Electromagnetic induction lab report answers

temple university physics

Electromagnetic Induction

In 1831, Michael Faraday experimentally showed that a changing magnetic field induces an electromotive force, or emf. An emf is a type of voltage and is aptly named, being a force that moves electrons – and thus makes current flow. The import of this discovery is apparent when you consider just how useful these emfs are (and how easy they are to generate). They are used in your phone charger (a transformer), in running the stereo in your car (an alternator), in electric motors, and credit card readers.

In one of his experiments, Faraday placed a loop of wire in a magnetic field (or B-field) and when he moved the wire through the field he found that an emf was generated. Interestingly, he found that the opposite arrangement (moving the magnetic field and holding the wire loop fixed) achieved the same result. Furthermore, simply alternating the sign of the B-field generated an emf in the wire loop. All of these results were reconciled by the realization that whenever there is a change in the magnetic flux going through a wire loop, an emf is generated:

(1)

where is the emf, is the magnetic flux, and is the number of wire loops through which the flux is passing. The derivative in Equation 1 arises because the rate of change of the magnetic flux determines the strength of the emf generated (so a constant flux doesn’t generate an emf). The negative sign is introduced to show the direction of the induced current with respect to the magnetic field. This effect, the generation of an emf by changing magnetic flux, is called electromagnetic induction.

Learning Goals for this Laboratory:

· Understand the factors that affect the magnitude and direction of induced voltages.

· Observe effect of adding iron core to solenoid

· Understand how transformers are constructed and their principle of operation

Apparatus
power supply, Pasco solenoid coil set, switch, galvanometer, two digital multimeters, resistance box, iron filings, small compass, round magnet

Part I. emf Generated by Motion of a Permanent Magnet
In this experiment, we will observe the effect of moving a permanent magnet near the core of a passive solenoid. This is a virtual lab that uses the PhET simulation Faraday’s Electromagnetic Lab which can be found here.

1. Our setup is simple: a coil of wire connected to a light bulb. Open the PhET simulation and click on the Pickup Coil tab. You should see a coil of wire (a solenoid) attached to a light bulb and a permanent magnet nearby.

2. Now do the following experiments, record the results in the data section of your report.

a. Move the magnet in and out of the pickup coil. Observe how the speed at which you move the magnet affects the brightness of the lightbulb.

b. Switch the Pickup Coil indicator from the lightbulb to the meter (this option is on the right-hand side of the simulation). The meter indicates the magnitude and direction of the emf voltage induced on the coil. Observe how the sign of the emf is affected by the orientation of the magnet. You can easily flip the orientation of the magnet by clicking the Flip Polarity button on the right.

c. Change the number of loops in the pickup coil and observe how this affects the magnitude of the voltage induced on the coil.

Question 1. What happens to the sign of the voltage when you reverse the orientation of the permanent magnet?

Question 2. How does changing the number of loops change the amount of current induced in the coil?

Question 3. Why don’t you get a deflection when you leave the permanent magnet stationary inside the solenoid?

Part I conclusion: simply waving a magnet around near a wire makes current flow! No power supply required!

Part II. The Transformer
Transformers are very useful devices for changing voltage from one value to another, such as the step-down transformers that take the high voltage from power lines and convert it to lower voltages for use in the home. (They are those big gray cylinders, also known as ‘pole pigs,’ that you see on the utility poles.) Transformers are ubiquitous, but they all follow the same basic induction principle that we saw in Part I of this lab: a magnetic flux induces an emf and if the emf is part of a complete circuit, a current will flow as a result. The setup for Part II is a little different; instead of generating the magnetic flux using a permanent magnet as we did above, we will use an electromagnet. The electromagnet is simply a solenoid coil with current passing through it and is called the primary. A second coil (cleverly termed the secondary) will be placed nearby, and because magnetic flux is passing through it an emf will be generated just like we saw in Part I.

1. Switch to the Transformer tab of the PhET simulation. You should see the primary coil attached to a battery. Next to the primary coil is a secondary coil with its ends connected to the ends of a resistor to make a complete circuit. Change the indicator from the light bulb to the voltage meter.

2. Carry out the following experiments, record your observations as your data.

3. Place the primary coil near the secondary coil. Notice that current is flowing in the primary, so it is acting as an electromagnet.

Question 4. Is there voltage induced in the secondary when the primary is sitting stationary? Is this consistent with your earlier observations with the stationary permanent magnet?

4. Change the current source of the electromagnet to AC so that the magnetic field of the primary is continuously changing. Is there current in the secondary now? If you don’t see current in the secondary, try moving the primary closer or increasing the number of loops in the secondary (pickup) coil.

5. Move the primary farther away and observe the voltage on the secondary.

6. Observe how increasing the loop area allows you to move the coils farther apart and still obtain a voltage on the secondary.

Question 5. What do you think is going on to cause the change in voltage in the secondary when you move the coils apart? Why is this affect reversed by increasing the loop area? (Hint: look up magnetic flux in your text and identify its role in Equation 1).

Question 6. You may have heard that you should unplug your phone charger when not in use to save electricity. Why is this? (What is a phone charger?)

Question 7. Transformers are simply a pair of coils like those we see in this experiment. They’re used to transform a source voltage to a different more desirable voltage. Real transformers usually have an iron core inside them that focuses the magnetic field in order to increase the magnetic flux passing through the secondary. What advantages would be provided by the iron core and the increased flux?

Summarize the results of your experiments in your report making sure to note how the magnitude, sign, and rate of change of the magnetic flux, and the number of turns in the secondary affect the induced emf. The grading rubric below on the next page is modified from the standard rubric and will be used for this take-home lab report.

Sections

A (90-100%)

B (80-90%)

C (70-80%)

D/F (0-70%)

1. Overall Presentation & Organization

10%

Clear and concise organization, formatting, and language.

Minor organizational problems. Text is unedited.

Several organizational issues. Text is in rough draft form.

Major problems with organization.

2. Introduction

10%

1-3 sentences summarizing the main goals and how they are obtained by experimentation.

Not concise. Goals unclear.

Poor understanding of the goals. Generic statements.

Cursory or missing introduction. Copied from Manual

3. Data

Calculations

Fitting

30%

Clear, concise equations and calculations. Care taken with units. Averages and standard deviation from the mean reported as necessary.

All essential data displayed in graphical or tabular format. No excessive data given. All axes labeled with units included.

Graphs fit with appropriate functions (lines or curves) with fitting parameters reported.

Calculations mostly correct.

Excessive detail or repetition of similar calculations. Excessive raw data included.

One of items at left poorly labeled or missing.

Major calculations missing or clearly needs improved organization.

Some essential data missing or plotted incorrectly.

More than one fit or equation missing.

No calculations given. Data reported without showing work.

Major issues with data presentation.

No fitting done.

4. Questions

30%

Correct and complete answers to all questions in lab manual.

Mostly correct and complete answers to questions.

Incomplete answers or several wrong answers.

Mostly incorrect or missing answers.

5. Results and Discussion

20%

Summary of conclusions drawn with references to values obtained.

Where appropriate, discussion of:

1) actual vs expected results

2) fitting results

3) reproducibility

4) effects from sources of error/precautions

Conclusion not comprehensive. Missing one of the items at left.

Only cursory summarization.

Conclusion inappropriate or missing.

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