Ohm’s Law Lab
Advanced Physics / Physics
In this activity, you will explore Ohm’s Law and learn about resistance and several ways of measuring
the resistance of a resistor.
Part 1: Ohm’s Law using a resistor
Items Needed for this part:
Ammeter to measure charge flow (current)
Voltmeter to measure voltage (energy per charge)
25 Ω Resistor
Connecting Wire (alligator clips and wire)
Switch
Six ‘D’ batteries
One of the most important laws of electricity is Ohm’s Law. Ohm’s Law states the following: The
potential difference (voltage) across an ideal resistor is proportional to the current through it.
In equation format, we say: V = IR, where the voltage (V) is measured in volts, the current (I) is
measured in Amperes (amps, A) and the resistance (R) of the resistor is V/A, simplified as Ohms (Ω).
Your setup will
have a few
different looking
things than the
picture. You have
batteries holders
to hold the
batteries. You
will build the
circuit with one
battery, placing
the ammeter, resistor, and switch in series with the battery. The voltmeter is placed in parallel with the
resistor.
The ammeter you will be using has three scales
(5, 50, and 500). The “500” is a good scale to
use for this part. This means that the maximum
current for that scale is 500 mA (milli-amps).
The red “500” connector is the + (or hot) end of
the ammeter. A wire should go from the +
connection of the battery holder to this red
connector. A wire leaves the ammeter at the
black (-) connector and should go the resistor
which we will call the + side of the resistor.
Ohm’s Law and Light Bulb Lab
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From the other end of the resistor (we’ll
call it the negative side), run a connecting
wire to the switch. From the other end of
the switch, run another wire to the
negative post of the battery holder.
Placing voltmeter in parallel with the
resistor means that your group places a
wire from the + end of the resistor to the +
(or red) connection of the voltmeter and
another wire from the – end of the resistor
to the black (or -) connector on the
voltmeter.
This voltmeter has only one scale (0-10
Volts)
If you have problems building the circuit, please ask. Once you get the circuit connected, we can start
taking data. We will read the current and the voltage as we go from one battery up to six batteries.
Once you have one battery and the circuit built correctly, you can close the switch and take voltage and
current readings. Remember, you are using the 500 mA scale of the ammeter and will read the top
scale of the ammeter. You’ll have to convert this to Amps on the data table (1000 mA = 1 A). Once
you get the readings, open the switch, and add another battery.
Part 1: Ohm’s Law using a resistor DATA TABLE
No. of batteries
Voltage (V)
Current (mA,
milliamps)
Current (A or
Amps) Divide
mA by 1000
1
2
3
4
5
6
Stop after you take data for six batteries and touch the
resistor. Be careful—its hot! Why is it hot and what does
this mean in terms of energy conversion? To what form is
the electric energy changing?
Once you get all the data, graph (using Logger Pro) current
(in A) on the x data and the Voltage on the y axis. You
should get a pretty nice linear graph when you calculate the
slope! And this is what Ohm’s Law is—a relationship
between the Voltage and the Current for an electric device.
We say that resistors are OHMIC—the relationship between
Ohm’s Law and Light Bulb Lab
y = 26.002x
10
Voltage (V)
____________________________________
Part 1: Ohm's Law (MY DATA)
8
Voltage (V)
6
Linear
(Voltage (V))
4
2
0
0
0.2
0.4
Current (Am ps)
p. 2
the flowing charge (current) and how much energy each charge has (Voltage) is linear. (V = IR).
The slope is a measure of the resistance of the resistor. I chose a resistor with a resistance of 25 Ohms
(Ω). When I graphed my data, I ended up with a slope or resistance of 26.002 Ω.
What did you get for the value of your slope? _________ Ω
Write down YOUR Ohm’s Law equation here: ________________________
Most resistors are constructed to be within 5% or 10% of their stated values. What is your % error for
your data?
______________ % [Do you remember how to get % error? {[(25-your value)/25]*100}]
The method above is a great way to find the resistance of an unknown resistance. By measuring the
current through the resistor and the corresponding voltage drop, we can use Ohm’s Law to measure
this resistance.
Another method of finding the resistance of an
unknown resistor is to use a MULTIMETER. A
multimeter is a device that can measure many
different things: current, voltage, and resistance.
There are two cords attached to the multimeter.
Make sure the red cord is attached to the V ΩmA slot
and the black cord is placed in the COM port. Turn
the dial to the correct scale in the Ω area (in this case,
we are going to re-measure the 25 Ω resistor so you
can turn the dial to the 200 scale. This means we can
measure resistances up to 200 Ω. I like to place
alligator wires on the multimeter cords. This makes
them easier to attach to the resistor.
Go ahead and attach your multimeter to the 25 Ω resistor
and measure its resistance. I get around 25.7 Ω when I
did this. You probably will find yours a bit different. If
you are having problems, make sure your connections
and alligator clips are on tightly.
What value did you get for your resistor using the
multimeter?
________________________
You can also use the multimeter to measure currents and
voltages. In fact, why don’t you try to use it to measure the voltage of one of your batteries? Remove
the resistor and then turn the dial to “20” in the DCV area. Put the red cord to the + terminal of the
battery and the COM cord to the – terminal of the battery. You should get around 1.5 V for the voltage
of one of the D cells.
Make sure you turn the dial of the multimeter back to OFF when you aren’t using it as there is an
internal battery in the multimeter and this prolongs the life of this battery.
Ohm’s Law and Light Bulb Lab
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One last method of obtaining resistances of
resistors: COLOR-CODING
Many resistors are color-coded. Maybe you have
seen color bands around resistors (see pic). Many
resistors have 4 color bands.
The fourth band is called the
tolerance band and it is usually
gold or silver. The one that I
have in my picture (top right)
has a gold band. You read the
other three bands from left to
right starting on the opposite
side of the tolerance band.
So the colors bands on the
resistor I have in the top-right
picture is brown-black-black.
So, what do the colors mean?
Check the chart at the right.
Brown means 1 and black
means 0. The first two bands
is the number and the third
band is the multiplier or the
exponent.
So, brown-black-black would
be 10 x 100 = 10 Ω. Because
we also have the gold band, the
complete statement of its
resistance would be 10 Ω ±
5% (9.5-10.5 Ω).
See the next page
Ohm’s Law and Light Bulb Lab
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Grab three of the color-coded resistors and try your luck at seeing the color bands. It can be hard
to see them and especially so for those among us with color-blindness (including me!) You can check
the resistor with the multimeter as verification.
There is a nice info here: http://www.tangibletechnology.com/audiobasics/resistors/R_Color_Code.html
Here is an applet for you: Select the colors and it will figure the resistance for you!
http://www.electronics-lab.com/articles/basics/resist/index.html
Unknown Resistor #1
Resistance using the color bands = ____________________ Ohms
Check it with the multimeter: Resistance = _______________________ Ohms
Was it close? ___________
Unknown Resistor #2
Resistance using the color bands = ____________________ Ohms
Check it with the multimeter: Resistance = _______________________ Ohms
Was it close? ___________
Unknown Resistor #3
Resistance using the color bands = ____________________ Ohms
Check it with the multimeter: Resistance = _______________________ Ohms
Was it close? _____________
List the three ways to measure a resistor’s resistance as done in this part of the activity:
1) _______________________________________________________________
2) _______________________________________________________________
3) _______________________________________________________________
Ohm’s Law and Light Bulb Lab
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