Ohm’s Law Lab Introduction
Slowly read through this page as a group. As a group, create a sketch on a whiteboard to make sense of the paragraph. Then,
individually reproduce your groups sketch in your lab notebook being sure to add labels.
Charge is fundamental principle of matter. 1 electron has a charge of -1.6x10-19 coulombs. Our charged particles are made up of
positive and/or negative charges taking on this elementary value of 1.6x10 -19 C. These elementary charges are made up of quarks
which allow physicists to explain the differences between two elementary charges on a more fundamental level.
Current – The flow of positive charge through a circuit (only electrons move; therefore, current is always in the opposite direction as
electron flow) each second. Current is measured in coulombs/second a.k.a. amp. 6 amps = 6 coulombs/second = six coulombs each
one second.
Ex. Current would be similar to the number of cars driving past a single point in one second. A higher current means that
more cars drive past that point per second. Higher current does not necessarily mean that these cars move faster. They could
simple be driving closer together or a wider road.
Measuring Current – Because current is the amount of charge flowing through a wire, to measure current at a point, you
must allow all of that current to ALSO flow through the ammeter. To accomplish this, an ammeter has very little resistance
so that it does not slow down the current flow. The ammeter must be placed in line with the current you want to measure.
Voltage – This term simplifies “electric potential difference.” Electric potential is positive near more positive charges than negative
charges. Electric potential is negative near more negative charges than negative charges. Therefore, the difference in electric potential
(voltage) is the difference to your proximities at these two locations. The units of electric potential are joules/coulomb.
Ex. Electric potential is zero in a room filled with an even distribution of positive and negative charge. Electric potential is
zero in the middle of a room with positives on one side and negatives on the other. Electric potential is very positive on the side of the
room with lots of positive charges and negative on the side of the room with lots of negative charges. The difference in electric
potential between the two sides of the room is therefore twice as large as the magnitude of the individual electric potential of each
side.
Measuring Voltage – Because voltage is the difference in electric potential between two points, you want to make the
measurement by touching these two points. The voltmeter has a very high resistance, so it only lets in a tiny amount of current. Just
enough for it to give you a reading of the electric potential difference (voltage) between these two points.
Resistance – A perfect conductor with no resistance allows electrons to flow freely from particle to particle to particle. Copper is a
very good conductor and this is why we use it in our household wires. All wires at normal temperatures have resistance. This impedes
the smooth flow of electrons from particle to particle. The unit of resistance is the ohm Ω.
Ex. A conductor with resistance is similar to a perfectly smooth road with a bunch of construction cones scattered randomly
on it.
Measuring the resistance of a component using a multimeter (one meter with a built-in ohmmeter, voltmeter, and ammeter) is
not that complicated of a process. The multimeter simply applies a small voltage from its battery across a component. If there is lots of
charge flowing, this means that there is a low resistance in the component. If there is little flow, this means high resistance.
Series components – Components of the circuit are organized so that all the current traveling through one component must travel
through the next component because there are no other paths.
Ex. A highway under construction may have two construction zones one after the other. The speed of traffic through one
zone will be influenced by the difficulty to pass through the other. However, all cars must travel through both construction zones.
Parallel components – Components of the circuit are organized so that the electric potential difference across the components must
be the same.
Ex. A highway under construction may split into two separate roads for a mile and rejoin back together. There may be lots of
cars piled up in a traffic jam before this split and cars very spaced out moving faster after the roads join back together. One path may
allow cars to travel through it easier than the other. However, the difference between the car spacing before each path and the spacing
after each path must be the same because they are referring to the same spacing.
Circuit Diagrams:
The chart below should be sketched out for any circuit you work with in the lab. Most problems given to you in this class will be
drawn like this image on the right. It is very important that you learn how to create and read a circuit diagram.
Setup
Circuit Diagram
Circuit Components:
Voltage Supply – Voltage is represented as two parallel lines (sometimes four parallel lines). One end of the voltage supply will
always be represented as a longer line than the other end. The longer end represents the higher electric potential side of the battery. A
voltage supply will always be labeled by the number of volts V difference between the ends. E.g. 6V, 1.5V or 2V.
What does this tell you? The voltage supply tells you the difference in electric potential from the wires on one side of it to the
wires on the other side of it. It also tells you which side has higher electric potential.
Resistor – A resistor is simply a material with a significant resistance. A resistor is represented by four zigzag lines and should
include a label that tells you the amount of resistance in that resistor. In practice, we consider all non-zigzag lines in our circuit
diagram to have zero resistance.
