Question

Two large vertical and parallel metal plates having a separation of 1 cm are connected to a DC voltage source of potential difference X. A proton is released at rest midway between the two plates. It remains at rest in the air then X is:

• A. $1\times {10}^{-5}V$
• B. $1\times {10}^{-7}V$
• C. $1\times {10}^{-9}V$
• D. $1\times {10}^{-10}V$

Answer 1

Answer: (C)

$1\times {10}^{-9}V$

Force on proton is equal to force due to its mass
$qE=mg$
$q\frac{V}{d}=mg$
$V=\frac{mgd}{q}$
$={10}^{-9}V$