Pre-Lab 7 Assignment: Capacitors and RC Circuits

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Capacitors and RC Circuits v 0.31+ −CP1C+ −2VP2Figure 6: Same circuit as Fig. 5, with current and voltage probes added.Voltage 2 (V)Current 1 (A)Time (s)Step 8: Sketch the graphs on the axes above. Indicate on the graphs the times at which thepositions of the switch was changed, i.e. the time at which the switch was moved fromposition 2 to position 1, and the time at which it was moved back to position 2 again.Question 3.5 Does the actual behavior over time observed on the current graph agree withyour sketches in questions 3-2 and 3-3? Comment on any features of the graph that you didnot expect or account for in your sketch. Explain.Question 3.6 Use the graph of potential difference across the capacitor to explain why the bulblights up when the switch is moved from position 1 to position 2 even though in this position ofthe switch the bulb is connected to the capacitor with no battery in the circuit. Also explain whythe brightness of the bulb changes with time.As you have seen in the lab activity on understanding resistance, a light bulb does not havea constant resistance. Instead, its resistance is temperature dependent and increases when thePHYS-204:Physics II Laboratory 13
Capacitors and RC Circuits v 0.3current through the filament heats it up. For more quantitative studies of the behavior of acircuit with resistance and capacitance, you should replace the bulb with a 10Ω resistor.Activity 3.2: The Rise and Decay of a voltage in an RC CircuitStep 1: Replace the light bulb in the circuit shown in Fig. 6 with a 10Ω resistor. Move theswitch to position 2. Begin taking the data. When the graph lines appear on the screen,move the switch to position 1. When the current and voltage data stop changing withtime, move the switch back to position 2.Voltage 2 (V)Current 1 (A)Time (s)Step 2: Sketch the graphs on the axes above. Indicate on the graphs the times at which thepositions of the switch was changed, i.e. the time at which the switch was moved fromposition 2 to position 1, and the time at which it was moved back to position 2 again.Question 3.7 Do the graphs you sketched for the resistor appear similar to those for thelight bulb? Are there any significant differences?The time interval for the voltage across the capacitor to decay to 37% of its initial valueis known as the time constant. The time constant of a circuit containing a resistor and acapacitor (commonly called a RC circuit) depends on the values of the capacitance andresistance in the circuit.Step 3: Use the Examine feature in the Analysis menu to determine the time and the initialvoltage across the capacitor at the moment the switch was moved to position 2.Thendetermine the time at which the voltage was 37% of this initial value. You have nowdetermined the time constant of your RC circuit.Initial voltage: time:37% of initial voltage: time constant:Careful measurements of the dependence of the potential difference V across a capacitorvs. time t, for a capacitor (C) discharging through a resistor (R) should reveal an exponentialdecay behavior for the potential difference across the capacitor. Such exponentialPHYS-204:Physics II Laboratory 14
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Pre-Lab 7 Assignment: Capacitors and RC Circuits

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