Ohm Law and Resistor Circuits Lab Report

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1 Experiment 11: Ohm’s Law Names: __________ Date: __________ Experiment 11: Ohm’s Law and Resistor Circuits – online version OVERVIEW Be sure to read this section before coming to lab. In this lab you will investigate the relationship between current and voltage for a Resistor. You will also look at Currents and Voltages for resistors connected in Series and Parallel. Carbon resistors Instead of using lightbulbs in this lab, you will use carbon resistors. A typical carbon resistor contains compressed powdered carbon in the form of graphite, suspended in a hard glue binder. It is usually encased in a plastic shell with a color code painted on it. The graphite material provides a relatively large resistance to the flow of electric current. A B Cutaway view of a carbon resistor showing (A) the color stripe, and (B) a cross-sectional view of the graphite material. PROCEDURE Part 1: Ohm’s Law 1. Set up a circuit to investigate the relationship between voltage, current, and resistance. Use the simulation at https://phet.colorado.edu/en/simulation/circuit-construction-kit-dc. Click on “Lab”. In the box that says “Show Current”, select the “Conventional” view. Build a basic circuit with a battery, resistor, and wires (see the diagram on the next page). To connect two different circuit elements, drag one object until its dotted circle overlaps the dotted circle on a different object. To disconnect two circuit elements, click on the connection and then click on the scissors that appear. To delete a circuit element, click on the object and then click on the trashcan that appears. Record the colors of the different stripes on the resistor. The fourth stripe is gold (which indicates the resistor quality and is unrelated to its resistance value). 1st band:_______, 2nd band:_______, 3rd band:_______, 4th band: _gold___ As you build your circuit, note that the default battery value is 9 V and the default resistor value is 10 Ω. After your circuit is constructed, double-click on the battery and lower its value to 5 V. The circled “V ” stands for the Voltmeter. The black lead on the voltmeter is labeled with a “–“ in the diagram. The red lead on the voltmeter is labeled with a “+” sign in the diagram. The circled “A” stands for the ammeter. Use the ammeter that is the farthest on the right. ©J. Cole, MiraCosta College, Oceanside, CA. Updated by E. Peters 2016. Reformatted by D. Brownell 2022. 2 Experiment 11: Ohm’s Law Notice that the voltmeter is connected across the resistor (one lead connected to each end of the resistor), while the ammeter is inserted into the circuit (“in line” with the resistor), so that the current flows through it. The ammeter must always be connected into the circuit so that current flows through it this way. Never connect the ammeter across a resistor or battery as if it were a voltmeter; this will cause the ammeter to blow a fuse or irreparably damage it internally. (+) A (–) (+) +V0 V (–) (GND) The “+V0” and “GND” connectors are just the (+) and (–) terminals of the battery. (–) Once everything is set up, observe the measured values of voltage across the resistor and the current flowing through the resistor. Then reverse the leads for the voltmeter, putting the black lead at the top next to the ammeter, and the red lead at the bottom. Does the voltmeter value change (magnitude and/or sign)? Response: __________ Now put the leads back. Record your results in the table below, with appropriate units. Double click on the battery and increase the voltage of the battery by 1 V. Record the new reading on the voltmeter and the ammeter. Repeat, increasing the setting by about 1 V each time, until you have filled out the table. Voltage (V) Current (A) Question: As the voltage across the resistor increases, does the current increase, decrease, or stay the same? Response: __________ Use Excel or Google sheets to make a plot of current I (^y-axis) vs. voltage ΔV (^x-axis). Question: Does the relationship between current and voltage appear to be linear? Response: __________ What are the units of the slope of the graph? ©J. Cole, MiraCosta College, Oceanside, CA. Updated by E. Peters 2016. Reformatted by D. Brownell 2022. 