM.
e. Answer the following questions:
 Does the resistor(s) have a constant resistance? Why or why not? If not, when
can one assume that a resistance is constant?
6-1

Does the resistor obey Ohm’s law? Why or why not?
Part 2: 6.3V Incandescent Lamp
a. Replace the 1k resistor in part (1a) with a 6.3V-incandescent lamp. Also, replace
the power supply with the Capstone signal generator and drive the lamp with a 1.0V
triangle voltage waveform at 0.01 Hz.
b. In Capstone, create a graph of the output current versus time, then drag the output
voltage icon over the time axis of this graph to convert it to a live IV graph.
c. Answer the same questions as in Part (1e). In addition, answer the following:
 How do you know when the light bulb is heating up or cooling down?
 Explain the shape of the curve using physics concepts.
Part 3: Temperature Dependence of the Resistivity of a 32 Gauge Copper Wire
a. Create in Excel a spread sheet with parameters i(mA), V(mV), (108 .m), T(0C),
and T(0C). Calculate in Excel the resistivity  = VA/iL where
 A and L is the gauge area (diameter of 0.202 mm) and length of the wire coil
 V is the voltage across and i is the current through the wire coil
b. Place the wire coil into a beaker containing enough water to cover the entire coil
and then put it onto a hot plate. Add a thermometer to read the temperature.
c. Set the power supply voltage so that the initial measured current is close to 5 mA.
Measure the initial current, voltage and initial temperature of the water.
d. Turn the hot plate to a low to medium heat and measure the current, voltage and
temperature at 10˚C intervals from 20˚C to 70˚C. Complete the Excel spread sheet
in Part (4a).
e. Plot the resistivity versus T. Determine the resistivity at room temperature
(0)thy and the temperature coefficient of resistivity thy from the slope and yintercept.
f. Compare the predicted (0)thy & thy with the accepted values of (0)accepted = 1.69 
108   m and accepted = 4.3  103 ˚C1 using a percent difference. How do they
compare?
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