Resistance and Resistivity Part 2
Resistance is useful...
CONTENTS
1
Objectives ............................................................................................................................................. 1
1.1
Prerequisite skills and Knowledge ................................................................................................ 1
1.2
Research Skills .............................................................................................................................. 1
1.3
Learning Objectives ...................................................................................................................... 2
2
Pre-Experiment ..................................................................................................................................... 2
2.1.1
3
Prepare for the Experiment ................................................................................................... 2
Laboratory Manual................................................................................................................................ 3
3.1
Pre-Experiment Discussion ........................................................................................................... 3
3.1.1
Resistance and Conductance ................................................................................................. 3
3.2
Materials Check Off List............................................................................................................... 4
3.3
Safety and Waste Disposal Protocols............................................................................................ 4
3.4
Experimental Procedure ................................................................................................................ 4
3.4.1
Predict the Effect of Resistor Shape on Resistance .............................................................. 4
3.4.2
Plan Your Time ..................................................................................................................... 6
3.4.3
Perform Experiment .............................................................................................................. 6
3.5
Post-Lab Assignment .................................................................................................................... 6
3.5.1
Poster Rubric ......................................................................................................................... 7
1
1.1
OBJECTIVES
PREREQUISITE SKILLS AND KNOWLEDGE
Students should have completed Part 1 of Resistance and Resistivity.
1.2
RESEARCH SKILLS
After this lab, students will have had practice in:
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Using a Vernier scale caliper
Measuring current and voltage
Reading a circuit diagram
Propagating error
Testing a prediction
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1.3
LEARNING OBJECTIVES
After this lab, students will be able to:
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Describe the effect of shape on the flow of current through a resistor
Describe the relationship between voltage, current and resistance in circuits that obey Ohm’s Law
Translate between a simple circuit diagram and a circuit
Differentiate between resistance and resistivity
Relate a current to the motion of charged particles
Design an experiment to explore an element of Play-Doh resistance
2
PRE-EXPERIMENT
How does the shape of a resistor affect the flow of electric current through it? Will electric current flow
more quickly through a resistor shaped like a rectangular block or one shaped like a donut? In this
experiment you will explore the effect that shape has on the flow of current through a Play-Doh object.
In the previous experiment, you measured the dependence of resistance on the length and cross-sectional
area of a cylindrical resistor. In this experiment, you will have the opportunity to test a variety of noncylindrical shapes and to experiment a bit further into the motion of charged particles through a
conducting medium.
How does the resistance of a cylinder-shaped resistor depend on length and cross-sectional area?
2.1.1
Prepare for the Experiment
Review the results you collected in the previous experiment, Resistance and Resistivity 1, and make sure
you can calculate the resistance of a cylinder with a given resistivity and propagate error as described in
that lab manual.
Look at the lab manual for tomorrows experiment. Propose a plan for the experiment you would like to
perform and come up with a schedule; record the plan and the schedule in your lab notebook.
Prepare an Excel worksheet to facilitate the data analysis for the experiment.
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3
3.1
LABORATORY MANUAL
PRE-EXPERIMENT DISCUSSION
Q1. Consider a cylinder with a fat neck, a cylinder with a narrow neck, a donut, or a rectangular slab.
What features of the resistor shape will influence the resistance?
Q2. Which features will have the greatest effect? Explain your reasoning.
Q3. Consider a donut shaped resistor. What effect should this shape have on the resistance?
3.1.1
Resistance and Conductance
In these experiments Play-Doh is treated as a resistor, yet even a resistor must allow some current to flow
to allow the resistance to be measured. Play-Doh is a commercial version of the traditional homemade
play dough (flour, salt and water).
Q4. What constituents of Play-Doh carry the charge so that current can flow through it?
Q5. Conductance is the inverse of resistance. Why do you think there is a different name for this
property? Under what conditions do you think it would be more appropriate to use conductance
rather than resistance?
Q6. You may have noticed that the Play-Doh became warmer as the current flowed through it. What is
happening at the molecular level to cause this increase in temperature?
