If the magnetic field is parallel to the positive $y-$axis and the charged particle is moving along the positive $x-$axis (Figure), which way would the Lorentz force be for

$(a)$ an electron (negative charge),

$(b)$ a proton (positive charge).

900-2

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The velocity $v$ of particle is along the $x$ -axis, while $B ,$ the magnetic fleld is along the $y$ -axis, so $v \times B$ is along the $z$ -axis (screw rule or right-hand thumb rule). So, $(a)$ for electron it will be along $-z$ axis. $(b)$ for a positive charge (proton) the force is along $+z$ axis.

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