Magnetic Induction. A science experiment for data loggers and oscilloscopes. Part of the collection of educational experiments from Pico Technology which includes experiments for the DrDAQ data logger with built in sensors for light, temperature, sound (dB and waveform) and pH.

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 Pico Magnetic Induction Kit
A PC based oscilloscope with PicoScope software running. We used the ADC-212 but any of our PC based scopes would have been suitable.

Experiment Setup

The tube-coil arrangement is mounted vertically. We used a clamp and stand.

The coil is connected to channel A of the oscilloscope.

PicoScope was set channel A to the 500mV range (the correct range will depend on the output from your coil).

The timebase was set to 5ms/div

The trigger was set to 'repeat' so that each time the magnet was dropped down the tube, the captured waveform was 'frozen' on the display

The trigger level was set to +50mV (rising) and the trigger delay was set to -50% (pre trigger) so that the waveform would be in the middle of the screen

Figure 1 :- Setup of Equipment

Carrying out the Experiment

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!
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.
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 magnets 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.

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