Download Ohm`s Law

PHYSICS 125 LAB 4: OHM’S LAW
Goal: This lab investigates how Ohm’s law relates the current and voltage in ohmic materials.
Background reading: Review section 17.3 (Resistance and Ohm’s Law).
Equipment needed:
DC power supply
Connecting wires (with banana plugs, on wall rack)
DPDT switch (with spade lugs to banana adapters)
2 Digital multimeters (Fluke D810 or equivalent)
Resistors (10, 15, and 25 ohm) and one light bulb (in Edison socket with banana posts)
Theory:
There is usually a simple relationship between the voltage V applied across a given piece of
material and the current I that flows as a result. When the current is proportional to the voltage,
the ratio is called the resistance of the material: R = V / I and is measured in units of Ohms
(abbreviated with capital omega  ). In some materials, this proportionality is not observed,
and we call these materials non-ohmic to distinguish them from ohmic materials which do obey
Ohm’s Law. Thus Ohm’s Law is not a fundamental law of nature that applies to all materials.
Experimental Procedure:
1. Set up the circuit as shown in the diagram, starting with a 25  resistor. One of the DMMs
will be used as an ammeter to measure current through the resistor, and should be set for DC,
mA/A, 2000mA with the pushbuttons, and the connecting wires should be plugged into the
COMMON and the mA inputs. The other DMM will be used as a voltmeter to measure the
voltage across the resistor, with the pushbuttons set for DC, V, and 20V, and the connecting
wires should be plugged into the COMMON and the V inputs. Have your instructor check the
circuit after you set it up, and before turning on power. Leave the switch open to start with.
Switch
Power
Supply
Resistor
R
V
A
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Variation of current with voltage
2. After the instructor has checked the setup, turn the voltage knob on the power supply fully
counterclockwise, which will reduce the voltage to zero. Then close the switch and slowly turn
up the voltage until a voltage of approximately 1 V is indicated on the voltmeter. Observe the
current on the ammeter, and record the measured voltage (which might be a bit different from
1.00 V) and the measured current. Repeat this measurement for several more voltages, for
example 2 V, 3 V, 4 V, and 5 V, and record the results in the table. Open the switch when you
are finished with each measurement. This will prevent the resistor from getting too hot, due to
the power dissipated in the resistor (this would be P = I V). Sometimes these resistors get too
hot to touch, depending on the voltage, current, and the time that current flows through them.
3. To analyze the data, there are two possible methods. One would be to simply calculate the
ratio V / I for each measurement, in order to get a value of R, and calculate the average value of
R for the five measurements. You will find that the resulting R is not precisely the value (for
example, 25 ) that is printed on the resistor (which is called the nominal value) since in
manufacturing there is some variation. The value of R from your measurements might vary as
much as 5% or 10% depending on the so-called “tolerance” of the resistor.
4. Replace the 25  resistor with another one, for example 10  and repeat the measurements.
Calculate the value of resistance from your measurements for this resistor using a graphical
method; see the next step.
5. Another way to analyze the data is to plot the values of V (on the vertical axis) versus I (on
the horizontal axis) on a sheet of linear graph paper or by using a computer program like Excel.
Then draw a straight line which is a good fit to the data points. Obtain the slope of the line by
picking two points near the ends of the line, and calculate the slope by dividing the “rise” by the
“run”. The slope of this line is the resistance of the resistor. Your instructor will show how this
can be done using the “trend line” option in Excel. (An alternative is “Graphical Analysis.”)
Unknown resistor
6. Now we will measure an “unknown” resistor, using the same procedure as above, with five
different voltages. Insert the “unknown” resistor in place of the resistor from the last set of
measurements, and make the current-voltage measurements, recording them in the data table.
Use one of the two methods of analysis to determine the resistance of the “unknown” resistor.