3 Experiment 11: Ohm’s Law Response: __________ Add a “Linear Trendline” to your plot and record its slope (including units): slope m = __________ Include a screenshot of your graph with this lab writeup. Screenshot: 2. Measure the current and voltage across the battery a different resistor. Disconnect the resistor from the circuit and set it aside. Bring in another resistor from the left. Double click on the resistor. Use the slider to change the value of the resistor to some new value. Record the resistance value and the colors of the bands. The fourth band is gold as before. Resistance: __________ Ω 1st band:_______, 2nd band:_______, 3rd band:_______, 4th band: _gold___ Place this new resistor in your circuit where the other resistor used to be. Take five readings of current and voltage as before. Record your measurements below. Voltage (V) Current (A) Use Excel or Google Sheets to make a plot of current I (^y-axis) vs. voltage V (^x-axis) as before. Add a linear trendline (“best fit”) to the plot, and record its slope (including units): slope m = __________ Attach a screenshot of your graph to this lab writeup. Screenshot: ©J. Cole, MiraCosta College, Oceanside, CA. Updated by E. Peters 2016. Reformatted by D. Brownell 2022. 4 Experiment 11: Ohm’s Law Reading resistor color codes The color codes on the side of the carbon resistors tell what the manufacturer claims that the resistance of their resistor is: Fourth Band (Tolerance) Color First band Second band Third band Black 0 0 ×100 = ×1 Brown 1 1 ×101 = ×10 ± 1% Red 2 2 ×102 = ×100 ± 2% Orange 3 3 ×103 = ×1000 Yellow/Gold 4 4 ×104 Green 5 5 ×105 Blue 6 6 ×106 Violet 7 7 ×107 Gray 8 8 ×108 White/Silver 9 9 ×109 ± 5% ± 10% The first two bands are the first two digits of the resistor’s value, and the third band tells you how many zeroes to add at the end. The fourth band tells you how accurate the resistance is. For example, if a resistor’s bands are Red, Green, Orange, and Gold, it would have a resistance of 25000Ω ± 1250Ω. The gold band indicates that the resistance is good to ±5%, so the resistor’s actual value out of the package could be anywhere from 22,750Ω to 25,250Ω. Note that if the 4th band is missing, then the tolerance is the least accurate, good to only ±20%. More Examples: _ _ _ _ = 220Ω ± 11Ω; _ _ _ _ = 4700Ω ± 235Ω, etc. 3. Check the resistance of your resistors. Take the two resistors that you used in the previous section. Read their resistances using the color code, and record the resistances in the table below in the column on the left. Once you’ve recorded their claimed values based on the manufacturer’s color code bands, double click on each resistor to see its actual value and record in the table below. ©J. Cole, MiraCosta College, Oceanside, CA. Updated by E. Peters 2016. Reformatted by D. Brownell 2022. 5 Experiment 11: Ohm’s Law From color code Actual value Question: Look at the slopes of the I vs. ∆V best-fit lines which you plotted before. Can you find an algebraic relationship between the slopes m and the resistances R of your resistors? Describe the relationship here. Response: __________ Question: Suppose you have measured the resistance of a resistor and the current passing through it. Based on your measurements with the two resistors, write down an equation that you can use to predict the voltage drop across a resistor, given the current through the resistor and the resistance of the resistor. Response: __________ Equation: ΔV(I , R ) = __________ Note: the relationship between voltage and current in a circuit is known as Ohm’s law (named after Georg Ohm, a German physicist from the early 1800’s). Part 2: Resistors in series or parallel 4. Find currents and voltages for two identical resistors in series. Select two 10 Ω resistors. Prediction: If you were to set up the circuit shown below (do not set it up yet!), with the power supply +V0 set at 5.0 V, give numerical predictions for the following values (use the equation you found above to help you reason out the answers). Draw dashed arrows on the diagram next to the battery and each resistor showing the direction you expect for the current. R1 +V0 R2 GND Predicted voltage drop ΔV1 across resistor one (R1) = _______________ V Predicted voltage drop ΔV2 across resistor two (R2) = _______________ V Predicted current I1 through resistor one (R1) = _______________ A ©J. Cole, MiraCosta College, Oceanside, CA. Updated by E. Peters 2016. Reformatted by D. Brownell 2022. 