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(D) The magnetic force on a moving charge is given by $\vec{F}_m = q(\vec{v} 	imes \vec{B})$.For an electron,$q = -e$. Given the velocity $\vec{v}$ i

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normal to the plane of the paper and into the plane of the paper

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(D) The magnetic force on a moving charge is given by $\vec{F}_m = q(\vec{v} 	imes \vec{B})$.For an electron,$q = -e$. Given the velocity $\vec{v}$ is in the $+x$ direction and the magnetic field $\vec{B}$ is in the $+y$ direction,the magnetic force is $\vec{F}_m = -e(v\hat{i} 	imes B\hat{j}) = -evB\hat{k}$.This force acts into the plane of the paper (along the $-z$ direction).To keep the electron undeviated,the net force must be zero,so the electric force $\vec{F}_e$ must balance the magnetic force $\vec{F}_m$. Thus,$\vec{F}_e = -\vec{F}_m = +evB\hat{k}$.Since $\vec{F}_e = q\vec{E} = -e\vec{E}$,we have $-e\vec{E} = evB\hat{k}$,which gives $\vec{E} = -vB\hat{k}$.The direction of the electric field is along the $-z$ direction,which is normal to the plane of the paper and into the plane of the paper.

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