Question

A sphere of mass $50\text{g}$ is suspended by a string in an electric field of intensity $5{\text{NC}}^{-1}$ acting vertically upward. If the charge on the sphere is $4\times {10}^{-3}\text{C}$ then the tension in the string is $(g=10{\text{ms}}^{-2})$

• A. $520\times {10}^{-3}\text{N}$
• B. $520\times {10}^{-2}\text{N}$
• C. $520\text{N}$
• D. $480\times {10}^{-3}\text{N}$

Answer 1

The correct option is D $480\times {10}^{-3}\text{N}$


From the FBD of the bob,

$T+F_e=W$

$T+Eq=mg$

$T=mg-Eq$

$=(50\times {10}^{-3}\times 10)-(5\times 4\times {10}^{-3})$

$=(500-20){10}^{-3}$

$T=480\times {10}^{-3}\text{N}$

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> Hence, (d) is the correct answer.