A. Ohm's Law:

Purpose: Study Ohm's law by investigating the relationship between current and voltage in a simple resister and in the filament of an incandescent light bulb.

Apparatus: PC w/interface, 2 patch cords, 2 alligator clips, 10-ohm resistor, and light bulb.

Background

Ohm discovered that when the voltage ( potential difference) across a resistor changes, the current through the resistor changes. He expressed this as; I = V/R, where I is current, V is voltage (potential difference), and R is resistance. Current is directly proportional to voltage and inversely proportional to resistance. In other words, as the voltage increases, so does the current. The proportionality constant is the value of the resistance. Since the current is inversely proportional to the resistance, as the resistance increases, the current decreases. 

A resistor is "Ohmic" if as voltage across the resistor is increased, a graph of voltage versus current shows a straight line (indicating a constant resistance). The slope of the line is the value of the resistance. A resistor is 'non-Ohmic' if the graph of voltage versus current is not a straight line. For example, if resistance changes as voltage changes, the graph of voltage versus current might show a curve with a changing slope.

For certain resistor, the value of its resistance does not changes appreciably. However, for a light bulb, the resistance of the filament will change up and down. At high AC frequencies, the filament doesn't have time to cool down, so it remains at a nearly constant temperature and the resistance stays relatively constant. At low AC frequencies (e.g., less than one Hertz), the filament has time  to change temperature. As a consequence, the resistance of filament changes dramatically and the resulting change in current through the filament is interesting to watch.

In the first part of this activity, investigate the relationship between current and voltage in a simple ten-ohm resistor. In the second part, investigate the relationship between current and voltage in the filament of a small light bulb.

Procedure:

a. 10-ohm resistor

1. Connect the banana plug patch cords into the 'OUTPUT' ports on the interface.

2. Plug in alligator clips on the other ends of the patch cords and connect the 10-ohm resistor. 

3.  Double-Click "DataStudio",  Click "Open Activity",  Double-Click "Library",  Double-Click "Physics Labs", and Double-Click "P48-Ohm's Law". 

4. Open the signal generator window by double-clicking the "output-voltage" in the data window and change amplitude to 2.5 V, frequency to 0.3 Hz, and  sample rate to 50 Hz. Close the signal generator window.

5. Double-Click Voltage versus Current Scope in the Displays window, maximize the window, and click "Start".

6. Observe the trace of voltage versus current for a few seconds and then stop measuring data, after capturing the whole graph.

7. Adjust the V/div and A/div until the graph fills most of the display.

8. Describe the Voltage versus Current relationship for the 10-ohm resistor in the data table.

9. Click "Smart Tool", and use the right-arrow key to move the smart tool until the current value reaches 0.05A. Now use the up-arrow key and move the smart tool until it reaches the Voltage versus Current line. At this point read the voltage value and record your result on the data table.

10. Repeat the above procedure for other values of current and complete the data table.

b. Light Bulb

1. Click "File", click "open activity", and without saving open a new screen for P48-Ohm's law. 

2.  Double-Click " output-voltage" in the data window and change amplitude to 2.5 V, frequency to 0.3 Hz, and  sample rate to 50 Hz. Close the signal generator window.

3. Remove the 10-Ohm resistor and the alligator clips and connect the light bulb.

4. Double-Click Voltage versus Current Scope in the Displays window, maximize the window, and click "Start". 

5. Observe the trace of voltage versus current for a few seconds and then stop measuring data, after capturing the whole graph.

6. Adjust the V/div and A/div until the graph fills most of the display.

7. Print a hard-copy of the graph.

8. Click "Start" again, and by observing the trace, identify the increasing current and decreasing current parts, for positive current and voltage, on the hard-copy of the graph.   

9. Describe the Voltage versus Current relationship for the light bulb in the data table.

10. Click "Smart Tool", and use the right-arrow key to move the smart tool until the current value reaches 0.02A. Now use the up-arrow key and move the smart tool until it reaches the Voltage versus Current line. At this point read the voltage value and record your result on the data table.

11. Repeat the above procedure for other values of current and complete the data table.

DATA:

a. 10-Ohm resistor

Describe the Voltage versus Current relationship for a 10-ohm resistor below.

____________________________________________________________________________

Current and Voltage Data for a 10-ohm resistor.

 Find the average value of the measured resistance = ____________

                                                Accepted value    =  10 Ohm

                                                % Error                = ___________

b. Light Bulb

1. Describe the Voltage versus Current relationship for a light bulb below.

____________________________________________________________________________

Current and Voltage Data for a light bulb:

2. What is the difference in resistance values for increasing current and decreasing current?

 ____________________________________________________________________________

3. The slope of the graph for the light bulb is not symmetric. Why is the trace on the scope different when the filament is heating up compared to the trace when the filament is cooling down?

____________________________________________________________________________

____________________________________________________________________________

____________________________________________________________________________

Conclusion: (about 1/3 of a page) 

B. Resistance

Purpose: To investigate the resistance of metal wires using a digital multi meter.

Apparatus: DMM, metal wires: one long (nichrome) and two short, connector box, micrometer, and meter stick.

Theory: Resistance, R of a metal wire of length l and cross-sectional area A is given by:

Procedure:

1. Set the DMM to measure resistances and connect two banana-plug wires from it to the connector box. 

2. Connect one end of the long metal wire to one of the terminals of the connector box.

3. Measure 10 cm length of wire, (do not cut the wire) and connect it to the other terminal so that there is exactly 10 cm of wire between the terminals.

4. Record the resistance value and repeat procedure-3 for other lengths.

5. Plot a graph R versus length, and determine its slope.

6. Measure the diameter of the wire with a micrometer and calculate the resistivity of the metal.

DATA

Slope of R versus l, graph:_________________

Diameter of wire = __________________    Cross-Sectional area of wire =________________

Measured resistivity of wire = _________________

Accepted resistivity of wire = 100 x 10^-6 ohm.cm.

                                    % Error = __________________

Now that you have seen what happens to the resistance as the length increases, can you predict what will happen to the resistance as the diameter increases?

PREDICTION:_________________________________________________________________

______________________________________________________________________________

You can check your prediction for the effect of diameter, by doing the following investigation, for a thin and thick wire of equal length of the same material:

Measure the resistance of the thin wire. Then measure the resistance of the thick wire.

EXPLAIN YOUR RESULTS:______________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Question: Electricity is transported across states using power lines with thick wires. Explain why?

______________________________________________________________________________

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