In the early 1900s we did not know what the charge
of the electron was
Millikan figured that if the force exerted by
an electric field was strong enough, it could balance the
weight force due to gravity
By measuring the field strength needed to
balance many different droplets of charged oil, he figured out that the
charges were multiples of \(1.5924(17)\times10^{−19}C\)!
Collect a Styrofoam Balls and Electric Fields sheet
from the front.
Come up the front and observe the behaviour of the
balls as the voltage is varied.
Use your observations and our discussion to answer
Task 1-2.
Ngohe/Task
Homework booklet Electric Fields A5
In 1909 Millikan used two oppositely charged metal plates to keep a
charged oil drop falling at terminal velocity when he was experimenting
to find the charge of an electron. A modified form of his experiment
keeps an oil drop stationary.
d) Discuss how it was possible to make the oil drop
stationary between the plates. (E)
In your comprehensive answer you should:
identify the forces acting on the oil drop
describe how the forces can combine to cause the
oil drop to be stationary
explain what type of charge the oil drop must have
in order to remain stationary.
Whakatika
Achieved:
Identifies the two forces
OR says that the forces must be balanced
OR Top plate should be positive and the charge on
the oil must be negative, or vice versa.
Merit:
Identifies the forces AND that the forces must be
balanced, as the charged drop is stationary.
Excellence:
Explain that the electrical force must be upwards
to balance the downward weight force, as the oil drop is stationary
AND For the electric force to be upwards, the
charge on the oil drop must be negative.
