Download may11_slides

Physics 2005…in 2009
Auguste Coulomb
Coulomb’s Torsional Balance
How did mankind begin to find out about the atom?
BIG SURPRISE FOR
RUTHERFORD:
MOST PARTICLES WENT
STRAIGHT THROUGH
THE GOLD FOIL.
Explaination:
“much of the atomic volume
was now assumed to
contain only the very light
electrons, which could not
produce much deflection.
Accordingly, many alpha
particles would pass
through the atom, deflected
only slightly by the electrons
and the long-range
Coulomb interaction with
the nucleus”
How did mankind measure charge?
Milliken
first let oil drops fall until they reached terminal velocity.
Electric force
Measured the terminal velocity.
at terminal velocity the oil drop is not accelerating.
So the total force acting on it must be zero, gravity+drag=0 ,
This lets us calculate the mass of each oil drop
Oil
Gravity
Finally, use an electric field to stop the falling –then the force of
Gravity is exactly cancelled by the electric force, which determines
The charge.
x
Milliken’s actual drawing, he measured e=1.5924(17) x 10^−19 coulombs
which is a touch low, due to an error in the viscosity of air