6 Experiment 11: Ohm’s Law Predicted current I2 through resistor two (R2) = _______________ A Predicted current I0 through the battery (+V0) = _______________ A Question: Explain your reasoning. Response: __________ Now check your predictions. Set up the circuit in the simulation, and measure the voltage drops across the two resistors: Measured voltage drop ΔV1 across resistor one (R1) = _______________ V Measured voltage drop ΔV2 across resistor two (R2) = _______________ V Discuss in your group (if appropriate) on where you should put the ammeter to read the current values you need. Caution: Remember that the voltmeter is connected across the resistors (one probe clipped to each end), but the ammeter must be placed into the circuit in series with the resistor (as part of the circuit either before or after the resistor its measuring). If you connect the ammeter incorrectly, you could blow a fuse in the meter! If you are not sure about the connections in your circuit, consult the instructor. Before you connect the circuit with the ammeter, use the small circuit diagrams below to draw in the location(s) where you will place the ammeter for each measurement. Label the diagrams to indicate which reading goes with which diagram (you may not need to use all of the diagrams). R1 +V0 GND R1 R2 +V0 GND R1 R2 +V0 R2 GND Set up the circuit(s) you have sketched, with the +V0 lever set to 5 V. Record the currents as positive numbers, and draw solid arrows next to the battery and each resistor showing the direction you found for the current. Measured current I1 through resistor one (R1) = _______________ A Measured current I2 through resistor two (R2) = _______________ A Measured current I0 through the battery (+V0) = _______________ A ©J. Cole, MiraCosta College, Oceanside, CA. Updated by E. Peters 2016. Reformatted by D. Brownell 2022. 7 Experiment 11: Ohm’s Law Question: Did your readings match your predictions? If not, explain any differences. Response: __________ Question: What happens to a battery’s current when it flows into a set of resistors in series, and how are the currents related? What happens to a battery’s voltage when applied across resistors in series, and how are the voltages related? Response: __________ 5. Find currents and voltages for two identical resistors in parallel. Use two 10.0 Ω resistors. Prediction: If you set up the circuit shown below (do not set it up yet), with the +V set for 5.0 V, give numerical predictions for the following values. Draw arrows next to the battery and each resistor showing the direction you expect for the current. +V0 GND R1 R2 Predicted voltage drop ΔV1 across resistor one (R1) = _______________ V Predicted voltage drop ΔV2 across resistor two (R2) = _______________ V Predicted current I1 through resistor one (R1) = _______________ A Predicted current I2 through resistor two (R2) = _______________ A Predicted current I0 through the battery (+V0) = _______________ A Question: Explain your reasoning. Response: __________ Now check your predictions. Set up the circuit in the simulation, and measure the voltage drops across the two resistors: ©J. Cole, MiraCosta College, Oceanside, CA. Updated by E. Peters 2016. Reformatted by D. Brownell 2022. 8 Experiment 11: Ohm’s Law Measured voltage drop ΔV1 across resistor one (R1) = _______________ V Measured voltage drop ΔV2 across resistor two (R2) = _______________ V Decide where to put the ammeter to measure the current through the first resistor (R1). (Be sure to think carefully about how you would set up the ammeter to read the current through one resistor only. Consult the instructor if you have any questions.) Indicate the ammeter’s location on the first small diagram below. Set up the circuit in the simulator, and record the current in the blank below. Repeat to find the current through the second resistor (R2), and through the battery (+V0). In each case, indicate where you placed the ammeter on one of the diagrams below. +V0 GND R1 R2 +V0 GND R1 R2 +V0 GND R1 R2 Measured current I1 through resistor one (R1) = _______________ A Measured current I2 through resistor two (R2) = _______________ A Measured current I0 through the battery (+V0) = _______________ A Question: Did your readings match your predictions? If not, explain any differences. Response: __________ Question: What happens to a battery’s current when it flows into a set of resistors in parallel, and how are the currents related? What happens to a battery’s voltage when applied across resistors in parallel, and how are the voltages related? Response: __________ Part 3: Resistors in series and parallel Now I want you to explore. Try the following circuit ideas, and then come up with your own to try. • Connect the battery and ammeter directly without any resistors. What happens to the battery? • Connect two resistors in parallel, then a third in series with them. Explore the currents as a function of the resistor values. Are you able to predict the total current from the battery based on the rules you’ve already discovered, as a function of the three resistances and the voltage of the battery? ©J. Cole, MiraCosta College, Oceanside, CA. Updated by E. Peters 2016. Reformatted by D. Brownell 2022.
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Experiment 11: Ohm’s Law

