Project: Metal Detector
Kick Off Questions
- What would happen inside a metal if it is placed inside a magnetic field?
- Draw out a sine graph on your whiteboard. What is the amplitude of the graph? What is the frequency?
- What steps do you follow to use the $V_{ind}$ equation?
Main Problem
When you dug up Dr. Watersa's secret jellybean stash, you were immediately surrounded by the herd of EM-boartigers. You were quite alarmed at first, but the EM-boartigers ignored you and focused on the jellybeans, the black licorice ones specifically. Since black licorice jellybeans are disgusting anyway, you were happy to let the EM-boartigers have them.
After eating the jellybeans the EM-boartigers became very friendly, even letting you pet them – although you got some pretty ferocious static shocks in the process – and then wandered away. They seem to have completely disappeared, since no one in Lakeview has seen them in weeks now…maybe they just wanted the jellybeans all along? But just as you were getting ready to leave Lakeview and return home, a new problem popped up. The S.P.A.R.T.A.N. financial auditors say they need proof of these weird creatures, or they will disallow your expense reports for the whole time you've been trapped in Lakeview and you'll be on the hook for thousands of dollars in costs.
Desperate, you turn to Manny for help since he's been right about everything all along. He says his research on the boartigers suggests that their excrement has a very high percentage of nickel in it. If you can MacGyver together a metal detector (similar to this one), maybe you can find some…evidence…to give to the auditors.
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.
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 Up Questions
- What would a magnetic field versus time graph and $V_{ind}$ versus time graph look like when there is *not* metal present?
- 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?