Datalogging in Science

Electromagnetic Induction (Faraday’s Law)

To demonstrate that a current is induced in a coil of wire when a magnet moves relative to the coil.

Apparatus

Bar magnet

Helmholtz coil

Current Sensor 0 - 100mA

Retort stand

Overview

A bar magnet is dropped through a coil.

The coil contains a large number of turns of wire.

A pulse of current is induced in the coil, which is recorded using a current sensor.

The pulse is displayed on the computer monitor using the Graph software

The pulse might resemble that shown below:

The graph may be small and it may be necessary to Zoom in, to see it.

If required the trace can be smoothed to assist in easy analysis.

N.B. A +/-1 Volt Voltage Sensor can be used instead of the 100 mA Current Sensor.

Procedure
1. Connect a 100mA current sensor to a datalogger and to a coil containing a large number of turns (e.g. 800).

2. Connect the datalogger to the computer
and launch the Graph software, setting it to record current
in Fast mode.

3. Set a trigger value so that recording only begins when a current
above a certain threshold level is detected.

4. Begin recording data and then drop the bar magnet through the coil.

5. The software generates a graphical representation of the pulse of current that was induced in the coil.

6. Save the results before analyzing the outcome.

7. Use the analysis features of the software to measure Peak values, duration of pulses and areas under each curve, etc

Results
Record your findings from above and use them to explore the following questions:


Questions

1. What happens if the south pole of the magnet enters the coil first, rather than the north pole?


2. What happens if the positive and negative leads from the current sensor are interchanged?


3. Is the latter portion of the waveform always of greater area than the former portion? Why?


4. What is the difference in milliamps between the crest and trough of the waveform?

Further work

1. Try a stronger magnet.

2. Try a coil with a different number of turns.

3. Try lowering the magnet through the coil rather than letting it fall freely.

Footnote

The coil
If constructing your own coil use a large number of turns and make sure the central hole is large enough for a magnet to easily pass through.

The magnet
The magnet needs to be quite strong to obtain a good pulse of current. An ALNICO magnet is recommended but any capable of lifting 3 or 4 linked crocodile clips will be suitable.

Soft landing
A soft landing for the magnet needs to be provided. Repeated hard blows to a magnet will reduce its strength.

Data Harvest users

1. Connect the sensor to the datalogger and the logger to the computer
2. Then click the Set Up icon to the right of this message.
3. When the software opens, click the Play button.