MadSci Network: Physics

Re: How was it determined that a coulomb is the charge of 6.24X10^18 electrons?

Date: Sun Aug 26 18:23:10 2007
Posted By: Martin Smith, Engineering, B.E., M.EngSc., Uni of Qld / airline pilot
Area of science: Physics
ID: 1184187395.Ph

The value for the charge of an electron was determined using the so called
Millikan Oil Drop experiment.

Very basically charged droplets of oil are suspended in an electric field.
 The electric field is known and the weight of the droplet is calculated. 
Now the force (which equals the weight) suspending the droplets is due to
the field (known) and the charge (unknown) on the droplets.  So the charge
on that droplet can be determined.  The charge is made up of "free"
electrons on the oil droplet.

The experimented is repeated many many times.  Eventually a discrete
integer value can be determined that is a factor of all the various
different charges the oil droplets had, and of the difference in charge the
droplets had.  The charge, and the difference in charge, must be an integer
number of electrons.    This number is thus the charge of a single electron.

Read more on it here

Martin Smith

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