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The cathode ray is bent upward toward the plate by electric field and downward by the magnetic field.

The first part isn't what I am confused about. However, the second part is. How did the magnetic field bend the cathode ray downward?

  • 2
    $\begingroup$ A charged particle moving in a magnetic field experiences a force of $q v \times B$. You can readily verify that in the configuration pictured the deflection will be up/down (depending on the charge of the particle). $\endgroup$
    – Jon Custer
    Nov 4, 2014 at 17:39
  • $\begingroup$ Can you annotate the formula $\endgroup$ Nov 4, 2014 at 17:49
  • 2
    $\begingroup$ $q$ is the charge on the particle, $v$ is the velocity (vector), $B$ is the magnetic field (vector). So, with the particles traveling in $x$ (left to right), and the magnetic field aligned along $y$ (into/out of the figure), the direction of the magnetic force on the moving charge is $\pm z$ depending on the sign of the charge $q$. $\endgroup$
    – Jon Custer
    Nov 4, 2014 at 17:53
  • $\begingroup$ Ever heard of Hall-effect? This is it! $\endgroup$
    – user36790
    Dec 1, 2014 at 12:28

1 Answer 1


Both electric and magnetic field apply a force on charged particles (roughly saying this is not always valid as you will see in case of magnetic field.) It's a vast topic but I would summarize it quickly, note that the explanation here involves mostly non-relativistic (small speed in comparison with light where things change drastically) explanation:

Electric Field: Electric field can apply force on charged particles moving or not, proportional to the charge of the particle given as ${\bf F}=q{\bf E}$ [$\bf F$:force vector, q:charge, $\bf E$: electric field vector ]You can note that force on positively charged particles is in same direction of electric field and on negatively charged particles in the opposite direction.It increases the energy of a particle if it is in same direction of velocity or acting on a particle at rest and decreases when opposite but increases after a long times in the absence of other forces.

Magnetic Field: Magnetic field can apply force only on moving charged particles ( which themselves vary the flux associated with the electric field produced by themselves due to continuos movement, where the flux is (roughly) like the amount of water flowing in a pipe or number (actually proportional) of electric field lines in a region ).The force is given by ${\bf F}=q{\bf v\times B}$ [$\bf F$:force vector, v:velocity, q:charge, $\bf E$: magnetic field vector ] One important thing to note that force is always perpendicular to the velocity and thus it does no work and the energy of particle is conserved only the direction is changed.


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