Electron Specific Charge

Electrons have a specific charge of 1.6 ⋅ 10 ^ -19 C. But how do we know this? Scientists have worked hard to figure it out. Knowing the specific charge of an electron is really important for physics. It helps us understand how electric fields work and how electrons move through them.

How was the specific charge of an electron determined?

In 1897, JJ Thomson conducted experiments with cathode rays, or as they were known then, gas discharge tubes. Thomson discovered the existence of negatively charged particles which he called 'corpuscles'. Even though this was the first observation of subatomic particles, they were not universally accepted at the time.

JJ Thomson working on his experiment with cathode rays

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JJ Thomson made progress by measuring the ratio between an electron's charge and its mass (qe / me), and estimating an electron's mass to be about 5.56 ⋅ 10 ^-4 times that of a hydrogen atom. However, he could not determine the exact charge of a single electron. Other scientists like George FitzGerald and Walter Kaufmann also experimented with electricity and magnetism, but only found that a charge is a continuous variable. These results were important, but not enough to explain the photoelectric effect.

In 1909, Robert Millikan and Harvey Fletcher conducted 'the oil-drop experiment' to determine the specific charge of a single electron. Today, it is known as Millikan's experiment.

Millikan's oil drop experiment

In Millikan's experiment, two metal plates were stacked on top of each other with an insulating material in between. The insulator had three holes for light to enter and one for examination. A potential difference was applied across the plates to create a uniform electric field. Some were charged due to friction from the nozzle when they were sprayed. An ionising radiation source, like an X-ray tube, could also be used to charge the droplets.

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Where:

m is the mass of the drop in grams

g is the gravitational constant, 9.8 m/s^2 at Earth's surface

qe is the charge of the electron in Coulombs

E is the electric field in Newton/Coulomb

This also meant that the drop was allowed to fall at its terminal velocity (v) when the voltage source was turned off. The mass of the drop was determined by how rapidly it descended when the voltage was turned off. Since we know that the voltage (V) was adjusted to balance the forces on the drop, and the electric field (E) was a product of the voltage applied, we can show it with the equation below. d is the distance between the plates in meters.

The charge of the electron may be estimated using the rearranged equation below after the mass of the drop is known.

V is the voltage that holds the drop stationary.

Millikan had measured the charge of the electron qe to an accuracy of 1 percent and had raised it by a factor of 10 to a value of -1.60⋅10^-19 C within a few years.

The importance of the specific charge of an electron

Determining the specific charge of an electron is one of the turning points in physics, which led to several new discoveries. Let's have a look at the role this discovery played.

Since the charge of an electron is related to its mass, determining the specific charge of an electron also meant that the mass of an electron was determined.

With the determination of the specific charge of an electron, the existence of subatomic particles was universally accepted. (Electrons being the first subatomic particles to be discovered.)

The specific charge of an electron is 1.6⋅10 ^-19. Robert Millikan and Harvey Fletcher conducted the oil-drop experiment in 1909 to determine a single electron's specific charge. The experiment involved placing two horizontal metal plates on top of each other with an insulating substance between them, pierced with four holes. The experiment not only determined the specific charge of an electron but also its mass. Understanding the specific charge of an electron and its mass helped in comprehending the structure of an atom. The discovery of the proton and the neutron further enhanced this understanding. Today, we know that the proton-electron mass ratio is mp/me = 1836.15, which sheds light on the roles of subatomic particles and how much of the atom's mass they comprise.