Question

An electron is moving in south direction and it is deflected in upward direction then direction of magnetic field present at this place is :-

• A. North
• B. South
• C. East
• D. West

Answer 1

Answer: (D)

West

Answer 2

The Lorentz force on an electron is $\vec{F} = -e(\vec{v} \times \vec{B})$. Given electron velocity $\vec{v}$ is South and force $\vec{F}$ is Upward. Since the force is opposite to $\vec{v} \times \vec{B}$ for an electron, $\vec{v} \times \vec{B}$ must be Downward. Using the Right-Hand Rule for cross product, if $\vec{v}$ is South and $\vec{v} \times \vec{B}$ is Down, then $\vec{B}$ must be West. Alternatively, using Fleming's Left-Hand Rule, the effective current direction is North (opposite to electron's Southward motion). With force Upward and effective current North, the magnetic field direction (forefinger) points West.