True or false on the magnetic force

Problem

True or false? (a) The Lorentz force is perpendicular to the velocity. (b) A charge at rest in a magnetic field feels no magnetic force. (c) Magnetic poles can be isolated. (d) Magnetic field lines are always closed. (e) The magnetic field has units of N/C.

Solution

• (a) TRUE: $\vec F = q\,\vec v \times \vec B$ is always perpendicular to $\vec v$ (and to $\vec B$).
• (b) TRUE: the magnetic force depends on velocity; if $v = 0$ then $\vec F = 0$.
• (c) FALSE: magnetic monopoles do not exist; north and south poles are always inseparable.
• (d) TRUE: magnetic field lines are always closed (the field is solenoidal, zero flux through any closed surface).
• (e) FALSE: $\vec B$ is measured in tesla, T $=$ kg/(A·s$^2$). The unit N/C is that of the electric field.

$\ev{\text{T, T, F, T, F}}$

Links

Topics: Magnetism Concepts: Lorentz force · Magnetic field