Download Millikan`s Idea Robert Millikan was a scientist who studied electricity

Millikan’s Idea
• Robert Millikan was a scientist who studied
electricity.
• During the early 1900’s, scientists thought
of electricity as a fluid.
• They had the concept of CHARGE, but not
of ELECTRONS!
• Millikan set about to find out if there were
‘bits’ of charge
Things Millikan Knew
• Acceleration due to gravity
• An object with a net force of 0 moves with a
constant velocity
• Droplets can be charged using x-ray’s
• “Stokes’ Theorem” – mass of droplets
• How to find the electric field between two plates
Millikan’s Apparatus
Millikan sprayed oil droplets into a chamber with
charged plates at the top and bottom
The droplets come out with a CHARGE on them
They areThey
attracted
to one plate or the other
are attracted to one plate or the other
depending on their charge!
Millikan’s Apparatus
Millikan noticed that when he adjusted the electric field,
some of the droplets would not move UP or DOWN.
WHY???
Fg must be equal to Fe
forces
on an
object
WhatWhat
do we
knowact
about
these
twoinforces?
gravitational and electric fields?
Equations
Millikan looked at this using
the equations:
Fe  Eq
Fg  mg
He noted that droplets
only stopped moving
when the electric field
was set to
certain strengths.
Eq  mg
mg
q
E
What does this suggest?
Electrical “Bits”
• Millikan did many observations, collecting
thousands of data samples for charge on oil
droplets.
• This experiment has been repeated by hundreds
of physicists (and university physics students)
with the same results.
• Charge ONLY occurs in
multiples of 1.6 x 10-19C