Download PHYS-1200 PHYSICS II SPRING 2004

Rensselaer Polytechnic Institute
Introductory Physics II
PHYS 1200 Fall 2008
Activity 9
RL and RC circuits
In this experiment, you will observe the time dependence of voltage in driven RL and RC
circuits.
Equipment:
Mobile Card (Oscilloscope function)
Mobile Card (Function generator)
Electronic Breadboard
0.1 microFarad capacitor
1 milliHenry inductor
1000 ohm resistor
Background:
Capacitor
When a capacitor is charged to voltage V0 and then allowed to discharge across a resistor, the
resulting voltage is V (t )  V0 e  t / RC (see for example Chapter 27.09 in Halliday, Resnick,
Walker 8th ed.) In our circuit, we will charge through a diode, which allows current to flow
one way but not the other. A positive voltage phase, the diode allows current to charge the
capacitor. When the voltage is negative, the diode effectively isolates the capacitor from the
source.
In the following circuit, the capacitor charges through the diode and discharges through the
resistor.
diode
function
generator
C
R
oscilloscope
ch 2
1) Setting up your Mobile Card Oscilloscope
 Set up your Mobile Card and enable your oscilloscope. Attach a red wire lead to the
A1+ contact of the Card and a black wire to A1- contact two over from the A1+
contact.
 Set Function Generator 1 to 100 Hz, 2V peak to peak, and Waveform: Square Wave.
 Enable Channel 2 on the Oscilloscope
o Channel 2 - on; Volts/div - 1V; Coupling - DC; Input - AWG1 (Function
generator)
o TRIGGER: Mode - Auto,; Source - Ch2;Slope - Falling.
o Horizontal: Time/Div - 1 millisec; Mode - Y-T
 You should have a nice single step in a square wave on the screen
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(C) 2009 Rensselaer Polytechnic Institute
Rensselaer Polytechnic Institute
Introductory Physics II
PHYS 1200 Fall 2008
2) Setting up the capacitor circuit.
You will want to construct a circuit like the one in the figure above. This is easier when you
use an electronic breadboard like the one in your kits. The principle of the breadboard is
simple. A wire can be pushed into each hole in the breadboard and is thus connected to
common terminal strips inside the breadboard. In your breadboard each set of five holes is
connected together. See the Appendix for details.
Here's a suggested layout.
Ch2
FG1
In the figure, FG1 is the output from the function generator on the Mobile Card. The function
generator outputs are labeled AWG1 and AWG2 and are approximately in the middle of the
connector strip. Connect between AWG1 and GND
3) Capacitor measurements
 Set up Channel 1 on the oscilloscope to measure A1 DIF. (Leave Channel 2 on so you
can use it to control the sweep trigger.) Set the Volts/div to about 250 mV.
 With the Horizontal Time/Div set at 2 ms you should see two cycles. The charging
stage should rise rapidly and the decay stage should drop off slowly.
 Sketch the Ch1 waveform on your paper.
 Increase your sweep speed to about 200 s/div so you can see the slow decay across
the screen. (You may find it useful to STOP collection so you don't have to chase a
moving target.)
 Enable cursors and move Ch 1 cursors to measure the maximum and minimum Ch 1
voltage on the screen.
 Record the minimum and maximum voltages ___________
__________
 Choose two points on the decay curve that are about 75% and 25% of the range from
maximum to minimum. Use the CH 1 time and voltage cursors to measure the time
and voltage of those points.
 Record the time and voltage of those points.
__________ __________

__________ __________
 V V 
Estimate the RC decay constant using: RC  (t25  t75 ) ln  75 25  .
 V75  Vmin 
_________
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(C) 2009 Rensselaer Polytechnic Institute
Rensselaer Polytechnic Institute

Introductory Physics II
PHYS 1200 Fall 2008
Is the value of RC consistent with the values for RC you were given at the start of this
lab?
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(C) 2009 Rensselaer Polytechnic Institute
Rensselaer Polytechnic Institute
Introductory Physics II
PHYS 1200 Fall 2008
Section 4 Homework Exercise
When a resistor and inductor are wired in series, the inductor limits the rate of change of the
V
current. For the decay portion of the cycle, VR  0 et / RL with  RL  L / R .
R
 Rewire the circuit to reproduce the series LR circuit in Fig. 20.21 in Halliday, Resnick,
Walker.
Here's a suggested layout.
Ch1
FG1




Set the horizontal time base to 2 ms/div and Ch1 to 1 V/div. Sketch the waveform you
see.
Expand the time base so you can see the decay curve. (About 1 to 2 s is ok.)
Stop the collection so you have a fixed screen and use the cursors as above to
determine the L/R time constant.
Is this time constant consistent with your expectations from theory and the numbers
you were given at the start of this document?
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Rensselaer Polytechnic Institute
Introductory Physics II
PHYS 1200 Fall 2008
 Appendices
Electronic Breadboards
Electronic breadboards have lots of simple connectors for electronic components. Holes in rows
and columns are usually connected to one another so it's easy to connect components to each
other. The interconnect layout for a typical breadboard is shown here. (A few typical devices
are also shown to indicate how they might be inserted in the breadboard.)
The leads of most components can be pushed straight into the holes. ICs are inserted across the
central gap with their notch or dot to the left.
The double top and bottom strips usually carry power or common signals - typically about 25
pins are connected in a row. The groups of five running vertically are called terminal strips and
make it easy to connect devices together.
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