Non-ohmic resistance
7. Now replace the resistor with a light bulb. Measure the voltage and current as you vary the
voltage from 1 V to about 19 V in steps of 2 V and record them in the data table. At the highest
voltage, the filament in the light bulb will probably glow red hot, but it is inside the glass and
won’t burn up since it is surrounded by inert gas. Plot the V and I values on linear graph paper
or using Excel as in step 5. You should be able to see that the points do not fall along a straight
line. Therefore it does not make sense to calculate R as a slope. Also, by calculating V / I for
each measurement, you will see that the resistance obviously depends on temperature.
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PHYSICS 125 LAB REPORT: OHM’S LAW
Name _____________________________
Variation of Current with Voltage
Data Table for Resistor 1
Resistor 1 data:
Nominal value of R = _______________
Voltage V
Current I
R = V/I
measurement 1
measurement 2
measurement 3
measurement 4
measurement 5
Average of values of R = _________________________
Data Table for Resistor 2
Resistor 2 data:
Nominal value of R = _______________
Voltage V
Current I
measurement 1
Obtain R from a
graph by finding the
slope of the trend line.
measurement 2
Attach the graph
to this report.
measurement 3
R = _____________
measurement 4
measurement 5
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Unknown resistance measurement
Unknown R data:
Voltage V
Current I
R = V/I
measurement 1
measurement 2
measurement 3
measurement 4
measurement 5
Calculated value of “unknown” R = _______________
Non-ohmic resistance measurements (also plot this data)
Light bulb data:
Voltage V
Current I
measurement 1
measurement 2
measurement 3
measurement 4
measurement 5
measurement 6
measurement 7
measurement 8
measurement 9
measurement 10
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R = V/I
Non-Ohmic Devices - Diodes
Semiconductors, especially the junctions between two different types of materials, can have nonohmic behavior, which is the basis for the manufacture of diodes. A diode will behave
somewhat like a switch which allows current to flow in one direction, but not in the opposite
direction. In this part of the lab you will make voltage and current measurements on an LED.
Set up the circuit as shown in the diagram. The diode in this circuit is called a light-emitting
diode (LED). You will see it emit red light when current goes through it in one direction. The
device itself is very small and is encased in red plastic, with two wires coming out of one side.
The other side is rounded to allow the red light to come out in a range of directions. This LED
will be connected in series with a 1000  resistor to limit the current (and prevent the LED from
being overloaded or burned out by mistake). The resistor is much smaller than the ones you used
earlier in the lab, and can’t handle much power, but we don’t expect more than about 10 mA in
this circuit (if we limit the voltage on the power supply to about 10 volts). You may notice that
the resistor has colored bands around it, brown, black, and red, which indicate the resistance
according to a color code (brown signifies 1, black signifies 0 and red signifies two zeroes more).
After the instructor has checked the setup, turn the voltage knob on the power supply fully
counterclockwise, which will reduce the voltage to zero. Then close the switch and slowly turn
up the voltage until a voltage of approximately 0.5 V is indicated on the voltmeter. Observe the
current on the ammeter (it will probably be zero), and record the voltage (on the voltmeter, not
the power supply) and the measured current. Repeat this measurement for several more voltages,
for example 1.0 V, 1.4 V, 1.52 V, 1.54 V etc. and record the results in the table on the reverse
side of this sheet. The diode “turns on” at about 1.5 V and you will see the current begin to
increase. At this point you need to use smaller voltage increments to see the detail in the current
vs. voltage curve (the I-V curve). Continue these measurements up to about 1.8 V, and do not
exceed 10 mA current in the LED.
You should also try to put current through the diode in the opposite direction (perhaps by
reversing the wires going to the power supply) and you will see it doesn’t light up, even for 2 or
3 V. Don’t exceed 5 V on the diode.
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One way to analyze the data is to plot the values of I (on the vertical axis) versus V (on the
horizontal axis) on a linear graph, called an I-V plot. You will notice that the I-V plot turns up
sharply at about 1.5 volts and the diode will then conduct a wide range of current without much
change in the voltage across it. So it behaves like a switch which “turns on” at about 1.5 volts
and then keeps almost the same voltage drop across it as the current increases.
Voltage (V)
Current (mA)
Voltage (V)
Current (mA)
Plot the data using Excel or Graphical Analysis on one of the lab computers and attach it to your
lab report.
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