# Number of electrons when something is charged

1. Feb 10, 2013

### jperentosh

When something is charged, is the number of excess electrons a large or small number when compared to the total number of atoms?

2. Feb 11, 2013

### Khashishi

On Earth, usually very small.

Consider a 2600 farad, 2V ultracapacitor, which is one of our best devices for storing charge. We can store 5200 coulombs, which is about 3.2 * 10^22 elementary charge units. The ultracapacitor has a mass of about 0.5kg, which is probably somewhere along the lines of 10^25 atoms, roughly.

Consider lightning. Wikipedia says that an average bolt of negative lightning carries 15 coulombs, which is 9.4 * 10^19 elementary charge units. On the other hand, a cubic meter of air has on the order of 10^25 atoms.

Last edited: Feb 11, 2013
3. Feb 11, 2013

### Staff: Mentor

When you charge ordinary objects, e.g. by rubbing a balloon with fur, the amount of charge involved is on the order of tens of nanocoulombs (10-8 C), i.e. about 1011 electron charges. I leave it to you to estimate the number of atoms in a typical balloon, but it's surely many orders of magnitude larger.

We measure the charge in an introductory lab in which students charge up two balloons, then suspend them on strings hanging from the same point. The balloons repel to form an inverted V. By measuring the distance between the balloons and the length of the strings, and assuming the charges are equal, one can calculate the charge using a force diagram for one of the balloons.

Last edited: Feb 11, 2013
4. Feb 11, 2013

### ZapperZ

Staff Emeritus
Please note that when you say "something is charged", it need not always be due to excess of electrons. It can easily be due to shortage of electrons.

Zz.