Meter – A meter in a circuit is a tool for measuring a value. The above circuit diagram represents a voltmeter that measures the
electric potential difference between the ends of resistor1. Looking at a circuit diagram, you can always tell the type of meter based on
the units of what it is measuring. An ammeter would be represented by a circle with an A inside because it measures amps (A). An
ohmmeter would be represented by a circle with an Ω inside because it measures ohms (Ω).
Circuit tools:
Current – The current (I) is not always drawn on a circuit diagram. Current is represented by an arrow that gives a direction and the
number of amps (coulombs/sec) that the electricity flows. Some problems will not refer to the current, so until you need it, you don’t
have to draw it on your circuit diagram. You can always add it on later. The current is not a circuit component like those listed
above.
Node – A node can be a useful tool and is often focused on by simply drawing a dot on the node to make it more obvious. A node is
any point where the current splits or joins together. There are two nodes in the circuit diagram above. These occur where the wires
from the voltmeter meet with the wires near R1.
Nodes can be a useful thinking tool when analyzing current because whatever current is coming into the node must also equal
the current that is leaving the node. This is similar to two streams combining into one large one. You know that if one steam enters the
node with 30 gallons of water per second, and one enters with 20 gallons of water per second, the water leaving the node must be 50
gallons of water per second.
Loop – A loop can be seen above as the voltage source + the wire + resistor1 + the wire + resistor2 + the wire. You should be able to
follow a full loop with your fingertip around your circuit.
A loop can be a useful thinking tool because each point in the circuit has a specific electric potential. The electric potential
may get higher or lower as you cross components. However, after completing a full loop with your fingertip, you must be back to
where you started and therefor at the same electric potential that you started at. This tells us that all the positive and negative changes
in electric potential (∆V) around the loop added up to zero. ∆V voltage source + ∆Vwire + ∆Vresistor1 + ∆Vwire + ∆Vresistor2 + ∆Vwire = Zero!
After reading the first two pages of “Ohms Law Lab Introduction” as a group, check to be sure that you have the following materials
to use in this lab:
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Wires
3 miniature light bulbs
1 light bulb holder with 3 openings
1 D-battery in holder.
Voltmeter
Ammeter
We will begin by learning how to use the voltmeter/ammeter to make measurements.
1.
Place one light bulb (resistors) in series with the battery. [To connect a light bulb, simply use the alligator clips and clip to
the ends of the light bulb raw wires, not the plastic]. Touch the ends of the alligator clips to the ends of the battery. This will
be the simple circuit we are working with. Be sure you can light the bulb before proceeding.
a. Measure the voltage across the light bulb by using the voltmeter. Hook up alligator clips to the voltmeter by clipping
two different wires one to the + and one - . By touching the ends of your voltmeter wires to the exposed wires on
each side of the light bulb. The voltmeter needs to be attached so that the + side is hooked closest to the + side of the
battery. *If you hook up the voltmeter wrong the needle will read less than zero.
b. Sketch a setup and circuit diagram chart for this setup. Include values for your voltage across the resistor.
Setup
Circuit Diagram
a.
b.
Measure the voltage across the battery by touching each side of the battery.
Sketch a setup and circuit diagram chart for this setup. Include values for your voltage across the battery.
Setup
Circuit Diagram
c.
One of these is the cause of the other. Explain why the battery’s voltage causes the light bulbs voltage.
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Your circuit diagram should use the symbol V to mean multiple different things. Explain the differences between
these “V”s. ___________________________________________________________________________________
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Measure the voltage difference across one of the wires that connects the battery to the light bulb. Using the lab
introduction materials, explain why the value that you get for this measurement makes sense. __________________
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Measure the current through the light bulb by changing your meter to an ammeter. Ammeters must be wired in
series with the component you wish to measure the current of. Include these measurements on your circuit diagram.
Setup
Circuit Diagram
d.
e.
f.
g.
2.
Explain the difference between the placements of the voltmeter when measuring voltage as opposed to the ammeter
when measuring current.
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h. Before measuring the current through the battery, decide as a group what you expect it to be based on the reading.
i. Measure the current through the battery. Was it what you expected? _______________________________
We are going to be analyzing a new circuit now. Place two light bulbs (resistors) in parallel with each other. Then connect a
battery in series with these parallel bulbs.
a. This time do not include the meters in your circuit setup/diagram or else you would have to draw a new circuit
diagram for each time you make a new measurement as you did in the previous setup.
b. Sketch out the setup and circuit diagram
Setup
Circuit Diagram
c.
3.
Measure the voltage across the following:
i. Voltage across battery: ∆Vbattery = ___________
ii. Voltage across bulb 1: ∆Vbulb1 = ____________
iii. Voltage across bulb 2: ∆Vbulb2 = ____________
d. Explain using the reading why voltage bulb 1 and voltage bulb 2 do not add up to voltage battery as one may expect.
