PH 1120 Lab 7: Magnetic Induction

Objectives

Background

The Earth's magnetic field has a direction, as do all magnetic fields. Measuring the field while orienting the measuring probe in different directions will change how much of the field is measured along that direction.

The probe does not generally register the largest field strength in a horizontal orientation. Because the Earth itself creates a magnetic field similar to the field that would be produced by a large bar magnet buried way down at the center of the Earth, the field lines this far north of the equator have substantial vertical components (whereas field lines are pretty much parallel to the Earth's surface at the equator). The building can also distort field lines due to the large amount of structural material made of iron, causing the directions of the field lines to be noticeably different from textbook specification.

Moving a magnet in and out of a coil of wire will produce an EMF (or voltage differential) in the coil, which in turn creates a current. This is known as electromagnetic induction, described by Faraday's low of induction and Lenz's law.

Rapid, repeated induction is one way to generate an alternating voltage, with which an alternating current can be generated. An even more convenient way of doing this (used by your local electric power company) is to rotate a coil in a constant magnetic field, making the angular orientation between a coil and magnetic field vary in a regular, periodic manner. This is how an electric generator works.

Changing how quickly the magnetic field is introduced to a coil will change the magnitude of the induced voltage, but the time integral of the voltage is equal for a given change in magnetic field.

Note that the Vernier probe gives measurements in tesla, the SI unit. Another common unit of magnetic field strength is the gauss; one gauss is equal to 0.1 millitesla.

Procedure

The following steps will show the directionality of the Earth's magnetic field and yield a numerical value for the strength of the field.

  1. Connect the Hall probe and ensure that it is set to the 0.3 mT sensitivity setting.
  2. Collect data (first Logger Pro Template) while orienting the probe in all possible directions. Note the directions for which the field reading is a maximum and minimum.
  3. Orient the probe at a right angle to the directions of maximum and minimum field measurement and zero the reading.
  4. Collect data and sweep the probe from the perpendicular direction to the maximum direction until you have a clear maximum reading. Then sweep the probe 180° until you have a clear minimum reading. The difference between min and max values is two times the value of the Earth's field strength.

Next, we will explore how a changing magnetic field induces a voltage in a wire coil.

  1. Connect the voltmeter and hook it up to a wire coil.
  2. Collect data (second Logger Pro template) while inserting a magnet rapidly halfway into the coil, then removing it rapidly.
  3. Move the magnet in and out of the coil at different speeds, noting how the induced voltage changes.
  4. Repeat the last two steps with the magnet flipped, noting how reversing the polarity changes the induced voltage.
  5. Collect data while moving one half of the magnet in and out of the coil repeatedly. Note how changing the speed of the motion affects the induced voltage.
  6. Move the whole magnet in and out of the coil and note how the induced voltage behaves.

Finally, we wil explore what changes or stays the same when the speed or polarity of the magnet is changed.

  1. Collect data (third Logger Pro template) while moving the magnet rapidly in and out of the coil, making a positive peak and a negative peak.
  2. Measure the area under the positive peak using the Area button and compare it to the area under the negative peak.
  3. Move the magnet rapidly into the coil, but remove it more slowly. Measure and compare the areas under the peaks.
  4. Drop the magnet fully through the coil, ensuring that it does not bounce back into the coil. Again measure and compare the area under the peaks. Also note the time duration of the two peaks.

Data Checklist

Logger Pro Files and Lab Report

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