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SOTM LAB: P18R
I.
TEACHER NOTES & GUIDELINES
TITLE OF LAB: Induced EMF
DEVELOPERS OF LAB
Mike Spear JD743, Phil Ousley JD733,
Nancy Moreau JD730
OVERVIEW OF LAB
DESCRIPTION
In this lab students investigate the induction of an EMF using a simple coil
(solenoid) and bar magnet. Pasco's voltage sensor and interface allow students to try a
number of variations as they discover the relationships between induced EMF and the
strength of the magnetic field, the speed the magnets move and number of turns of the
coil. (Use PASCO Experiment P41)
CURRICULUM CONSIDERATIONS
This lab should be done after students are familiar with concepts of voltage and
magnetic fields. The lab may be followed by a discussion of induced currents and
electrical power generation.
SAFETY CONSIDERATIONS
Care should be taken when magnets are dropped. Foam pads are to be used
under the apparatus.
BACKGROUND INFORMATION
A. SCIENTIFIC VIEWPOINT
Faraday's Law of Electromagnetic Induction states the ideas that students should
come to understand by carrying out this activity. The induced EMF is directly related to
the number of turns and the change in magnetic flux density. The EMF is inversely
related to the time it takes for the flux density to change. In this lab the velocity with
which the magnet falls through the coil is related to the time it takes for the flux
density to change. The greater the velocity the smaller the time. The negative sign in
Faraday's Law (indicating that the induced EMF resists the flux change) may be ignored
in this activity. Do not expect students to come to Faraday's law as a conclusion.
B. COMMON MISCONCEPTIONS
Students believe that induction has something to do with the National Honor
Society. Students think that electrons are attracted or repelled by magnets. Some think that the
mere presence of a magnetic field will induce an EMF. Most think that EMF is a force (but the
units tell us otherwise!).
OBJECTIVES
Students will observe the EMF induced in a solenoid as they vary the strength of the
magnetic field, the speed of movement of the magnets and the number of turns in the
solenoid.
The pupils will design an original investigation that will allow them to describe the
relationships between strength of magnetic field, speed at which the magnets move, and
the number of turns in the solenoid. Learners will then organize their data and
conclusions for presentation to fellow students.
EQUIPMENT/MATERIALS
PROVIDED BY SOTM
Laptops, PASCO interfaces and cables, solenoids, and foam pads (optional
to determine speed: photogates), bar magnets.
PROVIDED LOCALLY
Ring stands, single pulleys, string and tape.
ADVANCE PREPARATION
Clean lab tables that have electrical outlets provided for each group .
II. PRE-LAB
PRE-LAB EXERCISE TO ELICIT STUDENTS' PRIOR KNOWLEDGE AND
MISCONCEPTIONS
Give students the following prelab questionaire:
1. Is magnetism electricity? What is electricity?
2. How is electricity generated?
3. How can electricity be measured?
4. How does an electric guitar pickup work?
5. Why did Jimi Hendrix wind his own “pickup coils” on his Fender Stratocaster?
6. What is meant by electrical induction?
DISCUSSION OF PRECONCEPTIONS
Summarize student responses on blackboard.
1. Could someone explain the difference between electricity and magnetism?
2. What might be the relationship between electricity and magnetism?
3. How do electric guitars work?
4. In what unit would emf be measured?
5. Discuss how electricity can be used to produce magnetism and electricity can be
used to produce magnetism.
III. EXPLORATION OF SCIENTIFIC PRINCIPLE
& INTRODUCTION OF EXPERIMENTAL
PROTOCOL
PROBLEM
How does changing the strength of the magnetic field, the speed at which
the magnets move, and number of turns of the solenoid affect the emf induced?
EXPERIMENT AND TECHNICAL OPERATION OF EQUIPMENT
Demonstrate how an emf is produced when a magnet is dropped through the coil.
Show how a graph may be produced using the Pasco interface, cable,
voltage sensor and
laptop. Also show with a voltmeter or galvanometer that an emf (and current) may be produced.
IV. ELABORATION OF SCIENTIFIC PRINCIPLE:
INQUIRY-BASED STUDENT INVESTIGATION
PROBLEM
How is the rate of change of magnetic flux affected by changes in B, v, and N (number
of turns). How is the emf induced affected by the rate of change of magnetic flux?
HYPOTHESIS OR PREDICTION
Discuss with your group members how you think changes of v, B and N affect the rate
of change of flux and emf induced. Record your hypotheses in three parts (v, B and N).
EXPERIMENTAL DESIGN
1. What are the variables that you will be measuring?
2. What measuring techniques will you use
3. How will you control the rate of fall of the magnets in order to get distinct variations
in the speed (v) of fall?
4. What tables and graphs will you need to create?
5. List safety precautions that must be observed.
6. Check with your instructor before actually doing your experiment.
Checkpoint (Teacher checks students' experimental design for feasibility)
PLAN FOR DATA COLLECTION & ANALYSIS
1. What data are you going to collect?
2. How are you going to organize your data tables, graphs, etc?
3. Check with your instructor before actually doing your experiment.
Checkpoint (Teacher checks students' plan for feasibility.)
CONDUCTING THE EXPERIMENT
Carry out your investigation.
Checkpoint (Teacher monitors students' investigations in progress.
ANALYSIS OF DATA
Discuss your results as a group and redo any data collection. Analyze your results
Checkpoint (Teacher checks students' analysis.)
DISCUSSION OF RESULTS
COMPARE
Compare your results with other groups, identify any discrepancies and explain
what you think is the significance of the differences between groups. Identify the relationship
between each variable and the induced EMF.
PERSUADE
Present a summary of your initial hypotheses and your final conclusions
explaining any change in your thinking due to the results of your experiments.
RELATE
Can you explain how an electric guitar produces sound? What would be the
importance of this phenomenon to people who live near high voltage transmission lines?
V.
EVALUATION
POST-LAB SURVEY OF STUDENTS' CONCEPTIONS
Have students retake the Pre-Lab Exercise.
Compare pre-lab and post-lab responses.
TRADITIONAL
Provide at least three problems involving Faraday’s law of magnetic induction:
EMF induced = - N delta B/delta t
ALTERNATIVE
This material is based upon work supported by the National Science Foundation under Grant No. ESI 9618936. Any opinions,
findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the
views of the National Science Foundation.