Can the magnitude of a magnetic field be negative?
If we consider an electron in a mag field,
using r=mv/qB, the negative charge on the electron gives a negative answer. Do I drop the minus sign?
To get this you used the Lorentz force equation, a vector equation, then dropped the vectors and equalled it to a scalar centripetal acceleration as a scalar. Ignore the negative, and make certain you know the direction of curvature from the right hand rules.
If course. B is a vector, it can be + or -
Your equation is a magnitude type equation that does not really do the vector computations correctly to decide what direction r is in.
You have to do that yourself.
You need to go back to the vector equation for force on a charged particle in a B field:
F = q (v X B)
here q = -e
so F = -e (v X B)
figure out the direction of v X B with the right hand rule
Your force on the electron causing it to curve is in the opposite direction.
The magnitude of a vector quantity represents its size or magnitude, which is always positive or zero. For a magnetic field, the magnitude represents the strength or intensity of the field. Therefore, the magnitude of a magnetic field can only be positive.
In the equation you provided, r = mv/qB, the negative charge of an electron is represented by the negative sign on the charge q. However, this negative sign does not make the magnitude of the magnetic field negative. The magnitude of the magnetic field (B) itself should always be a positive value.
To clarify, the equation gives you the magnitude of the magnetic field, not its direction. If you want to find the direction of the magnetic field, you need to consider the sign of the charge (positive or negative) and the direction of the other variables in the equation (velocity, mass, and radius). The negative sign on the charge q will affect the direction of the magnetic field, but it does not change the fact that the magnitude of the field is always positive.