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(D) The force on a moving charge in a magnetic field is given by the Lorentz force formula: $\vec{F} = q(\vec{v} 	imes \vec{B})$.Here,the charge of t

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South

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(D) The force on a moving charge in a magnetic field is given by the Lorentz force formula: $\vec{F} = q(\vec{v} 	imes \vec{B})$.Here,the charge of the electron is negative $(q = -e)$.The velocity $\vec{v}$ is towards the east ($+\hat{i}$ direction).The magnetic field $\vec{B}$ is vertically downward ($-\hat{k}$ direction).Using the cross product: $\vec{v} 	imes \vec{B} = (v \hat{i}) 	imes (-B \hat{k}) = -vB (\hat{i} 	imes \hat{k}) = -vB (-\hat{j}) = vB \hat{j}$ (which is North).Since the charge is negative,the force $\vec{F} = -e(vB \hat{j}) = -evB \hat{j}$.The direction $-\hat{j}$ corresponds to the South direction.Alternatively,using Fleming's Left-Hand Rule: Point the index finger downward (magnetic field),the middle finger towards the west (since the electron moves east,the conventional current is west),then the thumb points towards the South.

An electron is travelling horizontally towards the east. $A$ magnetic field in the vertically downward direction exerts a force on the electron along:

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