A. OHM's LAW

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

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

Theory: Georg Simon Ohm (1787-1854), a German physicist, discovered Ohm’s law in 1826. This is an experimental law. Valid for both alternating current (ac) and direct current (dc) circuits.

When you pass an electric current (I) through a resistance (R) there will be a an electric potential difference (V) created across the resistance as shown below.

Ohm’s law gives a relationship between V , I and R as follows.

                                V = I R

Units: V------> volt (v), I------> ampere (A), R-----> ohm or v/A or Ω.

For ohmic resistances, V versus I is a linear relationship, and they have a constant resistance. Resistance can be calculated using the Ohm’s law, R = V/I. The slope of the V versus I, line will also give the resistance, R.

For non-ohmic resistances, V versus I is a non-linear relationship, and they have a varying resistance. The resistance at a particular point can be calculated using Ohm’s law, R = V/I, where V and I are the voltage and current at that point.

In the first part of this activity, investigate the relationship between current and voltage in a standard 10-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. Write down the following properties of the signal.

Type of signal_______________    Amplitude_____________    Frequency______________

5. Double-click "Measurements and Sample Rate" and change the sample rate to 50 Hz. Close the signal generator window.

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

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

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

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

10. Click "Smart Tool", and use the arrow keys or mouse to move the smart tool until the current value reaches 0.05A. Now 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.

b. Light Bulb

1. Remove the 10-ohm resistor, connect the light bulb, and click "Start'.  

2. Describe what you observe in the scope display and the behavior of the light bulb. Explain your observation using the slope of the Voltage versus Current graph and resistance of the filament of the light bulb. If you need to observe it again, click "Start" again.

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3. Determine the resistance of the filament when the bulb is fully lit, by finding the slope of the Voltage versus Current graph.

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4. Double-Click " output-voltage" in the data window and change the frequency to 0.3 Hz. The sample rate is already set to 50 Hz and the amplitude is at 3 volt. Close the signal generator window. Maximize the scope display 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. Describe what you observe in the scope display and the behavior of the light bulb. Explain your observation using the resistance and temperature of the filament of the light bulb. If you need to observe it again, click "Start" again.

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8. Print a hard-copy of the voltage versus current graph for the filament of the light bulb.

9. Click "Start" again, and by observing the trace, identify the heating up and cooling down parts, for positive current and voltage, on the hard-copy of the graph.   

10. Use the "Smart Tool" to record five pairs of Current and Voltage values in increasing order, for positive current and voltage, and complete the data table for the light bulb.

DATA:

a. 10-Ohm resistor

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

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Current and Voltage Data for the 10-ohm resistor.

 Find the average value of the measured resistance = ____________

                                                Accepted value    =  10 Ohm

                                                % Error                = ___________

b. Light Bulb

Current and Voltage Data for a light bulb:

Q1. What is the difference in resistance values during heating up and cooling down.

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Q2. Why is the trace on the scope different when the filament is heating up compared to the trace when the filament is cooling down?

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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 the above procedure for other lengths.

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

6. Print a hard-copy of your graph.

7. Measure the diameter of the wire with a micrometer, calculate the cross-sectional area, and calculate the resistivity of the metal.

DATA

Slope of R versus L, graph:_________________

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

Experimental resistivity of wire = _________________

Accepted resistivity of wire = 1.00 x 10^-4 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:_________________________________________________________________

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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:______________________________________________________

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

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