# Expression for magnitude of magnetic force

## Homework Statement

A particle with charge q moves with velocity v near a long, straight wire which has a current I flowing through it which generates magnetic field B. Give an expression for the magnitude of the force on this particle in terms of q, v = | v | and the magnitude of the magnetic field strength B. Make sure you define any other variables that you introduce.

F = q(v x B)

## The Attempt at a Solution

Could it be as simple as just quoting F = q(v x B)? I feel as though I may be missing something as this would seem a little too straight forward.

Cheers guys.

lightgrav
Homework Helper
no, that writes it in terms of the velocity vector and the magnetic field vector.
They want you to write it in terms of the scalar speed and the magnetic field strength (scalar).
(hint: you WILL need to introduce a new variable that you'll need to define)

Thanks a lot for the reply!
Is it somehow related to the formula for magnetic field strength at a given distance from a wire: B= [PLAIN]http://scienceworld.wolfram.com/physics/pimg170.gifi [Broken] / 2pi r? Im struggling to see from my books what else could be relevant.

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lightgrav
Homework Helper
they don't want your "Making B" equation ... that would write the Force in terms of I, not B.
what does that little "x" in your formula mean?

Ahh so you could take the dot product rather than the cross product of the vectors and theta would be the new variable?

lightgrav
Homework Helper
multiplication of scalars is not really a "dot product", it is simply multiplication.
A "dot product" multiplies the parallel components of 2 vectors, yielding a scalar.
This "cross product" multiplies the 2 vectors' perpendicular components, yielding a 3rd (perp) vector.

This is what I got, would appreciate some input:

F = qvBsin(theta)

lightgrav
Homework Helper
yes, good ... but since θ is a new variable, you need to define it (as they explicitly stated)