B Can we say that a charged balloon has a center of charge?

[Lotto]

TL;DR

We can say that the balloon has a center of mass circa in the middle. When we charge the balloon so that charge density is everywhere the same, can we say that the center of the total charge is in the middel as well?

Doing so, we can consider the balloon to be a point charge (approximately). Can we do it in this case, when there are only electrons on its surface? Or is it stupid and we can't do it under any circumstances?

 

[PeroK]

TL;DR Summary: We can say that the balloon has a center of mass circa in the middle. When we charge the balloon so that charge density is everywhere the same, can we say that the center of the total charge is in the middel as well?

Doing so, we can consider the balloon to be a point charge (approximately). Can we do it in this case, when there are only electrons on its surface? Or is it stupid and we can't do it under any circumstances?

The shell theorem tells you that a uniformly charge sphere or spherical shell creates the same external electric field as a point charge with the same total charge. And, the field inside a uniformly charged spherical shell is zero.

You should look up a proof of the shell theorem. It applies to electric fields and gravitational fields and, in fact, anything that obeys the inverse square law.

 

[Lotto]

The shell theorem tells you that a uniformly charge sphere or spherical shell creates the same external electric field as a point charge with the same total charge. And, the field inside a uniformly charged spherical shell is zero.

You should look up a proof of the shell theorem. It applies to electric fields and gravitational fields and, in fact, anything that obeys the inverse square law.

And if I have two charged balloons and distance between their centers of masses is $r$, can we say $F_\mathrm e=k\frac {Q_1 Q_2}{r^2}$?

 

[PeroK]

And if I have two charged balloons and distance between their centers of masses is $r$, can we say $F_\mathrm e=k\frac {Q_1 Q_2}{r^2}$?

As long as they are spherical and uniformly charged - and assuming the centre of mass is at the geometric centre of the circle - then yes!

 

[jbriggs444]

And if I have two charged balloons and distance between their centers of masses is $r$, can we say $F_\mathrm e=k\frac {Q_1 Q_2}{r^2}$?

To the extent that the balloons are spherical, have a uniform thickness (so a spherically symmetric mass distribution) and that the balloons are non-conducting so that a spherically symmetric charge distribution matching the uniform mass distribution is not affected by the approach of the other charged balloon, the answer is yes. The formula will work.

Note that there is no guarantee that the charge distribution will match the mass distribution. But I am assuming that you intend for the two to match.

 

[Herman Trivilino]

Doing so, we can consider the balloon to be a point charge (approximately).

Any distribution of point charges can be approximated as a single point charge if you are far enough away from it.