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Q7. Describe at a molecular level what is happening that allows current to flow through the Play-Doh
and how the current is resisted.
Q8. What would happen to Play-Doh’s conductive/resistive behavior if it were allowed to dry out?
3.2
MATERIALS CHECK OFF LIST
Each small group of (2-3) students will have:
Laptop computer with Excel and LabVIEW
Vernier SensorDAQ
Vernier Voltage Probe
Vernier Current Probe
5 oz container of Play-Doh
4 (four) 2 inch nails
4 (four) 1.5V batteries
1 four-battery holder
2 Alligator clip wires with pigtails
2 wires with banana plugs
4 Alligator clip adapters for banana plugs
flat piece of wood or hard plastic
Ruler
Vernier caliper
Each class will have:
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3.3
Steel wool or sandpaper for cleaning nails
SAFETY AND WASTE DISPOSAL PROTOCOLS
Make sure your lab-bench is dry; there should be no water anywhere on or near your workspace and the
equipment on it. Do not touch bare metal to your skin, and do not mishandle the probes. You will be
working with power sources which, if mishandled, might electrocute someone.
Dispose of scorched or irretrievably dry Play-Doh in the regular garbage containers. All other materials,
including squishy Play-Doh, should be saved to be used again.
3.4
EXPERIMENTAL PROCEDURE
In this experiment you will test your predictions of the resistance of unusually shaped resistors.
3.4.1
Predict the Effect of Resistor Shape on Resistance
So far the only shape you have tested is a cylinder. Choose one of the shapes below to investigate.
Narrow neck cylinder:
Fat neck cylinder:
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Rectangular slab:
Donut:
Your instructors may have plans for distributing shapes, so check with them before you start. Use all of
your Play-Doh to model the shape. You may want to compare ideas and results with a neighboring group.
Q9. What shape did you choose?
Q10. Describe your model of the flow of the current within the shape. How does it differ from the flow
through a cylinder of Play-Doh? Explain your reasoning.
Q11. What dimensional measurements (length, width, diameter, etc.) do you need to make to predict the
resistance of this resistor? Explain your reasoning.
Set up an Excel workbook to correspond to the measurements you chose to collect. Add the appropriate
labels to each of your columns. Collect the measurements.
Q12. Predict the resistance of this shape, based on the measurements you made in question 7. Support
you prediction with an equation that shows the calculation you made.
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3.4.2
Plan Your Time
Decide how long to spend on each part of the experiment. The times don’t need to be exact, but try to
keep to your schedule as much as possible.
Q13. Write your planned schedule here:
3.4.3
Perform Experiment
Now attach the nails and leads as you did on the previous day and measure the resistance of the shape.
Record your results in your data table.
Compare your results with your predictions.
Q14. Did your predictions bear out? Explain your answer.
Q15. What conclusions can you draw from your results?
Q16. Do your results suggest a second experiment? If so, what further data would you like to collect?
Q17. Compare your results with another group investigating the same shape.
If there is time, try a second experiment of your own devising, either to continue the same investigation,
or to start a new one. Answer the same questions (numbers 9-17) for your new experiment.
3.5
POST-LAB ASSIGNMENT
Prepare a laptop-sized poster (1 PowerPoint slide) to present your experimental results.
Include:
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An image of the shape you tested (or other experimental description)
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Your hypothesized current flow (diagram or description)
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Your predicted resistance based on initial measurements
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Results: was your prediction correct?
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Follow-up: What did you do then? Results?
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Surprises?
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Conclusion
Q18. Paste a snip of your slide here.
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3.5.1
Poster Rubric
The poster discussion will be graded using the following metrics.
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The question, methods and results are concisely described in a way that could be understood by a
typical X-Lab 1 student. (0-7 pt)
The images and figures are clear and uncluttered, and support the presentation of data and other
results. (0-7 pt)
Poster clearly explains the value of the investigation (0-7 pt)
Poster and presentation engage interest, curiosity, and imagination (0-7 pt)
Attendees act and speak professionally (0-7 pt)
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