# What happens to the voltage through an electromagnet if I drop a copper rod through it?

Discussion in 'Physics' started by PulseLED, Aug 12, 2015.

1. ### PulseLED Thread Starter New Member

Jan 24, 2014
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If I drop a magnet through a coil of copper it induces a voltage in the coil. So what happens to the voltage through an copper coil electromagnet if I drop a copper rod through it?

I can't (crudely) create a sensitive enough system to detect a change with copper, but when iron enters an electromagnet it creates a voltage spike on entry and a negative voltage on exit. What theoretically should copper do?

Jul 18, 2013
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What is the permeability of copper?
Max.

3. ### Papabravo Expert

Feb 24, 2006
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Nothing would be my first guess, and IIRC the answer to Max's question is goose egg!

4. ### ScottWang Moderator

Aug 23, 2012
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How is the voltage?
How is the current?
Where is the circuit?

nsaspook likes this.
5. ### PulseLED Thread Starter New Member

Jan 24, 2014
27
0
Magnitude of the values are not important, I was just asking to see if the expected response is similar to that of iron albeit more subtle. The question was asked on "ask physics" and the answer was that the voltage would drop, I'm questioning that answer seeing if there is also an increase in voltage like there is with iron.

6. ### PulseLED Thread Starter New Member

Jan 24, 2014
27
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Would the electromagnet not induce opposing eddy currents in the falling copper rod, slowing it down and create changes in the voltage of the coil?

7. ### crutschow Expert

Mar 14, 2008
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I believe all that dropping a copper rod would do is generate a voltage from one end of the copper rod to the other. If that rod is an open circuit then there is no appreciable current generated in the rod and thus little or no magnetic field to effect the solenoid field and voltage.

8. ### PulseLED Thread Starter New Member

Jan 24, 2014
27
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So it does with iron because? Iron increases the magnetic flux generated by the coil which then collapses when its removed?

What I'm having trouble getting my head around is that if a magnet is slid down a copper sheet it will slide very slowly due to eddy current formation, this is also "open circuit". I felt that surely these opposing currents would effect the voltage in the coil.

Jan 15, 2015
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Give this a read. Look at the chart and notice the permeability Max mentioned for Iron and for Copper.

Ron

10. ### crutschow Expert

Mar 14, 2008
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Iron reduces the magnetic flux resistance and thus the magnetic flux in the solenoid, because its magnetic permeability is much higher than air or copper.
I suppose that, depending upon the size of the rod, there could be some eddy currents generated that would have a small effect on the solenoid magnetic field.
Eddy current is affected by the area of the conductor so a sheet will generate more eddy currents than a rod in the same magnetic field.

11. ### nsaspook AAC Fanatic!

Aug 27, 2009
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2,356
I'll take swing, it's a little different from iron in a copper coil as the coil inductance changes very little due to the copper rod.
It's the same relative motion as the magnet in a copper pipe but flux from the small magnet is compact/strong, the flux from the air core copper coil is not.
The induction would happen mainly at the coil top and bottom where there is curling flux at the ends of the coil of the (maybe weak) magnetic field of the copper coil (co-axial magnet) because the falling copper rod is cutting flux lines inducing a changing current (Eddy currents), the changing current means there is also a changing voltage with charge moving in the highly conductive rod to neutralize any charge imbalance. The rod would be seen as a continuous set of ring conductors as it falls, drawing power from the coil into the rings that's dissipated by its resistance.

12. ### BR-549 Well-Known Member

Sep 22, 2013
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Permeability has nothing to do with it in this situation, unless there is established current in coil, which there is not.

Only the amount and speed of flux, entering the interior of the coil, will determine the voltage.