physics question?

Question

Object A is metallic and electrically neutral. It is charged by induction so that it acquires a charge of -2.5 10-6 C. Object B is identical to object A and is also electrically neutral. It is charged by induction so that it acquires a charge of +2.5 10-6 C. Find the difference in mass between the charged objects.

Answer 1

The difference in mass in the two spheres is caused by the difference in the number of electrons in each sphere.
When the spheres are charged, electrons move onto the sphere (in the case of a negative charge) or off the sphere (in the case of a positive charge). Each electron has a certain mass, so by dividing the difference in the charges on the spheres by the charge per electron, we can find the difference in the number of electrons. Then multiplying this by the mass of an electron we can find the difference in the spheres’ masses.

Difference in charge = (+2.5 E-6 C) – (-2.5 E-6 C) = (5.0 E-6 C)

Magnitude of charge on a single electron = 1.602 E-19 C

Difference in the number of electrons = (5.0 E-6 C) / (1.602 E-19 C per electron) = 3.125 E13 electrons.

The rest mass of an electron = 9.109 E-31 kg

Difference in mass of spheres = (3.125 E13 electrons) * (9.109 E-31 kg per electron) = 2.85 E-17 kg

So the difference in the masses of the two spheres is only about 2.85 E-17 kg, which for all intensive purposes is totally negligible.

http://www.physlink.com/Reference/PhysicalConstants.cfm