PHYSICS 124 EXPERIMENT NO. 3
DC CIRCUITS
The purpose of this laboratory is to observe the relation the relationship between
voltage drop across, and current through, electrical circuit component, You will also
gain familiarity with connecting circ
uit up and with voltmeter and ammeters
Discussion
Ohmic components obey Ohm's law of V = IR, where

V is the potential
difference, I is the current, and R is a constant (the resistance). Standard circuit resistors
are ohmic. In general, however, R ma
y depend on factors such as the temperature (e.g.,
light bulbs), the direction of current flow (diodes), or light intensity falling on the
component (light sensitive diodes).
You will use a voltmeter to measure V, the voltage drop across the component, an
d
an ammeter to measure I, the current flow through the component. You must keep in
mind that ammeters must be connected in
series
in circuits, while voltmeters must be
connected in
parallel
across the circuit component whose voltage drop is to be measured
.
The sketch shows the proper setup.
Technical note:
non

zero meter readings are obtained only when some current flows
through the meters; hence, the meters have
two
connections. A little reflection (or read

ing) will convince you that "good" voltmeter
s have a very large resistance compared to R,
and '£good" ammeters have very small resistance compared to R. This is in fact quite true
for your voltmeter, but only approximately true for your ammeter. The voltmeter should
not
be connected across the ammet
er because the ammeter has finite resistance.
Part I

Ohmic Components
The procedure is:
1.
Make sure the power supply is switched
off.
Connect the power supply, ammeter,
and voltmeter to one of the two resistors on your circuit board.
Be sure
to rec
ord
which board you have (numbered 1 through 12)
and
the position of the resistor in the
board (positions numbered 1 through 4). Have your lab instructor "OK" your setup.
2.
Measure V versus I in several steps up to 10 volts. Note that the knob on the powe
r
supply can be used to vary the voltage V.
3.
Plot V versus I. Indicate estimated errors in reading the meters. Is the resistor ohmic?
Can you explain any systematic errors suggested by the plot? What is the resistance R?
Part II

Non

ohmic Components
Re
peat the procedure of Part I using the light bulb instead of the resistor. From your
graph, does the bulb resistance increase or decrease with increased temperature? What is
the approximate bulb resistance (V/I) when V = 6 volts? At V = 6 volts, what power
is
dissipated in the bulb?
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