Capacitors in Circuits - Lab Experiment 7 | PHY 203, Lab Reports of Physics

Download Capacitors in Circuits - Lab Experiment 7 | PHY 203 and more Lab Reports Physics in PDF only on Docsity! Ph 203: General Physics III Lab 1 Instructor: Tony Zable Experiment # 7: Capacitors In Circuits PRELIMINARY QUESTIONS 1. Consider a candy jar, initially with 1000 candies. You walk past it once each hour. Since you don’t want anyone to notice that you’re taking candy, each time you take 10% of the candies remaining in the jar. Sketch a graph of the number of candies for a few hours. 2. How would the graph change if instead of removing 10% of the candies, you removed 20%? Sketch your new graph. 3. Consider an uncharged capacitor in series with a small light bulb, an open switch and a 3 V power source, as shown in the diagram below. Predict what happens to the light bulb when the switch is closed. Sketch a graph of bulb brightness vs. time. 4. After a long time and the capacitor is completely charged, the switch is opened and the power source is removed from the circuit. Predict what happens to the light bulb when the switch is closed. Sketch a graph of bulb brightness vs. time. Phy 203: General Physics III Lab 7 - 1 1 F+- switch 3 V bulb 1 F switch bulb Ph 203: General Physics III Lab 2 Instructor: Tony Zable INTRODUCTION The charge q on a capacitor’s plate is proportional to the potential difference V across the capacitor. We express this with V q C  , where C is a proportionality constant known as the capacitance. C is measured in the unit of the farad, F, (1 farad = 1 coulomb/volt). If a capacitor of capacitance C (in farads), initially charged to a potential V0 (volts) is connected across a resistor R (in ohms), a time-dependent current will flow according to Ohm’s law. This situation is shown by the RC (resistor-capacitor) circuit below when the switch is closed. R C R e d B l a c k Figure 1 As the current flows, the charge q is depleted, reducing the potential across the capacitor, which in turn reduces the current. This process creates an exponentially decreasing current, modeled by V t V e t RC( )   0 . The rate of the decrease is determined by the product RC, known as the time constant of the circuit. A large time constant means that the capacitor will discharge slowly. When the capacitor is charged, the potential across it approaches the final value exponentially, modeled by V t V e t RC( )         0 1 . The same time constant RC describes the rate of charging as well as the rate of discharging. OBJECTIVES  Measure an experimental time constant of a resistor-capacitor circuit.  Compare the time constant to the value predicted from the component values of the resistance and capacitance.  Measure the potential across a capacitor as a function of time as it discharges and as it charges.  Fit an exponential function to the data. One of the fit parameters corresponds to an experimental time constant. MATERIALS Power Macintosh or Windows PC 10-F non-polarized capacitor Universal Lab Interface 100-k, 47-k resistors Logger Pro two C or D cells with battery holder Vernier Voltage Probe single-pole, double-throw switch connecting wires Pasco Electric Circuit Board 1-F non-polarized capacitor Small light bulb & socket Phy 203: General Physics III Lab 7 - 2 Ph 203: General Physics III Lab 5 Instructor: Tony Zable 3. Note that resistors and capacitors are not marked with their exact values, but only approximate values with a tolerance. Ask your instructor the tolerance of the resistors and capacitors you are using. If there is a discrepancy between the two quantities compared in question 2, can the tolerance values explain the difference? 4. What was the effect of reducing the resistance of the resistor on the way the capacitor discharged? 5. How would the graphs of your discharge graph look if you plotted the natural logarithm of the potential across the capacitor vs. time? Sketch a prediction. Show Run 1 (the first discharge of the capacitor) and hide the remaining runs. Click on the y-axis label and select ln(V). Uncheck the boxes for the Potential column. Click U se Word 6. 0c o r lat er t o v iew M aci ntos h pi ctu re. to see the new plot. 6. What is the significance of the slope of the plot of ln(V) vs. time for a capacitor discharge circuit? EXTENSIONS 1. Try two 10-F capacitors in parallel. Predict what will happen to the time constant. Repeat the discharge measurement and determine the time constant of the new circuit using a curve fit. Phy 203: General Physics III Lab 7 - 5 Ph 203: General Physics III Lab 6 Instructor: Tony Zable 2. Try two 10-µF capacitors in series. Predict what will happen to the time constant. Repeat the discharge measurement and determine the time constant for the new circuit using a curve fit. Phy 203: General Physics III Lab 7 - 6