OBJECTIVES
1. Become familiar with the behavior of some common circuit elements.
2. Observe the behavior of ohmic and non-ohmic conductors.
EQUIPMENT
Power supply, 1 k
Ω
resistors (four), 10
Ω
resistor, 6.3 V light bulb, diode, breadboard, wires, digital multimeters (two).
THEORY
Ohm discovered that that the voltage (potential difference) across a resistor is directly proportional to the current, that is, V
∝ i .
This is known as Ohm’s Law:
V
= iR where i is current, V is voltage (potential difference), and R is resistance. The proportionality constant is the value of the resistance. When an element (such as a resistor) is ohmic , the proportionality constant R is a constant and is said to be linear .
That is, when a graph of current versus voltage shows a straight line, the slope of the line is 1/R. An element is non-ohmic if the graph of voltage versus current is nonlinear .
For certain resistors, the value of its resistance does not change appreciably and is therefore ohmic. However, for a light bulb, the resistance of the filament will change as it heats up and cools down. As a consequence, the resistance of the filament changes dramatically and the filament is therefore non-ohmic.
Another example of a non-ohmic element is a diode. A diode is an electronic device that only allows current to flow in one direction once a certain forward voltage is established across it. If the voltage is too low, no current flows through the diode. If the voltage is reversed, no current flows through the diode. A light-emitting diode (LED) emits light as current passes through the diode in the forward direction.
In this lab, we will investigate the behavior of a resistor, a light bulb, and a diode by analyzing the graph of the current vs. voltage or I-V characteristic curve.
PROCEDURE
Part 1: Ohmic Conductors
Part 1A: 1 k
Ω
Resistor a) Using the breadboard, build a simple circuit consisting of a 1 k
Ω
resistor connected to a power supply. In your circuit, include two digital multimeters to measure the voltage across and the current though the 1 k
Ω
resistor.

b) Adjust the power supply until the voltage across the resistor is about 1.0 V. Record both the voltage across and the current through the resistor. c) Increase the voltage across the resistor to about 2.0 V and record the voltage and current. Continue to increase the voltage in 1.0 V increments until you reach 10.0 V.
Record all of your data into a table. d) When the voltage across the resistor is 10 V, watch the current through the resistor for about 30s. Does the current through the resistor change during the 30s? What does this tell you about the temperature of the resistor? e) Reverse the polarity of the voltage across the resistor by switching the leads from the power supply (change + to – and – to +). Check on the current for two or three different values of the voltage and compare with your previous results. Does the polarity of the
voltage across the resistor make any difference in the current through the resistor? f) Use Excel to make a plot of current vs. voltage. Is the resistor ohmic or non-ohmic?
How do you know?
g) Use Excel to fit the data to a straight line. From the slope of your line, calculate the resistance ( R exp
).
Use a percent difference to compare R exp
to R theor
. where R theor
. is the actual value of the resistor as measured by the digital multimeter.
Part 1B: 10
Ω
Resistor a) Using the breadboard, build a simple circuit consisting of a 10
Ω
resistor connected to the power supply. In your circuit, include two digital multimeters to measure the voltage across and the current though the 10
Ω
resistor. b) Adjust the power supply until the voltage across the resistor is about 0.5 V. Record both the voltage across and the current through the resistor. c) Increase the voltage across the resistor to about 1.0 V and record the voltage and current. Continue to increase the voltage in 0.5 V increments until you reach 5.0 V.
Record all of your data into a table. d) Now increase the voltage across the resistor to 10 V and watch the current through the resistor for about 30s. Does the current through the resistor change during the 30s?
What does this tell you about the temperature of the resistor?
DO NOT TOUCH THE RESISTOR – IT WILL BE HOT! e) Lower the voltage of the resistor back down to 1.0 V. Does the current through the resistor agree with the value that you got previously? Why or why not?
f) From your results for the two different resistors, can you conclude that all resistors obey Ohm’s law? That is, is the resistance of all resistors constant? If not, when can you assume that the resistance of a resistor is constant?

Part 2: Non-ohmic Conductors
Part 2A: 6.3V Light Bulb a) Build a simple circuit consisting of a 6.3 V light bulb connected to a power supply. In your circuit, include two digital multimeters to measure the voltage across and the current though the light bulb. b) Adjust the power supply until the voltage across the light bulb is about 0.5 V. Record both the voltage across and the current through the light bulb. c) Increase the voltage across the light bulb to about 1.0 V and record the voltage and current. Continue to increase the voltage in 0.5 V increments until you reach 5.0 V.
Record all of your data into a table. d) Use Excel to make a plot of current vs. voltage. Is the light bulb ohmic or non-ohmic?
How do you know?
Does the light bulb filament have a constant resistance? Why or why not?
Part 2B: Light Emitting Diode (LED) a) Using the breadboard, build a simple circuit consisting of a 1 k
Ω
resistor and an LED connected in series with a power supply. In your circuit, include two digital multimeters to measure the voltage across and the current though the LED . b) Adjust the power supply until the voltage across the LED is about 1.75 V. Is there any current flowing through the LED? c) Reverse the polarity of the voltage across the LED by switching the leads from the power supply (change + to – and – to +). Does the polarity of the voltage across the
LED make any difference in the current through the LED? d) Adjust the polarity of the voltage across the LED so current flows through the LED. e) Adjust the voltage across the LED to 0.5 V and record the current through the LED.
Increase the voltage to 1.0 V and then 1.5 V and record the current through the LED. f) Record the current through and the voltage across the LED for about 10-20 values between 1.5 V and 2.0 V. Record all of your data into a table. What is the turn-on voltage of the LED? g) Use EXCEL to plot current vs. voltage for the LED. Is the LED ohmic or non-ohmic?
How do you know?