# Q & A: Electrons in a Coulomb

Q:
how many units of charge (electrons) are there in one coulomb?
- sam (age 20)
Uni o’ Auckland NZ
A:
A collection of 6.2415 times 10 raised to the 18th power would have a charge of approximately -1 Coulomb.

Tom

(published on 10/22/2007)

## Follow-Up #1: How many electrons in one Coulomb

Q:
How do you calculate that one coulomb is equal to 6.24x10^18 electrons
- Mary (age 22)
Shawnee, OK USA
A:
The Coulomb is a derived unit.   That is, its value depends on the basic units of the metric system, the meter, the kilogram, the second, and the Ampere.   So one Coulomb is defined as the amount of charge collected by an electric current of one Ampere in one second.  The Ampere is defined as the amount of current required to produce a certain magnetic force between a pair of current carrying wires. You still need one more thing:  how many electrons per second equals one Ampere.  The Millikan oil drop experiment which measures the charge on a single electron provides that answer. (There are more modern experiments which get more accurate numers./mbw)     So in sum:  you need to define a consistent set of basic units that span all of the possible things you can measure.  You can explore this further by looking at the Wikipedia article:

LeeH

(published on 10/22/2007)