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Having trouble explaining this phenomena

  • #1

Homework Statement



I was doing an experiment to verify faraday's law

Where we were basically dropping a magnet through a coil and measuring the induced emf

Now here is the thing I cant explain;

we connected 2 coils in series and found the induced emf was the same as that of one coil

then we connected 2 coils in parallel and found that the induced emf was half that when using one coil.

How do you explain this... I thought it had something to do with the resistance but when I checked it using a bit of math for resistance in series and parallel I did not get a result like that from the experiment.

What is happening in terms of the physics here. Thanks



Homework Equations





The Attempt at a Solution

 

Answers and Replies

  • #2
PeterO
Homework Helper
2,425
46

Homework Statement



I was doing an experiment to verify faraday's law

Where we were basically dropping a magnet through a coil and measuring the induced emf

Now here is the thing I cant explain;

we connected 2 coils in series and found the induced emf was the same as that of one coil

then we connected 2 coils in parallel and found that the induced emf was half that when using one coil.

How do you explain this... I thought it had something to do with the resistance but when I checked it using a bit of math for resistance in series and parallel I did not get a result like that from the experiment.

What is happening in terms of the physics here. Thanks



Homework Equations





The Attempt at a Solution

Does it help to think that a single coil is a whole lot of loops joined in series, so to add a whole lot more loops in series should mean the same result - but given the physical size of the set-up, that emf may exist for a longer period of time.

Not sure of the parallel connection.
 
  • #3
Does it help to think that a single coil is a whole lot of loops joined in series, so to add a whole lot more loops in series should mean the same result - but given the physical size of the set-up, that emf may exist for a longer period of time.

Not sure of the parallel connection.
Yeah I kinda gathered that but im more unsure of the parallel result... I just cant seem to figure it out.
 
  • #4
PeterO
Homework Helper
2,425
46
Yeah I kinda gathered that but im more unsure of the parallel result... I just cant seem to figure it out.
I wonder if the parallel situation is giving you an "average" result? The magnet can't be in both coils at once [unless one fits inside the other] so at each phase you are getting "emf+0"/2.

One might argue that a magnet falling through a coil effectively produces a power supply, like a battery.

What happens when you connect two similar batteries in Series?
What happens when you connect two similar batteries in parallel?
 
  • #5
I wonder if the parallel situation is giving you an "average" result? The magnet can't be in both coils at once [unless one fits inside the other] so at each phase you are getting "emf+0"/2.

One might argue that a magnet falling through a coil effectively produces a power supply, like a battery.

What happens when you connect two similar batteries in Series?
What happens when you connect two similar batteries in parallel?
Well in series the voltages would add and the current capacity would stay the same

in parallel it would be the opposite

so your saying that the ε ≈ dBA/dt


so for ε/2... BA stays the same but the time the battery spends falling throught the loop approx. doubles since it is two loops.so N=2n/2l

Is this what your saying?

for the first case (series) it acts like this

2ε = dBA/2dt

thats why we had the same result.

second case (parallel)

ε = 2BA/2dt



???
 
  • #6
PeterO
Homework Helper
2,425
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Well in series the voltages would add and the current capacity would stay the same

in parallel it would be the opposite

so your saying that the ε ≈ dBA/dt


so for ε/2... BA stays the same but the time the battery spends falling throught the loop approx. doubles since it is two loops.so N=2n/2l

Is this what your saying?

for the first case (series) it acts like this

2ε = dBA/2dt

thats why we had the same result.

second case (parallel)

ε = 2BA/2dt



???
I have never actually considered this situation before, so don't have a definitive answer - I merely put forward some thoughts to prod you along.

This last consideration where you refer to rate of change of flux looks promising.
 
  • #7
I have never actually considered this situation before, so don't have a definitive answer - I merely put forward some thoughts to prod you along.

This last consideration where you refer to rate of change of flux looks promising.
Ok thanks
 
  • #8
NascentOxygen
Staff Emeritus
Science Advisor
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then we connected 2 coils in parallel and found that the induced emf was half that when using one coil.
Can you explain how you went about this? How you dropped a magnet through two parallel coils simultaneously?
 
  • #9
Can you explain how you went about this? How you dropped a magnet through two parallel coils simultaneously?
the coils were placed on top of each other so the length of the coil (x) is now 2x.

then we dropped the magnet through them so at some point I guess the magnet could have been in both coils at the same time since the length of the magnet was ≈x/2.

does this make sense.

??
 
  • #10
NascentOxygen
Staff Emeritus
Science Advisor
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1,068
the coils were placed on top of each other so the length of the coil (x) is now 2x.

then we dropped the magnet through them so at some point I guess the magnet could have been in both coils at the same time since the length of the magnet was ≈x/2.

does this make sense.

??
That's how you placed them for parallel coils. How did you place them for series connection?
 
  • #11
That's how you placed them for parallel coils. How did you place them for series connection?

the same way
 
  • #12
NascentOxygen
Staff Emeritus
Science Advisor
9,243
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the same way
http://imageshack.us/a/img717/4080/3110y.gif [Broken] ...... there lies your problem.

To succeed in demonstrating what you expect, then the EMFs in each coil will have to be identical, i.e., generated simultaneously. If the magnet has to pass through one coil before it enters the second, then (without careful analysis) it's anyone's guess how the result will pan out.

Were I to do it, I would try concentric windings.

What device were you using to measure the voltage?
 
Last edited by a moderator:
  • #13
http://imageshack.us/a/img717/4080/3110y.gif [Broken] ...... there lies your problem.

To succeed in demonstrating what you expect, then the EMFs in each coil will have to be identical, i.e., generated simultaneously. If the magnet has to pass through one coil before it enters the second, then (without careful analysis) it's anyone's guess how the result will pan out.

Were I to do it, I would try concentric windings.

What device were you using to measure the voltage?
just a voltmeter
 
Last edited by a moderator:

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