Lab & Pre-lab #5: Voltage

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Fundamentals of Circuits III: Voltage & Ohm’s Law v 0.1Question 3.1 What happened to the current in the circuit as the power supply voltagewas increased? How did this compare with your prediction?Question 3.2 How did the potential difference across the resistor change as the currentthrough the resistor changed? How did this compare with your prediction?Step 5: You can display axes for voltage vs. current on the top graph. Click on the word timebelow the horizontal axis, then select current in the dialog box that comes up. Close thedialog box. You will also need to change the maximum value on the horizontal axis. 0.5A would be an appropriate value. The axes should now appear as shown below.Voltage 1(V)Current 2 (A)Current 2 (A)Time (s)Reminder: We are interested in the mathematical relationship between the voltage acrossthe resistor and the current through the resistor. This can be determined from the graphby drawing a smooth curve that fits the plotted data points. Some definitions of possiblemathematical relationships are shown below. In these examples, y might be the voltagereading and x the current.PHYS-204:Physics II Laboratory 13
Fundamentals of Circuits III: Voltage & Ohm’s Law v 0.1yyyxxxy is a function of x, whichincreases as x increases.y is a linear function ofx which increases as xincreases according to themathematical relationshipy = mx + b, where b isa constant called the yintercept.y is proportional to x. Thisis a special case of a linearrelationship where y = mx,and b, the y intercept, iszero.These graphs show the differences between these three types of mathematical relationship.y canincreaseasxincreases, andtherelationshipdoesn’thavetobelinearorproportional.Proportionality refers only to the special linear relationship where the y intercept is zero,as shown in the example graph on the right.Step 6: Usethefitroutineinthesoftware toseeiftherelationshipbetweenvoltage andcurrentfor a resistor is a proportional one.Step 7: Sketch your graphs on the axes shown previously.Question 3.3 Describe in words, what the mathematical relationship between the potentialdifference across a resistor and the current through that resistor is. Explain, based on yourgraphs.When the relationship between potential difference and current for a resistor is linear, itmeans that current changes proportionally to the change in voltage and the value of this proportionalitydoes not change no matter what the voltage is. When such a linear relationshipexists between current and voltage, the mathematical relationship between current and voltageis known as Ohm’s law. We now define a quantity known as resistance. Resistance is definedas the slope of the voltage vs. current graph.If potential difference is measured in volts and current is measured in amperes, then theunit of resistance is the Ohm, which is usually represented by the Greek letter ”omega” Ω.Question 3.4 Write a mathematical equation that expresses the relationship between V, I, andR.PHYS-204:Physics II Laboratory 14
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Lab & Pre-lab #5: Voltage

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