# Homework Help: Question regarding the ''cross'' and ''circle'' signs in magnetism

1. Jan 3, 2014

### LegendF

1. The problem statement, all variables and given/known data

Hi there.

I've experienced another confusion.. This time regarding the signs used in the electromagnetical fields, the cross and the ''circle with a dot in it''.

2. Relevant equations

The issue here is, that my teacher always told us that for example X means that the vector quanity is going into the board, however in my book it keeps using that it means that it's going into the paper. Which one is correct? For example, if the X symbols currency or magnetical field, does it mean that the currency or magnetical field is going into the table or that the currency is going into the paper?

If an object is said to be perpendicular to the paper, can I interpret it as I rotate the object 90 degrees and then place it against the board and then get the direction of the force for example?

Best regards,
LegendF
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2. Jan 3, 2014

### Malverin

Circle means that the vector for intensity of magnetic field points towards you.
And cross points from you to the paper.

3. Jan 3, 2014

### SammyS

Staff Emeritus
If you place your book on a table, then a vector pointing into the paper is also going into the table.

When your teacher said that the X means that the vector is going into the board, think of placing your book up against the board. Then a vector pointing into a page of the book will also be pointing into the board.

4. Jan 3, 2014

### LegendF

But I don't get the same result if I assume that something is going into the board, as if it's going into the paper.

For example, this image:
http://phy214uhart.wikispaces.com/file/view/righthandrule.gif/82801767/righthandrule.gif

The current in the image is going upwards(out of the book/paper?), but how would I explain that using a board..? Would the current then go out of the board instead..? Wouldn't I then also have to turn the object 90 degrees to the left in order to be able to even place it against the board? :(

Best regards,

5. Jan 3, 2014

### Malverin

When the current goes out of the paper, magnetic field will rotate CCW.

6. Jan 3, 2014

### LegendF

Yes, but what about if we're talking about the board and not the paper? That's what's confusing me..

Best regards,

7. Jan 3, 2014

### tiny-tim

hi everyone!

i've been reading the last few posts: what is this board that you're talking about? where is it?

8. Jan 3, 2014

### LegendF

Hello there Tim! :D

At 2:48 he uses the method, at 2:04 he explains that ''cross'' means into the board and further on.

What my issue is that my teacher keeps telling me that something is going into the board, or out of the board, just like the video does, however my book keeps stating that something is going into or out of the paper.

Which is confusing me terribly for some reason..

Best regards,

Last edited by a moderator: Sep 25, 2014
9. Jan 4, 2014

### Malverin

The board is his "paper"
so it is the same.

Last edited by a moderator: Sep 25, 2014
10. Jan 4, 2014

### ehild

The teacher works on the board and you work on the paper (which is on the table). His world is vertical, your world is horizontal. The magnetic field is normal on the board, it must be also normal to your paper: that makes both worlds equivalent. They are just turned by 90°. The magnetic fields in his world and in your world are not the same, but they are the same with respect to the frame of reference in their own worlds. You draw the x,y axes horizontally on your paper, and imagine the z axis perpendicular to them, perpendicular to the paper (that is, vertical). The magnetic field is along the z axis, in or out of the paper.
The teacher draws the x, y axes on the vertical board, and imagines the z axis perpendicular to them, that is horizontal. The magnetic field is along his z axis, in or out of the board.

ehild

11. Jan 4, 2014

### tiny-tim

Hello LegendF!
ah, the video guy is saying "board" because he has a board!

as Malverin says: the board is his "paper"

"board" and "paper" mean the same thing …

they're whatever the diagram is on
that diagram uses a 3D view (using perspective)

so you simply draw the whole arrow (as you can see in the diagram) …

either a straight arrow or a circular arrow

the only time you need the cross or dot in a circle is in a 2D view, where if you drew the whole arrow you wouldn't be able to see the line part of it at all, since you're seeing it head-on!

12. Jan 4, 2014

### LegendF

So if I get a problem to solve, (determine the direction of the magnetic field), on my paper and if I imagined it being wrote on the board I would not get the same result as I would by determing the magnetic field by looking at my paper, but I would get the same result with respect to the frame of reference in their own worlds?

So if I get a problem to solve on a paper, I solve it on the paper, if I get it on the board, I solve it on the board, it's just all about the reference?

If the object is going upwards in my paper, then it would also ''rotate'' 90degrees on the board so it's perpendicular against it because it's perpendicular in my paper? If the current is going into the paper, then it would also go into the board? If the magnetic field is going into the paper then it also needs to go into the board and so on..?

Best regards,

13. Jan 4, 2014

### tiny-tim

yes

14. Jan 4, 2014

### ehild

YES

Think about an other example - a ball is thrown up. The teacher draws it on the table moving upward while gravity acts downward . But you draw the motion of the ball on the horizontal paper, and the direction of gravity as you draw it, is horizontal again. You show the motion of the ball by an "upward" arrow and gravity is represented by a downward arrow on the paper, but both arrows are horizontal in the real world. Somebody arriving from an other planet where
paper and pen are not used by the students would not understand why the students think that gravity acts horizontally.
I do not know, if they are available now, but all students will use such tablets in the near future that they can draw on the display, and the display can be hold vertically, like the table of the teacher....

ehild

15. Jan 4, 2014

### LegendF

I see, very good explanation.

It's all about a matter of reference then, where you place the object and then just make sure that the forces/vectors point out the same way based on the plane the object is on.

If a force is going out of the board, then it's going out of the paper, both forces are perpendicular to its plane.

Correct? :D

Thanks so much guys!

Best regards,