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Lab 5: 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:
___________ ___________ ___________
Numerical
value of this
color
Numerical
value of this
color
Phy 102: Ohm’s Law
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Take the numerical
value of the color
and write that
number of zeros.
Resistor value: In
the space below,
write the
resistance as read
by the color
bands.
4th color
(silver or gold)
5% or 10%
Find the
tolerance for
your resistance
value
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Write the values for R2 here:
1st color
2nd color:
3rd color:
Resistor value: In
the space below,
write the
resistance as read
by the color
bands.
___________ ___________ ___________
Numerical
value of this
color
Numerical
value of this
color
Take the numerical
value of the color
and write that
number of zeros.
4th color
(silver or gold)
5% or 10%
Find the
tolerance for
your resistance
value
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.
In the space below, 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.
In the space below is a drawing of 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 on page 2. 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.
(Chapter 17 of your text should be used as a reference.)
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:
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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.
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 102: Ohm’s Law
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