Laboratory Activity on Resistance and Ohm's Law | PHYS 212P, Lab Reports of Physics

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Physics Lab 212P-5 Resistance & Ohm's Law NAME: ____________________________________ LAB PARTNERS: ____________________________________ ____________________________________ LAB SECTION: __________________________ LAB INSTRUCTOR: __________________________ DATE: __________________________ EMAIL ADDRESS: __________________________ Physics Lab 212P-5 Software list Microsoft Excel Equipment List (all items marked with * are in the student kit, others are supplied at the time of the lab) Science Workshop Interface + voltage probes with alligator clips Digital multimeter + probes Carbon resistor Diode Measuring scale *Two 1.5 V batteries + a battery holder *Flashlight bulbs (one round, one long) + bulb holders *Hookup wires with alligator clips *Thick nichrome wire (bare wire with no insulation) *Thin nichrome wire (bare wire with no insulation) *Compass Lab Activity 2: Ohm's Law Ohm's law states that -- under certain conditions -- the voltage across an object varies linearly with the current through the object: i.e. V = IR. Another way to say this is that the resistance of the object is independent of the voltage applied across it. A device that obeys this law is said to show "ohmic" behavior and a graph of voltage versus current shows a straight line whose slope is equal to the resistance of the device. However, by convention, current versus voltage (an I-V plot) is usually graphed. In this case the slope is equal to the inverse of the resistance. Devices that deviate from this law, i.e. the voltage versus current curves exhibits a change in slope as the current is changed are said to show "non- ohmic" behavior. The resistance is still equal to the slope of the voltage versus current graph, but in contrast to the ohmic case, the resistance changes as the current is changed. The circuit needed to determine whether or not a given device obeys Ohm's Law is shown in the figure below. In this circuit, a variable voltage is applied across the device S, and the current and voltage through the device is measured. The voltage and current are measured using the Science Workshop interface. The ammeter and voltmeter for this experiment are built into the Science Workshop interface. A detailed understanding of the circuit is not necessary at this stage: it will be obvious to you how this circuit works once we get to circuits.  Set up the circuit shown above.  Plug the banana plug into the Output of the Science Workshop interface. S A V B A 5 V AC Waveform Voltage & Current Output of Science Workshop  Start Science Workshop  Click the "Sample V" button.  Set the Amplitude for 5.000 V.  Set the frequency for 0.4 Hz.  Click on the triangle waveform.  Click on the "Auto" button.  Click the "Sample I" button.  Click & drag the "Graph" icon to Output I.  Change the input on the x-axis to Analog Output.  Click the "sampling options" icon.  The default rate at which the program measures data is 10 Hz (i.e. 10 readings per second). Change this to 100 Hz.  Now, connect the carbon resistor (brown, black, brown) in the position S in the circuit.  Produce a graph that displays I versus VAB.  Click on the "REC" icon on your program screen.  After the current makes a couple of sweeps, stop recording data by clicking on the "STOP" icon.  Produce a linear fit to the graph.  Click on the "" icon. This should open a frame on the right side of your graph.  Click on the "" icon and select curve fit  linear fit from the pop-up menu.  Note the value of the slope of the linear fit  Include all relevant plots with your report. Q5. Is the resistor an "ohmic" or a "non-ohmic" device? From the I-V plot for the resistor, determine the value of the resistance. Measure the resistance value of the resistor using the DMM. How do these values compare to the stated resistance value (brown, black, brown) of 100 ? Rgraph = ___________ RDMM = ___________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________  Replace the resistor with the long light bulb included in your lab kit.  Produce a graph that displays I versus VAB. Include this graph with your report.  Change the voltage sweep to 0.015 V and produce a second graph. Use the linear fit function to estimate the low voltage resistance of the light bulb. Include this graph with your report. Q6. Does the light bulb have a constant resistance? Is the light bulb an "ohmic" or "non-ohmic" device? Explain your answer. _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ Q7. Why does the slope of the light bulb graph change? Explain (qualitatively) why the slope changed when a current was flowing through it. _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________ _____________________________________________________________________________