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?

Solution

## The correct option is D decrease by an amount too small to measure directlySuppose the positive charge has the large value $$1\mu C$$. The object has lost some of its conduction electrons, in number$$10^{-6}C(1 e/1.60\times 10^{-19} C)=6.25\times 10^{12}$$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 $$10^14$$ times smaller than the $$\sim 1g$$ mass of the coin, so it is an immeasurably small change.Physics

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