Question

A metallic coin is given a positive electric charge. Does its mass

• A. increase measurably
• B. increase by an amount too small to measure directly
• C. remain unchanged
• D. decrease by an amount too small to measure directly
• E. decrease measurably?

Answer 1

The correct option is D decrease by an amount too small to measure directly

Suppose the positive charge has the large value $1\mu C$. The object has lost some of its conduction electrons, in number
${10}^{-6}C(1e/1.60\times {10}^{-19}C)=6.25\times {10}^{12}$
And in mass
$6.25\times {10}^{12}(9.11\times {10}^{-31}kg)=5.69\times {10}^{-18}kg$
This is on the order of ${10}^{1}4$ times smaller than the $\sim 1g$ mass of the coin, so it is an immeasurably small change.