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# Callibrate a Hall Probe: Planning

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Introduction

Planning Exercise Design a laboratory experiment to calibrate an uncalibrated Hall probe and use it to investigate how the magnetic flux density mid-way between opposite poles of two permanent bar magnets varies with the separation of the bar magnets.

Calibrating the Hall probe

To calibrate the Hall probe, Helmholtz coils should be used as a uniform field is produced.  Helmholtz coils

The magnitude of the flux density in the region of uniform field is given by where B = flux density (T)

µ0 = permeability of free space (4π x 10-7 Hm-1)

n = no. of turns on coil

I = current in each coil (A)

R = radius of each coil (m)

Formula 1

The magnitude of the Hall voltage measured by a Hall probe is given by

where VH = Hall voltage (V) I = current in Hall probe (I)

B = magnetic flux density (T)

n = no. of electrons per unit volume (e/m3)

e = electron charge (1.6 x 10-19 C)

d = thickness of film in Hall probe (m)

Formula 2

Middle

Position the Hall probe in the centre so that it is perpendicular to the direction of the field.  1. Record the voltage reading from the milli-voltmeter.
2. Use Formula 1 to find a value for the corresponding magnetic flux density.
3. Increase the current going through he Helmholtz coil to 1.0A.
4. Measure the voltage reading and use Formula 1.
5. Repeat this for coil currents of 1.5A, 2.0A and 2.5A.
6. Record values for voltage and magnetic flux density using a data logger and create an automated graph.
7. Use this graph to find the corresponding magnetic flux density for any value of voltage shown by the milli-voltmeter. NB

• The current through the coils must not exceed 3A to avoid overheating.
• The current through the Hall probe must not exceed 0.4A.
• The Hall probe should be connected to a DC power supply so that a constant current is provided and therefore a constant temperature. This increases the reliability of the results as changes in temperature can affect the measurements.
• A milli-voltmeter increases the accuracy of the voltage readings.
• Run the positive and negative leads of the coils together so that the fields they are generating are cancelled out.  • A data logger ensures accurate results.

Conclusion

Read the voltage and use the graph created during calibration to work out the magnetic flux density.Increase the distance between the magnets to 0.02m.Read the voltage and use the graph to find the magnetic flux density.Take readings for distances of 0.01m, 0.02m, 0.03m, 0.04m, 0.05m and 0.06m.Record the results of magnetic flux density and create a graph of distance between magnets against magnetic flux density to find a relationship, which should follow the inverse square law.

NB

• Ensure that this experiment is performed away from other magnetic materials so that results are not affected.

References

• http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/helmholtz.html
• http://physicslabs.phys.cwru.edu/EM/Manual/pdf_version/MAG.pdf
• A-Level Physics 4th Edition by Roger Muncaster – Chapter 41, Pg 615
• A-Level Physics 4th Edition by Roger Muncaster – Chapter 41, Pg 637 http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/helmholtz.html

 A-Level Physics 4th Edition by Roger Muncaster – Chapter 41, Pg 615

 A-Level Physics 4th Edition by Roger Muncaster – Chapter 41, Pg 637

 http://physicslabs.phys.cwru.edu/EM/Manual/pdf_version/MAG.pdf

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