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A uniform electric field and a uniform magnetic field are acting along the same direction in a certain region. If an electron is projected in the region such that its velocity is pointed along the direction of fields, then the electron
A
Will turn towards left of direction of motion
B
Will turn towards right of direction of motion
C
Speed will decrease
D
Speed will increase
Detailed Solution
Magnetic force: $\vec{F}_m = q(\vec{v}\times\vec{B})$. The velocity is parallel to $\vec{B}$, so $\vec{v}\times\vec{B} = 0$ and there is no magnetic force; the electron does not turn.
Electric force: $\vec{F}_e = q\vec{E} = -e\vec{E}$. Since the electron has negative charge, this force is opposite to $\vec{E}$.
The velocity is along $\vec{E}$, so the electric force is opposite to the velocity.
The electron is therefore retarded along its line of motion.
Hence its speed will decrease.
Electric force: $\vec{F}_e = q\vec{E} = -e\vec{E}$. Since the electron has negative charge, this force is opposite to $\vec{E}$.
The velocity is along $\vec{E}$, so the electric force is opposite to the velocity.
The electron is therefore retarded along its line of motion.
Hence its speed will decrease.