2.1.2 Understanding Electrons

Question 1

Understanding Electrons

Answer 1

• Robert Millikan determined the chargeof an electron by observing oil droplets in an electric field.
• Today, the charge of an electron is accepted as 1.60 x 10^–19 C, and the mass of an electron is accepted as 9.11 x 10^–31 kg.

Question 2

Robert Millikan

Answer 2

• Robert Millikan determined the charge of an electron by observing oil droplets in an electric field.
• In the Millikan oil drop experiment, oil is atomized into small droplets, and then allowed to fall through a hole into the chamber. Droplets fall because of gravity. An x-raybeam knocks electrons off of air molecules, and some of those electrons become attached to the oil droplets. The negatively charged oil droplets feel an upward force from the electrical potential.
• With the proper choice of an electric field, FC (the Coulomb force) will exactly balance FG (the gravitational force), and the droplets will become motionless. This means that q = mgd/V, where q is the charge of a particular droplet, m is the mass of that droplet, g is the acceleration due to gravity, d is the distance between the plate and the droplet, and V is the electrical potential. Since the mass of the droplet can be determined by measuring the terminal velocity when the electric field is turned off, the only unknown is q.
• Through repeated measurements, Millikan found that the charge of each droplet was an integer multiple of 1.59 x 10–19 C. This was the charge of an individual electron.

Question 3

Thomson

Answer 3

• J. J. Thomson had previously shown that the charge-to-mass ratio of an electron was 1.76 x 1011 C/kg. Using this ratio and the experimentally determined charge, the mass could also be calculated.
• Today, the charge of an electron is accepted as 1.60 x 10^–19 C, and the mass of an electron is accepted as 9.11 x 10^–31 kg.

Question 4

Why were X-rays a necessary part of Millikan’s experiment?

Answer 4

The X-rays caused the oil drops to become charged.

Question 5

Millikan determined the charge of an electron, which is _____________ .

Answer 5

1.60 × 10^−19 C

Question 6

In his experiment, Millikan was able to determine the __________ of an electron. He then used the ratio discovered by Thomson to find the __________ of an electron.

Answer 6

charge; mass

Question 7

How did Millikan use counteracting forces in his oil-drop experiments?

Answer 7

He used an electric field to attract a charged oil droplet, thus counteracting gravity to keep an oil droplet suspended in air.

Question 8

Whose findings did Millikan use to help determine the mass of an electron?

Answer 8

Thomson

Question 9

With his oil-drop experiments, Millikan determined __________ .

Answer 9

the charge of an electron

Question 10

Who conducted experiments to calculate the charge and mass of an electron?

Answer 10

Millikan

Question 11

In order to calculate the mass and charge of an oil drop, what did Millikan first need to determine?

Answer 11

The terminal velocity of the oil drop.

Question 12

By determining the charge of an electron Millikan should also have been able to determine the charge of ___________ .

Answer 12

a proton

Question 13

What was the purpose of the microscope in Millikan’s oil-drop experiment?

Answer 13

to visualize the oil droplets