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e. Measure the current through the following:
i. Current through battery: Ibattery = ___________
ii. Current through bulb1: Ibulb1 = ____________
iii. Current through bulb2: Ibulb2 = ____________
f. Explain using the reading why current through bulb 1 and current through bulb 2 do add up to current through
battery.
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g. Without measuring, what should the current through the wire touching the positive side of the battery be? ________
h. Without measuring, what should the voltage across the wire touching the positive side of the battery be? _________
We are going to be analyzing a new circuit now. Place two light bulbs (resistors) in series with each other. Then connect a
battery in series with these series bulbs.
a. Again, do not include the meters in your circuit setup/diagram.
b. Sketch out the setup and circuit diagram
Setup
Circuit Diagram
c.
d.
e.
Measure the voltage across the following:
i. Voltage across battery: ∆Vbattery = ___________
ii. Voltage across bulb 1: ∆Vbulb1 = ____________
iii. Voltage across bulb 2: ∆Vbulb2 = ____________
Explain using reading why voltage bulb 1 and voltage bulb 2 do add up to voltage battery as one may not expect.
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Measure the current through the following:
i. Current through battery: Ibattery = ___________
ii. Current through bulb1: Ibulb1 = ____________
iii. Current through bulb2: Ibulb2 = ____________
f.
4.
5.
Explain using the reading why current through bulb 1 and current through bulb 2 do not add up to current through
battery as one may expect. _______________________________________________________________________
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g. Explain the differences that you notice in the bulbs brightness from part 2. _________________________________
h. Which has a greater effect on the brightness? Voltage or current? Clearly explain your reasoning. _______________
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i. Make a measurement of the voltage across both resistors. Explain why this is the value that one may expect.
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You made a measurement of the current through the battery in part 3.e.i when there were two light bulbs in series. Remember
how you found this value. You made a measurement of the current through the battery in part 1.i. when there was only one
bulb. Remember how you found this value. Imagine that the whole circuit in part 3.e.i was covered except for the battery and
you were able to measure the current through and the voltage across this battery. The measurements for current would be half
as much as you measured in part 1.i. How does this measurement tell you about the amount of resistance in the circuit? This
is a very important step so explain carefully. _____________________________________________________________
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We will now be working with a new circuit. Place 3 bulbs in series with each other and a battery.
Setup
Circuit Diagram
You have values of voltage (which should be the same for all because it is always measured across the battery) and current of
whole circuit from part 1 for a single bulb and from part 3 for two bulbs. Make measurements for the current through the loop
for 3 bulbs in series now. Then complete the chart below.
Voltage (V)
Resistance
Current (A)
One Bulb
One bulb
worth
Two Bulbs
same
Two bulbs
worth
Three Bulbs
same
Three bulbs
worth
The graph should look similar to a graph that you may know as y = 1/x. This graph displays that:
Current is proportional to 1/Resistance.
If we were to keep resistance constant and double the voltage of the battery, we would notice that all current doubles. This
displays that:
Current is proportional to Voltage
6.
Measurements similar to this led Georg Simon Ohm to the model we now know as Ohm’s Law: I = ∆V/R
Use Ohm’s Law to figure out what the resistance of the miniature light bulbs you are using is. Show your work.
7.
Using basic algebra, please rearrange Ohm’s Law to solve for each variable. Any of these three relationships are valid.
I = ∆V/R
8.
∆V =
R=
We will now be working with a new circuit. Place 2 bulbs in series with each other (label as bulbs 1 and 2). Connect this set
of series bulbs with another bulb (bulb 3) in a parallel configuration. Connect this whole configuration in series with a
battery.
Setup
a.
9.
Circuit Diagram
Complete the chart below by measuring these values or calculating using Ohm’s law when you have two values:
Resistance (from part 6)
Voltage
Current
Bulb 1
Bulb 2
Bulb 3
Bulbs 1&2 combined
Bulbs 1,2&3 combined
b. Try to explain qualitatively why the resistance of 1&2 is greater than the resistance of 1,2&3 __________________
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We will now be working with a new circuit. Place 3 bulbs in a parallel configuration and place that parallel configuration in
series with the battery.
Setup
Circuit Diagram
a.
Explain the steps that one must take to measure the resistance of all three bulbs combined using only the ammeter
and voltmeter. Create bullet points with steps. Complete the steps as you go.
b.
c.
What is the equivalent resistance of all the resistors in this circuit? __________________
Explain how this value makes sense conceptually_____________________________________________________
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Use the concepts from part c to apply your understanding to a more difficult problem:
i. As a group, try to explain the reason that the equivalent resistance of the circuit in part 8 is exactly
2/3 of the resistance of a single bulb. Use the space below to model your thinking.
d.