1

Names: __________

Date: __________

Experiment 11: Ohm’s Law and Resistor Circuits – online version
OVERVIEW
Be sure to read this section before coming to lab.
In this lab you will investigate the relationship between current and voltage for a Resistor. You will also look
at Currents and Voltages for resistors connected in Series and Parallel.
Carbon resistors
Instead of using lightbulbs in this lab, you will use carbon resistors. A typical carbon resistor contains compressed powdered carbon in the form of graphite, suspended in a hard glue binder. It is usually encased in a
plastic shell with a color code painted on it. The graphite material provides a relatively large resistance to the
flow of electric current.

A

B

Cutaway view of a carbon resistor
showing (A) the color stripe, and
(B) a cross-sectional view of the
graphite material.

PROCEDURE

Part 1: Ohm’s Law
1. Set up a circuit to investigate the relationship between voltage, current, and resistance.
Use the simulation at https://phet.colorado.edu/en/simulation/circuit-construction-kit-dc. Click on “Lab”. In
the box that says “Show Current”, select the “Conventional” view. Build a basic circuit with a battery, resistor, and wires (see the diagram on the next page). To connect two different circuit elements, drag one object
until its dotted circle overlaps the dotted circle on a different object. To disconnect two circuit elements, click
on the connection and then click on the scissors that appear. To delete a circuit element, click on the object
and then click on the trashcan that appears. Record the colors of the different stripes on the resistor. The fourth
stripe is gold (which indicates the resistor quality and is unrelated to its resistance value).

1st band:__Brown_____, 2nd band:___Black____, 3rd band:___Black____, 4th band:
_gold___
As you build your circuit, note that the default battery value is 9 V and the default resistor value is 10 Ω. After
your circuit is constructed, double-click on the battery and lower its value to 5 V.
The circled “V ” stands for the Voltmeter. The black lead on the voltmeter is labeled with a “–“ in the diagram.
The red lead on the voltmeter is labeled with a “+” sign in the diagram. The circled “A” stands for the ammeter.
Use the ammeter that is the farthest on the right.

©J. Cole, MiraCosta College, Oceanside, CA. Updated by E. Peters 2016. Reformatted by D. Brownell 2022.

Experiment 11: Ohm’s Law

2

Notice that the voltmeter is connected across the resistor (one lead
connected to each end of the resistor), while the ammeter is inserted into the circuit (“in line” with the resistor), so that the current flows through it. The ammeter must always be connected into
the circuit so that current flows through it this way. Never connect
the ammeter across a resistor or battery as if it were a voltmeter;
this will cause the ammeter to blow a fuse or irreparably damage
it internally.

(+)

A

(–)

(+)

+V0
V

(–) (GND)

The “+V0” and “GND” connectors are just the (+) and (–) terminals of the battery.

(–)

Once everything is set up, observe the measured values of voltage across the resistor and the current flowing
through the resistor. Then reverse the leads for the voltmeter, putting the black lead at the top next to the
ammeter, and the red lead at the bottom. Does the voltmeter value change (magnitude and/or sign)?

Response: ____Yes,...


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