184_projects:metal_detector [Projects & Practices in Physics]

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==== Project: Metal Detector ====

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Life has gotten back to normal in Lakeview as the continued magnetic field seems to have driven away the boar tigers. There has been the added bonus that there has been a drastic reduction in car accidents due to the populace being afraid that the magnetic field will destroy their engines. However, tranquility is unsettled when the government agency P.E.R.L. comes to town in order to further investigate the boar tiger phenomenon. You are a local guide, part-time engineer and a high ranking member of the McGyver fan club who has been recruited to go into the forest surrounding Lakeview to help P.E.R.L in their investigation. Once deep in the forest, your party is ambushed by boar tigers and several of the team are captured and taken away. The remaining members of the team try to follow the tracks of the Boar Tigers but this becomes impossible after a time. However, you remember hearing from a friend of yours that did research on the boar tigers that their excrement has a very high percentage of nickel in it. You indicate to the group that you think you can McGyver a metal detector together to track your kidnapped colleagues. You have access to a whole bunch of 12 gauge copper wire, an alternating current power supply that can produce a maximum current of 2 A and has a frequency of 60 Hz, and a voltmeter that will measure an alternating voltage. If the voltmeter measures a voltage bigger than 50 mV, it will sound an alarm. You want to design your metal detector so that if there is not a metal nearby, the voltmeter will be below the threshold; however, if there is a metal nearby, then the measured voltage on the voltmeter will be above the threshold and set off the alarm.  

<WRAP info>
=== Learning Goals ===
  * Explain how a metal detector works (be able to explain what happens when a metal is present and not present)
  * On a microscopic level, explain what happens to the metal pieces when placed in a magnetic field.
  * Make a current vs time graph for the alternating current and for the induced current. Explain any similarities and/or differences in the graphs.
  * Use the right hand rule to check the direction of the induced current.
</WRAP>

Conceptual Questions:
  - What is design of a metal detector? (Hint: use your resources - *cough* google *cough*)
  - What would a magnetic field versus time graph and $V_{ind}$ versus time graph look like when there is *not* metal present?
  - What would happen inside the metal when it is placed inside a magnetic field?
  - What would a magnetic field versus time graph and $V_{ind}$ versus time graph look like when there *is* metal present?
  - If there was a lot of metal near your detector versus a small amount of metal, what would you expect to change?
  - What assumptions are you making in this problem?