Ascham trial: Lenz's law induction question (1 Viewer)

[martin]

Hi,
anyone want to explain question 23 of the Ascham 2002 trial:

A square sheet of aluminium is held horizontally and then dropped so that it enters a vertical uniform magnetic field. It then leaves the field and continues to fall to the floor.
Draw a graph showing the acceleration of the aluminium sheet against time before entering the field, in the field and after. Account for the shape of your graph.

Obviously before and after a = 9.8 But what about in the field? It seems to me that the falling conductor creates eddy currents that produce magnetic fields against the change, slowing the acceleration down. But is it linear or some kind of curve?

thanks, Martin

 

[superhubert]

the sheet would slow briefly, (curve) then continue through the field at a constant speed (linear) then accelerate again (curve)

 

[BlackJack]

Acceleration against time?

Line 9.8.... an instantaneous dip...*9.8 inside mag field...
* oohh.. problem. Is the saluminium wholly inside the mag field, or is it just partialy?
IF partially, then it drops to some point less than 9.8 degrees and moves (slowly, exponteential i think...) towards zero... if fully then returns to 9.8.

then there's another dip (or rise if partially) and reutrns to 9.8.

Reasoning, if Al wholly inside magfield... a current can not be set up as the entire field is uniform... there'd be a potential difference stopping further gflow of induced current.
If Al partially covered, induced circuit continues to exert a force resisting the change... depending on how good a conductor Al is and depending on velocity...

 

[Lazarus]

Hangon.


"held horizontally and then dropped so that it enters a vertical uniform magnetic field"

Won't it be moving parallel to the magnetic field?

Which means that none of the electrons in the aluminium will experience a force?

Which means that no eddy currents will be induced?

 

[SgtSlick]

BA BING - LAZ wins again

 

[Ace-Pilot]

Originally posted by Lazarus
Hangon.


"held horizontally and then dropped so that it enters a vertical uniform magnetic field"

Won't it be moving parallel to the magnetic field?

Which means that none of the electrons in the aluminium will experience a force?

Which means that no eddy currents will be induced?

If its parralel to the Mag Field then it wont induce any eddy currents.

But look , it says Held "HORIZONTALY" on the X axis plane right.

The Mag field is "Vertical" on the Y axis, going from up to down, Now we have a Flat sheet of metal (Surface area) facing downwards.

As it falls it interacts with the vertical magnetic field, therfore being perpendicular to each other, thus experiencing a force.

Aceleration would intially increase, as it enters mag field it would start to drop, then when it leaves out the other side it would increase again.

 

[Lazarus]

I don't follow your explanation, Ace-Pilot.


The sheet is perpendicular to the field lines but it moves parallel to them. There is no change in flux... ?

 

[Ace-Pilot]

Okay i'll try to graphiclly present what im interpeting this Q to be


---------
|...........|
|...........| <----- Metal Sheet on Vertical Axis (y-axis)
|...........|
---------


====== <----- Metal sheet on Horizontal Axis (X -axis)

^^^
| | |
| | | <------ Mag Flux lines (Vertical)
| | |
| | |


If the sheet is on the Horizontal axis then its surface area is facing the mag field thus causing eddy currents. Now i see where u gota argue that in reality if a sheet is dropped on its Horizontol it will automaticly switch to its vertical position due to air-resistance, but in high school physics reality physics dosn't exsist i guess. Yeah i see the dilema. If this was like a multiple choice I would presume it stayed on its horizontal axis as it fell, but if it was an extended response i would mention it flippin onto the vertical axis due to air resitance.

 

[Lazarus]

The surface area is facing the magnetic field, yes.

It is moving downwards through the field - that is, parallel to the field lines.

I am not arguing that the sheet flips due to air resistance or anything of the sort.

I can show you that it is impossible to calculate the induced current.

ε = ΔΦ/Δt

Φ_i = 0
Φ_f = BA

ΔΦ = Φ_f - Φ_i
ΔΦ = BA

Δt = 0 (the increase in flux occurs <i>instantaneously</i> when the sheet enters the field)

ε = ΔΦ/Δt
ε = BA/0
ε = undefined

 

[Ace-Pilot]

Hey but LAz,

If the sheet fell thru the mag fields vertically it would pass unaffected at a=9.8

what im saying is i think thr examiners want us to presume that the sheets is falling thru the field horizontally, not asking us the exact amount of current induced, but more of how the representation of a curve would look like.

???

 

[Lazarus]

You're missing my point.

How about this -

If the sheet is falling vertically, it still has a measurable width, right? It is exactly the same as the sheet falling horizontally except that it is a lot skinnier and a lot taller.

 

[Ace-Pilot]

Shiiiit, Sorry Laz, damn now i see how its Falling "With" the magnetic fields not "against" them thus no eddy curents induced.

 

[Lazarus]

no worries - good luck in your exam.

 

[Ace-Pilot]

yep thanx man.