# OHM'S LAW w/ CBL

## Gunn Physics

When a voltage is applied to a complete circuit containing resistance, current begins to flow. In an electrical circuit, the function of a resistor generally is to limit the flow of current in the circuit.

When current flows through a resistor, energy is converted from electrical energy into heat energy, and a potential difference appears across the leads of the resistor. This voltage will be studied as a function of the current flow in the circuit.

Ohm's Law describes the relationship between three important electrical quantities: I, the current, V, the potential difference, and R, the resistance. I = V / R, Ohm's Law, holds for a wide range of electrical devices and electrical phenomena.

In this lab, you will vary the amount of voltage being applied to two devices in series, one a standard value resistance and the second your "sample". The voltage across the standard value, 1000 W in the diagram below, can be converted into current by dividing by the resistance of the standard. The voltage across the sample can then be related to the current flow.

PURPOSE

• Observe Ohm's Law in a dynamic situation
• Compare the behavior of different electronic devices

MATERIALS

• D-cell batteries (2)
• Potentiometer (25 W)
• Resistors, diode, LED's
• Wires
• Clips to hold wires
• Voltage probes (2)
• CBL
• TI-82/83

BEFORE YOU BEGIN

1. If Ohm's Law holds true, how should a graph of Voltage vs. Current look for a given resistance? Explain.
2. If Ohm's Law holds true, how should graphs of Voltage vs. Current compare for different resistances? Explain.
3. If a Voltage vs. Current graph differed from your prediction in question (1), how might you interpret your results? Try to develop multiple interpretations.

PREPARE THE EQUIPMENT FOR DATA COLLECTION

1. Collect the necessary electrical components, wires, clips and batteries. Construct the circuit as shown in the diagram, but don't connect both wires from the batteries.
2. Connect the voltage probes to the CBL, and connect them to the circuit carefully, respecting the colors of the lead wires as shown in the diagram. This is important in order for the data to be interpreted properly.
3. Prepare the CBL for data collection by launching "OHMSLAW" under [PRGM]. The time has been set for 10 seconds.
4. During the collection of data, you will rotate the potentiometer from completely counter-clockwise to fully clockwise and back once. With practice this can take the full time allotted on the graph. Move the shaft of the potentiometer smoothly in each direction.

PROCEDURE

1. Record the value for your "standard" resistor, and what your "sample" is in the data table.
2. Connect the batteries to the circuit. Rotate the shaft of the potentiometer fully counter-clockwise in preparation for collecting data.
3. Start data collection. Follow the process described in the previous step (4). At the end of 10 seconds, data collection halts.
4. Obtain the graph of Voltage vs. Time. Sketch this graph for future reference.
5. Obtain the graph of Current vs. Time. Compare the shape of this graph with the previous one.
6. Now obtain the graph of Real V vs.Real I. Complete step (2) or (3) in the Analysis.
7. Change the sample to other choices given by your instructor. Repeat steps (1-6) for each. Make a total of 5 data runs and analyses.
8. When finished with the data collection, disconnect the batteries from the circuit and clean up your work area.

SUGGESTED SETTINGS

 Sample Standard Resistance 470 W Resistor 1000 W 2,200 W Resistor 1000 W Diode - 1N4002 470 W LED 470 W

DATA TABLE

 trial Sample Standard Resistance Slope of V vs. I Graph 1 2 3 4 5 6

ANALYSIS

1. What is the shape of the Voltage vs. Current graph for a resistor? What does this indicate about the nature of the relationship between these two quantities?
2. If the graph is a straight line, determine the equation. What is the slope, both value and units? Is this value related in any way to the size of the sample resistance?
3. Do all of your samples obey straight line Voltage vs. Current relationships? If not, describe what the shape of the graph indicates for each. Some outside reading may be necessary to interpret the shape.
4. Be prepared to discuss the results of your experimentation or answer questions about your results in a quiz format at the next class meeting.

NOTE: The same setup will be used with the ULI or PASCO interfaces. Replace the "CBL" in the diagram with the appropriate computer interface.