Photoelectric effect

The photoelectric effect is the emission of electrons from a usually metallic surface upon exposure to, and absorption of, electromagnetic radiation, such as visible light or ultraviolet radiation. An older term for the photoelectric effect was the Hertz effect, though this phrase has fallen out of current use.http://scienceworld.wolfram.com/physics/HertzEffect.html

Uses and effects

Solar cells (used in solar power) and light-sensitive diodes use the photoelectric effect. They absorb photons from light and put the energy into electrons, creating electric current.

Related Topics:
Solar cell - Solar power - Diode - Electric current

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Electroscopes

Electroscopes are fork shaped, hinged metallic leaves placed in a vacuum jar, partially exposed to the outside environment. When an electroscope is charged positively or negatively, the two leaves separate, as charge distributes evenly along the leaves causing repulsion between two like poles. When ultraviolet radiation (or any radiation above threshold frequency) is shone onto the metallic outside of the electroscope, the negatively charged one will discharge and collapse, while nothing will happen to the positively charged one. The reason is that electrons will be liberated from the negatively charged one, gradually making it neutral, while liberating electrons from the positively charged one will make it even more positive, keeping the leaves apart.

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Photoelectron spectroscopy

Since the energy of the photoelectrons emitted is exactly the energy of the incident photon plus the material's work function or binding energy, the work function of a sample can be determined by bombarding it with a monochromatic X-ray source or UV source (typically a Helium discharge lamp), and measuring the kinetic energy distribution of the electrons emitted.

Related Topics:
Monochromatic - X-ray - UV - Helium

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This must be done in a high vacuum environment, since the electrons would be scattered by air.

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A typical electron energy analyzer is a concentric hemispherical analyser (CHA), which uses an electric field to divert electrons different amounts depending on their kinetic energies. For every element and core atomic orbital there will be a different binding energy. The many electrons created from each will then show up as spikes in the analyzer, and can be used to determine the elemental composition of the sample. {{ref|spectroscopy}}

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Spacecraft

The photoelectric effect will cause spacecraft exposed to sunlight to develop a positive charge. This can get up to the tens of volts. This can be a major problem, as other parts of the spacecraft in shadow develop a negative charge (up to several kilovolts) from nearby plasma, and the imbalance can discharge through delicate electrical components. The static charge created by the photoelectric effect is self-limiting, though, because a more highly-charged object gives up its electrons less easily.{{ref|spacecraft}}

Related Topics:
Spacecraft - Volt - Static charge

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Moon dust

Light from the sun hitting lunar dust causes it to become charged through the photoelectric effect. The charged dust then repels itself and lifts off the surface of the moon by electrostatic levitation. This manifests itself almost like an "atmosphere of dust", visible as a thin haze and blurring of distant features, and visible as a dim glow after the sun has set. This was first photographed by the lunar surveyor in the 1960s. It is thought that the smallest particles are repelled up to kilometers high, and that the particles move in "fountains" as they charge and discharge. {{ref|moondust1}} {{ref|moondust2}}

Related Topics:
Lunar dust - Electrostatic levitation - Lunar surveyor

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