# Magnetic forces

In giving directions of magnetic forces, we sometimes use the term "out of paper" or upwards, but what really is the difference between them?

quasar987
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upward lies in the plane of the paper while "out of paper" is perpendicular to it.

mukundpa
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Perpendicular to the paper?? What do you thinK about normal?

If there's just a wire placing horizontally, how can you determine whether the force lies in the plane or out of the plane?

mukundpa
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That will depend on direction of magnetic field and that of current. Use Fleming's left hend rule.

quasar987
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perpendicular and normal are synonim

mukundpa
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but I think that perpendicular is to a line and normal is to a plane.

quasar987
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After a bit of research, I think both terms are good here.

Mathworld says for the definition of 'normal vector':

"The normal vector, often simply called the "normal," to a surface is a vector perpendicular to it."

So it makes sense to talk about a vector perpendicular to a surface (we knew that of course).

'Normal' is just a more sophisticated way of saying the same thing.

And it doesn't seem right to say that 'normal' and 'perpendicular' are synonim, because the term normal, as I understand it, can only be used when talking about a vector. So we can say that two planes are perpendicular to one another, but we cannot say that they are normal to one another.

sources:
http://mathworld.wolfram.com/NormalVector.html
http://mathworld.wolfram.com/Perpendicular.html
http://en.wikipedia.org/wiki/Surface_normal

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mukundpa
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So will it be correct to say that two vectors are normal to each other if angle between them is 90.

quasar987
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I would say 'no'. 'Normal' is only used to describe perpendicularity of a vector wrt a plane.

Have you seen this used in a textbook before? I think they mostly use the term 'orthogonal', which is a synomim to perpendicular when talking about simple object such as lines, vectors, planes, etc., but actually extend to more abstract mathematical objects as the generalisation of the notion of perpendicularity.