CSSA 2017 Trial multiple choice Q11 (1 Viewer)

[The_Owl]

The cut off part of the question says "Four identical planes are flying above the equator at the same time with the following speeds, directions, and altitudes."

The correct answer is C, but I would have thought that going east to west around the equator at a constant speed and altitude there would be no change in magnetic flux, as the magnetic field the plane is moving through is uniform.

 

[pikachu975]

So it's C or D as north to south or south to north is 0 change in flux due to flux lines being vertical.

East to west would be a faster change in flux as the Earth rotates to the east as well as the plane going left so they're both moving relative to each other. It's like if you ran at someone and they ran at you, the speed relative to each other increases.

Furthermore the altitude of C is 5000 m which is closer to the source of flux so it's cutting more lines.

Speed of C is higher than D so it's cutting flux faster.

 

[The_Owl]

So it's C or D as north to south or south to north is 0 change in flux due to flux lines being vertical.

Yes, the field lines are vertical, but the spacing between them changes as the plane goes from north to south, so the magnetic field strength should vary.

If you're going east to west, isn't that like moving a metal sheet within a uniform magnetic field? You only get eddy currents induced when the plate moves in or out of the uniform field, even though it is cutting lots of field lines. Like in this image:

 

[pikachu975]

Yes, the field lines are vertical, but the spacing between them changes as the plane goes from north to south, so the magnetic field strength should vary.

If you're going east to west, isn't that like moving a metal sheet within a uniform magnetic field? You only get eddy currents induced when the plate moves in or out of the uniform field, even though it is cutting lots of field lines. Like in this image:

The magnetic field isn't into the page though. The flux lines are up so cutting them horizontally results in a change in flux. Also for the vertical travel the plane will just be travelling along one flux line so although it will change in strength it won't be that much of a change in flux anyway.

 

[The_Owl]

The magnetic field isn't into the page though. The flux lines are up so cutting them horizontally results in a change in flux. Also for the vertical travel the plane will just be travelling along one flux line so although it will change in strength it won't be that much of a change in flux anyway.

If you look at the cross section of the Earth cut through the equator then the magnetic field is into the page.

If you travel around any of those circles then the magnetic field strength would be constant.

How can you have a change in magnetic field strength without a change in magnetic flux, given magnetic field strength is magnetic flux density, and the area that you are measuring over isn't changing?

 

[pikachu975]

If you look at the cross section of the Earth cut through the equator then the magnetic field is into the page.
View attachment 34241
If you travel around any of those circles then the magnetic field strength would be constant.

How can you have a change in magnetic field strength without a change in magnetic flux, given magnetic field strength is magnetic flux density, and the area that you are measuring over isn't changing?

The flux is up the page as seen in the question's diagram

 

[The_Owl]

The flux is up the page as seen in the question's diagram

The diagram in the question is showing a side view of the Earth, but for a plane going around the equator don't you need to look at a top down cross section (which is what I drew)?

 

[fluffchuck]

When you're moving north to south or south to north, you will be travelling on the same set of flux lines which means that there will be a minimal change in magnetic flux.
However, if you move west to east or east to west, you pass through a lot of flux lines which means that there is a great change in magnetic flux. Thus, you can eliminate A and B.
Now since the speed in C > speed in D, there is a greater change in magnetic flux in C than there is in D.
Thus, the correct answer is C.

 

[fluffchuck]

The diagram in the question is showing a side view of the Earth, but for a plane going around the equator don't you need to look at a top down cross section (which is what I drew)?

Yes, from what you have drawn, if you move the solid metal sheet (representing the planes in the question) through the magnetic field, there is a change in magnetic flux.

If you just move the sheet up and down the page, you can see that there is no change in magnetic flux.

 

[The_Owl]

Yes, from what you have drawn, if you move the solid metal sheet (representing the planes in the question) through the magnetic field, there is a change in magnetic flux.

If you just move the sheet up and down the page, you can see that there is no change in magnetic flux.

Which image are you talking about? I meant this one:

 

[fluffchuck]

Which image are you talking about? I meant this one:
View attachment 34242

I was referring to your first image.

Also, that the magnetic field lines drawn should be into the page rather than out of the page.

Using the crosssection.jpg image, if you move a plane around the Earth, you will see that there is a change in magnetic flux, and hence, the answer must involve moving from west to east or vice versa.

 

[The_Owl]

I was referring to your first image.

Also, that the magnetic field lines drawn should be into the page rather than out of the page.

Using the crosssection.jpg image, if you move a plane around the Earth, you will see that there is a change in magnetic flux, and hence, the answer must involve moving from west to east or vice versa.

If you are following the path of one of the circles of dots, the magnetic field is uniform because the dots in the circle are evenly spaced, so there shouldn't be any change in flux.

 

[spinach123456789]

mate, you're moving through a mag field, there is a change in flux...?

if you think that there is no change in flux, you should abandon past papers and go reread a textbook