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Unit 3: Atoms and Light—Exploring Atomic and Electronic Structure

Section 2: The Electron

Cathode Ray Tube Television, ca. 1953

Figure 3-2. Cathode Ray Tube Television, ca. 1953

© Wikimedia Commons, Public Domain.

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Cathode Ray Tube Television, ca. 1953

Figure 3-2. Cathode Ray Tube Television, ca. 1953

Cathode ray tubes found their greatest use in television sets and computer monitors. Inside the CRT, a tightly focused beam of electrons, steered by electromagnets, hits a detector on the inside face of the tube, which glows and creates the image as the beam is scanned quickly across and up and down over the detector. In color televisions, three separate beams light up red, green, and blue detectors. Today, cathode ray tubes are being replaced by sleeker flat screen technology.

The first indication that the atom could be divided into smaller parts stemmed from experiments with an early precursor to neon signs and televisions—cathode ray tube (CRT). (Figure 3-2) Sir Joseph John (J.J.) Thomson (1856–1940), a British-born physicist, sought to understand the glowing beam created within a CRT, and his experimentation resulted in the discovery of the electron.

In the late 1800s, scientists experimented by running high-voltage electricity through sealed glass tubes, which had been outfitted with two metal electrodes that pass through the glass casing and are separated by a distance of a few centimeters. When most of the air has been pumped out, and the electricity is applied, electrons are ejected from the negative electrode inside the tube and accelerate as they approach the positive electrode. When the high-speed electrons hit a detector screen placed inside the tube, they create a glow, making their path visible. The beam was called a "cathode ray" because its direction of travel was from the negative connector (the cathode) to the positive one; scientists called this apparatus a "cathode ray tube." When objects were placed in the path of the beam, they cast a shadow, providing evidence that some form of matter was passing through the tubes. (Figure 3-3)

Cathode Ray Tube

Figure 3-3. Cathode Ray Tube

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Cathode Ray Tube

Figure 3-3. Cathode Ray Tube

When high-voltage electricity is applied across the electrodes at either end of the tube, electrons are knocked off gas molecules inside the evacuated tube and travel toward the positive anode. Inside the tube, this stream of electrons hits a specially coated screen and is visible as a bright line. Early cathode ray tubes were perfected by the British physicist, William Crookes and further developed by J.J. Thomson.

Thomson named these particles "corpuscles." (Later, the scientific community decided to use a previously coined term: "electron.") Thomson inferred that his electrons are a part of every atom. He tried making the electrodes out of different metals, and they always produced the same results: a glowing beam. Then, he tried deflecting the beam and discovered that the beam curved in response to both magnets and electric fields. This supported the idea that the particles in the beam were negatively charged.

As a physicist, Thomson assumed that electrons emit light only when they are in motion. Since matter does not always glow, he devised a model of matter, which he called the "Plum Pudding Model," in which the electrons are evenly distributed and held in place by a positive pudding-like "goo." (Figure 3-4)

Plum Pudding Model of the Atom

Figure 3-4. Plum Pudding Model of the Atom

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Plum Pudding Model of the Atom

Figure 3-4. Plum Pudding Model of the Atom

The brown pudding is the positively charged substance that keeps the raisins, or electrons, in place. The positive forces push on all sides of the electrons, keeping them stationary.

Thomson's discovery of the electron was an enormous accomplishment because, for the first time, the atom was determined not to be the smallest indivisible particle of matter, but rather the atom itself had smaller, or subatomic, components. Thomson was not able to determine the mass or charge of the particle independently. However, using the deflection of the beam by an electric field, Thomson was able to measure the electron's mass-to-charge ratio.

By 1897, Thomson could conclude that the cathode rays were composed of particles more than 1,000 times smaller than the lightest atom, hydrogen. Another scientist, Robert Millikan, taking advantage of Thomson's calculated mass-to-charge, used the deflection of tiny drops of oil in another experiment to determine exact values of the mass and charge of an electron.

Glossary

Cathode ray tube

An evacuated tube with two electrodes inside it. High voltage electricity is applied to the negative electrode, creating a stream of electrons that travel to the positive electrode.

Electrons

Negatively charged subatomic particles.

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