Why electrons move up if the Magnetic field is Horizontal?

[mo0nfang]

<--Why electrons move up if the Magnetic field is Horizontal?

at 0:14 <-- the Magnetic field is Horizontal but the electrons are also moving horizontally...

 

[mfb]

The force is perpendicular to the motion and perpendicular to the electric field, with its strength proportional to the sine of the angle between the two directions.
In the first video the motion is vertical, therefore there is a horizontal force.
In the second video the direction of motion changes, but the horizontal component is along the electric field - it does not lead to a force. Only the vertical component of the velocity leads to a force.

 

[vanhees71]

I guess you mean magnetic field? The Lorentz force on a point particle is
$$\vec{F}=q \left (\vec{E}+\frac{\vec{v}}{c} \times \vec{B} \right).$$