Determining the magnetic field of a bar magnet experimentally?

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Discussion Overview

The discussion focuses on methods for experimentally determining the magnetic field of a bar magnet. Participants explore various techniques and tools that can be utilized for this measurement, including both practical setups and theoretical considerations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant suggests using a many-turn air coil to measure the magnetic flux by integrating the voltage output as the coil is pulled away from the magnet.
  • Another participant mentions that flipping the coil can double the voltage integrator output signal.
  • A participant expresses concern about their lack of experience with circuits and asks for simpler methods, indicating a preference for less precision.
  • One participant proposes creating an EMF by rotating the magnet in a solenoid, referencing Faraday's Law, and notes that this method is feasible if the bar magnet's dimensions meet certain criteria.
  • Various methods for measuring magnetic fields are listed, including Hall Effect, NMR, flux gate, rotating coil, SQUID, and flip coil, suggesting a range of approaches available for different applications.
  • Another method discussed involves measuring the oscillation period of the bar magnet as a torsional pendulum in Earth's magnetic field to calculate its magnetic moment and pole tip field strength.

Areas of Agreement / Disagreement

Participants present multiple methods and approaches for measuring the magnetic field, indicating a lack of consensus on a single best method. Some participants express confidence in their suggestions, while others seek simpler alternatives, highlighting differing levels of expertise and preferences.

Contextual Notes

Participants mention various methods without resolving the complexities or assumptions involved in each technique. The discussion reflects a range of practical and theoretical considerations that may affect the choice of method.

Who May Find This Useful

This discussion may be useful for undergraduate physics students, educators, and hobbyists interested in experimental physics and magnetic field measurement techniques.

Xyius
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I would I go about determining the magnetic field of a bar magnet experimentally? Any ideas?
 
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Use a many-turn (N > 100) air coil slightly larger than the bar magnet and slip it over the bar magnet down to the middle. Turn on a voltage integrator - see my post # 27 in

https://www.physicsforums.com/showthread.php?t=70290&highlight=flip+coil&page=2

and pull the coil off the end and away from the magnet. This measures the volt-seconds of the coil voltage output, which is equal to the magnetic flux Φ. This will measure the total flux Φ out of one end. Divide the measured flux Φ by the area A of the end of the bar magnet to get the flux density B.

Bob S
 
Awesome thanks a lot!
 
Xyius said:
Awesome thanks a lot!
You can double the voltage integrator output signal if you flip the coil over and slide it back on the bar magnet.

Bob S
 
I am currently in undergraduate Physics, and have little experience setting up circuits and I feel that is beyond by capabilities due to the lack of knowledge I currently have. Is there any easier way? It doesn't have to be extremely precise.

EDIT:

What if I create an EMF by rotating the magnet in a solenoid and using faradays Law?
 
Last edited:
Xyius said:
What if I create an EMF by rotating the magnet in a solenoid and using faradays Law?
If the bar magnet is less than ~2 times longer than it is wide, you can do it this way. The equipment for this is very similar to the voltage integrator/ flip coil method I describe above and in the referenced post. The flip coil can be built using parts at the local electronics store (voltmeter, dc power supply, soldering iron, op-amp, resistor, capacitor, wire, coil form).

Here is a list of various methods suitable for your magnetic field measurement application.

Hall Effect
NMR (nuclear magnetic resonance)
Flux gate
Rotating coil
SQUID
Flip coil

Look them up on the web. Here is another method.

Calculate the moment of inertia of your bar magnet.

Tie a thread to it and measure the oscillation period (torsional pendulum) in Earth's magnetic field.

Calculate magnetic moment of bar magnet. From dimension of magnet, calculate the pole tip field strength.

Bob S
 
Thanks A lot man, I can tell you know your stuff!
 

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