Download Phy122 L_Ohm

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Lab: Ohm’s Law
Theory:
In 1827, Georg Simon Ohm (1787-1854), a German physicist,
demonstrated a relationship between the current and the potential difference across a
number of different materials. This relationship that he was the first to demonstrate was a
linear relationship, or more specifically, a direct proportion. In honor of Georg Simon
Ohm, this relationship has been named Ohm’s Law.
You will remember that a linear function, when graphed, appears as a straight line. Its
equation can be written in the form y=mx+b. Where y and x are the variables, m is the
slope of the graph line, and b is the vertical axis intercept. When b=0, we refer to the
relationship between the variables y and x as a direct proportion, and the graph of y vs. x
goes through the origin (0,0).
Now, a variable can be any measured quantity that changes. In a simple circuit with a
given load, we may choose to vary the supply voltage so that the voltage (V) becomes a
variable. If we choose to measure the current (I) then it also becomes a variable. In order
to state the relationship between V and I we could write an equation, or write a verbal
description. We might show this relationship visually by graphing V vs. I. Each of these
different types of descriptions of the circuit are known as representations.
We will be using resistors in this lab. Resistors are made to have a particular resistance.
The marking for this rating are in colored bands on the resistor. You will need to learn to
read this color-code to find the “theoretical” or rated value for each resistor.
Reading a resistor color code.
Write the values for R1 here:
1st color
2nd color:
3rd color:
4th color
__________
__________
___________
(gold 5% or
silver 10%)
Numerical
value of this
color
Numerical
value of this
color
Take the numerical
value of the color
and write that
number of zeros.
Resistor value:
Write the resistance
as read by the color
bands.
Find the
tolerance for
your resistance
value
Write the values for R2 here:
1st color
2nd color:
3rd color:
4th color
__________
__________
___________
(gold 5% or
silver 10%)
Numerical
value of this
color
Numerical
value of this
color
Take the numerical
value of the color
and write that
number of zeros.
Phy 122: Ohm’s Law
Eyres
Resistor value:
Write the resistance
as read by the color
bands.
Find the
tolerance for
your resistance
value
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Procedure:
You will need a simple circuit with a stable supply voltage that can be changed, and two
resistors in series. Select a power supply and 2 resistors, then physically wire the circuit.
Draw a picture of your circuit. This is not a schematic but an actual sketch of what it
looks like, much like you would see if you took a picture with a camera.
Draw a circuit schematic of this situation.
Now, what is the range of the power supply that you’ve selected to use for this lab?
Calculate the maximum current you expect. Use the rated values for your resistors.
How do you use this information to set up your multimeter to measure current?
Add the 2 multimeters to the picture and to the schematic you drew. Make sure that your
drawing is clear as to how the meter is hooked into the circuit to make the measurements
you want. The symbols are a V or an A with circles around the letter.
Take several sets of V and I measurements for R1 and for R2 and for the total circuit.
Record them in a table that you create. Be sure to:
• Label each measurement with its variable and unit. You should know how
to get information about units from your multimeter.
• Include the uncertainty with each type of measurement. You will need to
take the necessary measurements to find the uncertainty.
Analysis:
Analyze your data so that you will be able to give an answer to the question posed in your
purpose. In the analysis section, be sure to include all of your reasoning. You should have
more than just tables and graphs. Include words that describe the relationship that you
see. Remember that you have given a prediction in the theory section of your report. You
should compare your results to that prediction and support your conclusion with
reasoning.
Include an uncertainty section. You should be able to use what you learned about derived
uncertainty in this lab to talk about uncertainty of your analysis.
Conclusion:
Conclude by summarizing an answer to the question posed in your purpose. Make your
summary as specific as possible and include a comparison to theory.
Phy 122: Ohm’s Law
Eyres
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