A charged rod is touched to a neutral object. When the rod is removed the object is found to have a positive charge. Let's say the object acquires a charge of +1 C (this is actually an extremely large charge, but let’s not worry about that). Is the object heavier or lighter than when it was neutral? If so, by how much?

1. Heavier
2. Heavier by just a small amount
3. The mass is unchanged
4. Lighter by just a small amount
5. Lighter

Charging an object almost always involves transferring electrons. If that is the case here the object must have lost electrons, making it lighter. You could figure out how many electrons it takes to give 1 C, and then determine the mass of that many electrons. An equivalent approach is to say that the charge-to-mass ratio of the electron is:

e m

=

1.60 x 10-19 C 9.11 x 10-31 kg

=

1.76 x 1011 C/kg

To produce a charge of 1 C we must transfer 5.69 x 10^-12 kg worth of electrons.