1 Basic DC Circuits - Physics

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7 Οκτ 2013 (πριν από 3 χρόνια και 8 μήνες)

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1 Basic DC Circuits
Introduction
This set of introductory lab exercises will provide some experience in using
breadboards to build circuits,show Ohmic and non-Ohmic behavior,and let
you test whether a Thevenin equivalent circuit is really equivalent.It will
also provide some experience in using the NI ELVIS II virtual instruments.
Procedure
1.Measure the series resistance R
s
of a 9-V battery.
(a) Build the circuit shown in gure 1,using a\decade resistance
box"for the variable resistance.Use a Digital Multi-Meter (DMM)
to measure the voltage.
Figure 1:Measuring the resistance of a battery
(b) Starting with the decade box at a very large resistance,record
the voltage measured by the DMM.
(c) Decrease the resistance until the DMM reading is half its initial
value.The variable resistance is now equal to the series resistance
of the battery.Report the value and show why this method works
in your writeup.
2.Explore Ohm's Law and resistance.
(a) Choose a resistor with a value somewhere between 10
2
and 10
3
Ohms.Measure the resistance of this resistor directly,using the
DMM.
(b) Build the circuit shown in gure 2,using the ELVIS II positive
variable supply as a voltage source.
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Figure 2:Measurement of Ohm's Law
(c) Make a plot of current versus voltage for this circuit.Find the
slope of this plot,and determine the resistance of the resistor
from that slope.
(d) Does your resistance calculated fromthis slope compare well with
the resistance measured by the DMM?With the resistance indi-
cated by the resistor's color code?
(e) Now repeat the activities in this section,but replace the resistor
with a small light bulb.Discuss your results.
Please note:The experiments in part 2 are ideally done using Lab-
VIEW control the voltage,measure the current,plot the results,and
even give you the curve-t results.If you've never used LabVIEWbe-
fore,it will take you longer to do it this way;but the stu you'll learn
will make the rest of your labs much easier in this and every other
upper-division physics lab.To get you started on learning LabVIEW,
gure 3 shows a LabVIEWprogram that does the experiment for you.
3.Get some experience with voltage dividers and Thevenin equivalent
circuits
(a) Design a voltage divider circuit that takes the ELVIS II 5Vsupply
as an input and puts out 2.0 V as an output.It should draw
somewhere between 50 and 100 mA.
(b) Build your design on a breadboard and test it.
(c) What is the Thevenin equivalent circuit for your voltage divider
and 5V supply?Calculate V
Th
and R
Th
,then build the Thevenin
equivalent using the ELVIS II variable supply for V
Th
.
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(d) Experimentally verify that the Thevenin equivalent is equivalent
by recording the voltage for both circuits at the outputs,for a
variety of load resistances.Using the decade resistance box as a
variable load may be helpful for this part.
Clean up!
Cleaning up after yourself is an often-neglected laboratory skill.Take this
opportunity to begin practicing the art.Return all resistors to their proper
bin if they're not too twisted up,put away all meters and other tools,strip
your ELVIS II board so it's ready for the next experiment,and generally
make sure everything is tidy and ready for the next experiment group.
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Figure 3:LabVIEWprogram to plot Ohm's Law
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