Magnetic induction using DrDAQ: Faraday’s Laws

Introduction

This experiment investigates magnetic induction, as governed by Faraday’s First Law:

When the magnetic flux threading a circut is changing, an E.M.F. is induced in the circut.

The experiment involves dropping a magnet through a coil of wire and observing the result on a PC-based oscilloscope.

Equipment required

• A DrDAQ data logger running PicoScope oscilloscope software.
• A Pico Magnetic Induction Kit

Figure 1: set up of equipment

Experiment setup

1. Mount the tube and coil vertically in a clamp and stand
2. Connect the coil to the pH input of the DrDAQ using the BNC to 4 mm test lead.

To perform this experiment with DrDAQ we have to use the custom ranges feature in PicoScope. To do this a file can be installed into PicoScope or the settings can be entered manually. Both are explained below.

Figure 2: custom range settings for the pH input

Manually configuring the pH input for this experiment

1. Select File > Settings > Custom ranges
2. Select Add in the custom range list
3. Enter the settings as shown in the image to the right (Figure 2)
4. Use the drop down menus for channel A to select pH and the range ±576 mV
5. The DrDAQ screen should now look identical to the image shown in Figure 3

Installing PicoScope setup files to configure the pH input for this experiment

1. Download the setup file and save it to the PICO directory
2. Start up PicoScope
3. Select File > Open in the PicoScope toolbar
4. In the Open file window change files of type to show Set up file (*.PSS) in the drop down menu
5. Select OK
6. Again, the DrDAQ screen should look identical to the sceeenshot shown in figure 3 below.

Figure 3: screen shot of PicoScope screen after installing set up files

Carrying out the experiment

1. Drop the magnet down the tube (with the north pole facing down) and observe the trace on the screen. Catch the magnet so it does not smash on the floor!
2. Before repeating the experiment, think about how the trace will change if you drop the magnet with the south pole facing down — try it and see if you are correct.
3. If the apparatus allows, experiment by moving the coil up and down the tube so the magnet travels different distances before entering the coil. Either PicoScope rulers or automatic measurements can be used to record the amplitudes.

Questions and discussion of results

Q1. Why are both negative and positive voltages generated as the magnet passes through the coil?
Q2. Why does the width (duration) of the pulse vary as the coil is moved up and down the tube?
Q3. What is the relationship between the speed of the magnet, as it travels through the coil, and the amplitude of the voltage produced from the coil?
Q4. What happens to the speed of the magnet as it passes through the coil?

Further study

Due to the small distance from the release point of the magnet to the top of the coil, the pulse produced by the induced E.M.F is asymmetric. The pulse only becomes truly symmetrical when the distance between the coil and the release point of the magnet is large. Why is this effect seen?

Measure the distance between the magnet's release point and the coil, use this to calculate the velocity. Repeat this for different distances and plot a graph of peak amplitude versus magnet velocity.

Use Lenz’s law to explain how the polarity of the magnet affects the direction of the induced current.