An electron moving in the +y direction,at right angles to a magnetic field,the magnetic force exerted on it is in the -x direction.The direction of the $\vec { B }$ field is in the

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The direction of the magnetic force on a moving charge is given by the right-hand rule, which states that if you point your fingers in the direction of the velocity of the charge (in this case, the +y direction) and curl them towards the direction of the magnetic field, your thumb will point in the direction of the force. Since the force is in the -x direction and the electron is moving in the +y direction, the magnetic field must be in the +z direction for the right-hand rule to apply (remembering that electrons are negatively charged, so the force direction is opposite to what you'd expect for a positive charge).

An Electron Moving in the +Y Direction,at Right Angles to a Magnetic