Lab & Pre-lab #5: Voltage

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Fundamentals of Circuits III: Voltage & Ohm’s Law v 0.1Question 2.2 Did your measurements agree with your predictions? Did closing andopening the switch significantly affect the potential difference across bulb A (by more thanseveral %?Question 2.3 Did closing and opening the switch significantly affect the potential differenceacross bulb B by more than several %? Under what circumstances is there a potentialdifference across a bulb?Question 2.4 Based on your observations, deduce a rule for the potential differences acrossdifferent branches of a parallel circuit. How do these relate to the potential difference across thebattery?Question 2.5 In analyzing your observations, would you deduce that the battery is a constantcurrent source which delivers essentially a fixed amount of current regardless of the componentsconnected to it or would you deduce that it is a constant voltage source which essentially appliesa fixed potential difference regardless of the circuit connected to it or do you think that neitherdeductions necessarily follow from your observations? Explain, based on your observations inthis and the previous lab.Question 2.6 What is the potential difference between two points on a short length of wirewhen there is no bulb, battery or resistor between the points? You may assume that the wirehas negligible resistance.Investigation 3:Ohm’s LawWhat is the relationship between current and potential difference? You have observed in previousinvestigations that there is a potential difference across a bulb or resistor only when thereis a current through that circuit element. So how does the potential difference depend on thecurrent? In order to examine this, you will need the following:• Computer with Logger-Pro software• Lab Pro interface• voltage probe• current probe• variable DC power supply• 10 Ohm resistorPHYS-204:Physics II Laboratory 11
Fundamentals of Circuits III: Voltage & Ohm’s Law v 0.1• connecting wires• alligator clipsThe circuit shown below contains a power supply. A power supply has the same purposeas a battery. Both raise the potential energy of the electric charges that flow through them;the battery uses chemical energy to do this, while the power supply uses electrical energy. Avariable DC power supply lets you adjust the potential difference. When you turn the dial, youchange the voltage between its terminals. Therefore, this circuit allows you to measure how thecurrent through the resistor varies as you change the potential difference across it.CP2DC Power Supply+−+ −+−VP1Figure 9: Circuit with a variable power supply to explore the relationship between current andpotential difference for a resistor.Prediction 3.1 What will happen to the current through the resistor as you increase the appliedvoltage from zero?Prediction 3.2 What will happen to the potential difference across the resistor as the currentthrough it increases from zero?Prediction 3.3 What do you predict for the mathematical relationship between the potentialdifference across the resistor and the current through the resistor?Activity 3.1: Current and Potential Difference for a ResistorStep 1: Open the experiment file called L4A3-1 (Ohm’s Law).Step 2: Zero both probes with nothing connected to them.Step 3: Be sure the power supply is turned off. Also, check to see that the voltage control(the knob on the left) is turned down all the way (counter clockwise). Connect the circuitin Fig. 9. Note that the current probe is connected to measure the current through theresistor, and the voltage probe is connected to measure the potential difference across theresistor.Step 4: Turn on the power supply. Begin graphing current and voltage starting with the powersupply set to zero voltage, and graph as you turn the voltage control up slowly to about3 volts.Warning: Do not exceed 3 volts!PHYS-204:Physics II Laboratory 12
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Lab & Pre-lab #5: Voltage

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