Download Lab 4 - Ohm`s Law - Physics Introductory Labs at Stony Brook

3/20/2017
Lab 4 ­ Ohm's Law [Stony Brook Physics Laboratory Manuals]
Stony Brook Physics Laboratory Manuals
Lab 4 - Ohm's Law
In this experiment you will measure electric current as a function of voltage for two different resistors and for a light bulb. You will
also measure the voltage and current for the two resistors connected in series and in parallel. A current vs. voltage plot is called the IV characteristic of the device. Ohm's Law is satisfied when the I-V characteristic is a straight line. The resistance R is defined as the
ratio of the applied voltage V and the resulting current I:
R =
V
I
where V is expressed in Volts (V), the current in Amperes (A), and the unit for R is the Ohm Ω
Equipment
Power supply
Board with components
Voltmeter
Ammeter.
Red and black test leads
Alligator clips
Method
The voltage across each device to be studied is varied by changing the output of a power supply (see Figure). Voltages and
corresponding currents are measured by meters. CAUTION: Check your circuit to make sure it is correctly wired. Set your
meters to their least sensitive (i.e. highest number) scales before you turn on the power supply. Later you may (and
should) switch to more sensitive settings as long as the meter is not overloaded. The voltmeter reads the voltage across the
device, but the ammeter reads the current through the device plus the current through the voltmeter. Is the current which passes
through the voltmeter significant? How can you check?
Procedure
A power supply provides current I at whatever voltage (according to its specified limits) is needed to drive that current through the
circuit connected to its two output terminals. An ammeter, always in series never in parallel, is used to measure I . The resistance of
the ammeter is “small”. The current I then passes through the “device under test” (DUT), which is in series with the power supply
and ammeter. A voltmeter, always in parallel with the DUT, never in series, measures the voltage drop V across the DUT. The
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3/20/2017
Lab 4 ­ Ohm's Law [Stony Brook Physics Laboratory Manuals]
resistance of the voltmeter is “large”. Notice that all four devices above are “two-terminal” devices: one input terminal and one
output terminal. The whole circuit above can be called a “series-parallel” circuit because it has three elements in series and one in
parallel.
The board which contains the various DUT's you will measure is shown below.
The board has 6 components on it: 4 color-coded resistors, one flashlight bulb (the one on your board may look a bit different from
the one in the figure), and one silicon diode. Note that each component is a 2-terminal device with each of its leads captured between
two brass nuts on the vertical brass “studs”. Make your electrical connections to the component of interest by using an alligator clip
at the end of a test lead to “grab onto” the correct brass stud.
Part I
1. Always turn the power supply down when building and modifying circuits. Connect the circuit as shown in Figure 1 with a
resistor as the device under test. Start at low voltage and slowly work your way up. Never exceed 10 Volts and 250 mA in this
circuit. Space your voltage and current readings between 0 and these maxima.
2. Measure the current read by the ammeter for at least 5 values of voltage between 0 and 10 Volts. Reverse the polarity of the
power supply and the meters and repeat these measurements for negative voltages and currents.
3. Plot current as a function of voltage for the resistor, showing experimental error bars on each point.
4. Determine the resistance of the resistor from your plot. Compare your experimental value for R (and its uncertainty!) against
the value given by the color code [http://en.wikipedia.org/wiki/Electronic_color_code] on the resistor.
Part II
1. Repeat the measurements you did before, but now with the light bulb in place of the resistor. Make most of your
measurements for small voltages below the voltage needed to make the bulb glow brightly; the bulb's resistance will change
dramatically as it get's hot.
2. Make a plot of I vs. V from your data Does the light bulb obey Ohm's Law? Can you explain the bulb's behavior?
Part III
1. Repeat Part 2 only now with the silicon diode. Since, as was noted early on in this Lab 4 manual, the diode is a “polar device”,
you must take data for both polarities. This means that you must take a series of data for one end of the diode “positive”, and
you must take another series of data for the other end of the diode “positive”. Do at least 5 measurements for each polarity.
2. Make a plot of I vs. V from your data. Discuss.
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