EC-3: Capacitors and RC-Decay

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Alan Underwood
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1 Your TA will use this sheet to score your lab. It is to be turned in at the end of lab. You must use complete sentences and clearly explain your reasoning to receive full credit. EC-3, Part I: Do not do this part EC-3, Part IIA: Do as in manual: explain below your prediction of ΔV across each cap Summarize here your predicted and measured values of ΔV Predicted ΔV Measured ΔV 1.0 µf cap 0.5 µf cap Part IIB: Do as in manual: explain your prediction of final ΔV across cap combination Voltage on isolated 0.5µF combination Calculated ΔV Measured ΔV 1

2 Now you will connect a resistor and capacitor to understand how a capacitor charges and discharges. Turn the DC voltage source on and set it for zero volts output. Connect the DC voltage source and electrometer to the 10 MΩ resistor and 1µF capacitor as shown below. For the electrometer, use a BNC to banana plug adapter connected to the input : DC voltage source - + Electro meter 30V 1000V 10 MΩ 1 µf Explain why the electrometer connected in this way can be used to measure the current in the circuit. Put the electrometer on the 30V scale. Quickly increase the voltage on the DC supply to 20 V and watch the electrometer. After the electrometer needle stops moving (you ll probably need to wait at least a minute), quickly turn the voltage supply to zero volts, and watch the electrometer again. Note that the electrometer measures the sign as well as the magnitude of the voltage across the resistor. Use your measurement to explain what the current in the circuit is doing after your sudden increase and decrease in the voltage. 2

3 To learn how to use the oscilloscope to observe RC decay, use Part A of lab E-6 on page 51 of your lab manual as a reference. Your TA will give you a quick intro to the oscilloscope Ground the oscilloscope channel you will use by setting the switch on the input s left to ground, and use the vertical position knob to set the zero volt position (trigger needs to be set to auto for this). Set the oscilloscope to 2V / div. Set the function generator to a square wave of about 1000 Hz frequency, and connect the 600 Ω output to the oscilloscope using two patch cords and the BNC to banana plug adaptor. Adjust the signal amplitude on the function generator until you see a signal a few divisions high, but probably drifting across the screen. Adjust the time base until you can see several periods of the square wave. Now set the trigger back to normal. You may need to adjust the trigger level knob to get a consistent trigger, and also may need to change the vertical scale and the time base in order to see the signal. 1) Draw a graph of the square wave produced by the function generator, marking divisions, units, and values on the x and y axes. 2) Use the oscilloscope to measure the period of the square wave for different frequencies and compare it to the reading on the function generator. Frequency setting (Hz) Measured period (seconds) 1 / period (seconds -1 ) 3

4 Now you will use the oscilloscope to measure the properties of RC circuits. You will need to connect the function generator, oscilloscope, resistor and capacitor. Connect the circuit elements as shown in the figure below. The function generator changes the voltage in the same way you were doing by hand previously, but many times per second. Function generator Oscilloscope Ω 600Ω 10 kω 0.5 µf The oscilloscope is now set up so that you can determine the time-dependent current in the circuit. Explain why the current is likely to depend on time. Set the function generator to a 15 Hz square wave. Using three different resistor-capacitor combinations, starting with the 10kΩ resistor and 0.5 µf capacitor, use the oscilloscope to measure the characteristic decay/saturation time (time after which the oscilloscope signal has decreased to 1/e of its initial value (e=2.718, so about one third). For larger resistors or capacitors, you may need to reduce the frequency to give time for the signal to decay. R ( Ω ) C ( F ) Measured time (sec) R x C ( sec ) 4

5 Set the function generator for a 15 Hz square wave. Connect the function generator directly to the oscilloscope and set the height of the square wave to 5V by changing the amplitude knob on the function generator. Now hook up the 10 kω resistor and the 0.5 µf capacitor as on the previous page. Connect channel A of the computer interface across the resistor instead of the oscilloscope. Make sure the oscilloscope is not connected it interferes with the comp interface. Record the data with the computer by clicking the start button, and then dragging a graph icon from the bottom left window onto the data. The vertical axis is in volts because that is what the computer measures. Transform the values to current in ma by using the calculator icon to divide the voltage by 10. Print out the graph and turn it in with this sheet. Why does the current alternate positive and negative? With the mouse select one decay from maximum to zero current on your Current (ma) vs Time (sec) graph, and find the area under the curve by selecting area from the statistics (capital sigma) menu at the top of the graph. Area under curve Value Units In the space below, calculate the expected value of this area from basic principles of the capacitor (you don t have to do any integration). How does your value compare to the measured one? 5

Lab 5: EC-3, Capacitors and RC-Decay Lab Worksheet

Lab 5: EC-3, Capacitors and RC-Decay Lab Worksheet , Capacitors and RC-Decay Lab Worksheet Name Your TA will use this sheet to score your lab. It is to be turned in at the end of lab. You must use complete sentences and clearly explain your reasoning to