Lab 6: Capacitors and Resistor-Capacitor Circuits Phy208 Spr 2008 Name _______________________________ Section ___________ 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. What are we doing this time? You will complete three related investigations. PART A: Build capacitor circuits on circuit board, investigating current flow and voltages. PART B: Build resistor-capacitor circuits, and measure time-dependent phenomena. PART C: Use these ideas to measure and investigate a crude cell membrane electrical model, investigating propagation of an action potential down the cell membrane. Why are we doing this? Capacitors are almost as ubiquitous as dipoles, showing up almost everywhere there is an insulator. Actually, capacitors have some similarities to dipoles, with equal and opposite charges on the electrodes. And they almost always show up in combination with some non-insulator — a resistor-capacitor circuit! What should I be thinking about before I start this lab? You should be thinking about the ideas of circuits you developed when looking at resistors last week, and the aspects of capacitors you looked at in lecture and discussion. In particular, how the voltage across the capacitor is related to the charge on it, and how the current in a circuit delivers charge to a capacitor.Lab 6 2 For the first part of the lab, you use the same circuit board as you did last week. The board is shown below: Holes connected by black lines are electrically connected by conducting wires, so all points connected by black lines are at the same electric potential. You build a circuit by plugging in resistors and capacitors across the gap between crosses. The resistors are built into plastic blocks with banana-plug connectors that exactly bridge the gaps. After you plug in a resistor, there will still be unused holes in each cross. You will use the remaining holes to connect the variable voltage source to supply your circuit with charge, and to connect the electrometer or Pasco interface to measure potential differences and currents at various points in the circuit. These 5 points connected together Resistor or capacitor block goes hereLab 6 3 A. Capacitor circuits. You use the electrometer to measure voltages. Connect the red probe to the electrometer ‘input’ connection, and the black probe to the electrometer ‘ground’ input with black coaxial cables (not the banana plug cables). You don’t need any adaptors: the coaxial cable connects directly to the probe and to the electrometer. 1) Series capacitors: you will build the circuit below and measure it, but first predict how the 20V provided by the voltage supply will split among the two series capacitors. V across C1: V across C2 Build the circuit below (note that the power supply is not connected). a) First, use a bananna plug cable to temporarily short out each capacitor to make sure there is no charge separation between the plates. Make sure the cable is removed before proceeding. b) Set the voltage supply to 20V. Touch the black and red voltage supply leads across the series circuit, then disconnect both the black and red leads from the circuit. c) Use the electrometer to measure the voltage drops across each capacitor individually with the supply disconnected. Compare these to your prediction. 30V DC voltage source 1000V 1.0 µF 0.47 µF C1 C2Lab 6 4 2) Parallel capacitors. Here you connect capacitors in parallel to see how they share charge. Start by discharging both capacitors. You first charge up C1 (the 0.5µF capacitor) to 20V, then disconnect the voltage supply. The electrometer will not measure correctly unless you disconnect the power supply. You then connect the 1 µF capacitor in parallel. a) Do a calculation here to predict the final voltage across the capacitors. Now do the experiment: b) Measure the voltage drop across each capacitor with the electrometer. Remember that the power supply is disconnected here. How does this compare with your predicted value? 30V DC voltage source 1000V 1.0 µF 0.47 µF C1 C2 Wire Wire Plug 1 µF capacitor in after making sure it is discharged, and after disconnecting the power supply.Lab 6 5 B. Resistor-capacitor circuits. Now you will connect a resistor and capacitor to investigate 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 1 µF capacitor exactly as shown below. Put the electrometer on the 30V scale, and make sure the voltage is still at zero V. Quickly increase the voltage on the DC supply to 20 V and watch the electrometer. After the electrometer needle stops moving (wait at least a minute), quickly turn the voltage supply to zero volts, and watch the electrometer again. Note that the electrometer measures both the sign and the magnitude of the voltage across the resistor, proportional to the current through the resistor. Remember that the voltage supply keeps a constant voltage between its two output terminals. Zero volts means that there is no potential difference between the red and black terminals: it is as if a wire connects them. 1) What is the direction of the resistor current after increasing the voltage 0V to 20V. 2) What is the direction of the resistor current after decreasing the voltage 20V to 0V? 10 MΩ 1 µF 30V DC voltage source 1000V Electrometer + - red black red blackLab 6 6 3) The current represents a charge flow. What happens to this charge after it goes through the resistor? 4) What do you think is happening to the voltage across the capacitor as a function of time? (you can’t measure this with the electrometer because of the way it is grounded). 5) How does the capacitor voltage affect the current through the resistor? 6) You saw that the current through the resistor changes smoothly with time. But it can be easier to think about this in short time steps. Fill out the following table to approximate the current as a function of time as you increase the voltage 0V to 20V. ΔQR is the charge that flowed through the resistor during the previous 2 sec. QC is the charge on the capacitor VC is the voltage drop
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