Photovoltaic cells (PV cells), or more commonly called solar cells or solar panels convert light into electrical current. The process is called the photovoltaic effect which is mostly the same as the photoelectric effect. In the photovoltaic effect, a "charge carrier", which is typically an electron, absorbs a photon and becomes "excited" to a higher energy level or a higher voltage. This increased voltage causes electrical current to flow.

Protons are part of the atomic nucleus and cannot normally have their energy changed and be made to move around by photon absorption.

Photons are the particles that make up all electrical magnetic radiation (light, radio waves, microwaves, etc).

"Holes" refers to a semiconductor model, and while PV cells are made of semiconductor material (silicon), holes and electrons are normally discussed when talking about transistors. It is worth mentioning that in the electron/hole model, when an electron is ejected from the PV cell, it does leave a hole, but that hole is promptly filled by a lower energy electron due to the current flow in the circuit.

Silly Hint: Energy from Light... EL - electrons

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Photoconductivity is the property of a substance to conduct electricity based upon the intensity of the light shining on it. Brighter light decreases resistance in the substance, resulting in better conductivity and vice versa.

Photoconductive materials are used in semiconductors.

Logical hint: Photoconductivity can be broken down into two parts: Photo means "light," and conductivity means how conductable something is. So photoconductive materials get more conductive when exposed to light.

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This feature is found in in some high-frequency transceivers, and is used to transfer electrical signals between two isolated circuits by using light. It's essentially an LED focusing light on a phototransistor.

An opto-isolator is primarily used to prevent high voltages (line spikes, RF, lightning, electrostatic discharge, etc) from damaging equipment.

https://en.wikipedia.org/wiki/Opto-isolator

Note that sometimes an opto-isolator is called an optocoupler or even an "optical relay". These are all terms for the same thing.

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The photovoltaic effect is the creation of voltage or electric current in a material upon exposure to light.

The standard photovoltaic effect is directly related to the photoelectric effect, though they are different processes. When the sunlight or any other light is incident upon a material surface, the electrons present in the valence band absorb energy and, being excited, jump to the conduction band and become free. These highly excited, non-thermal electrons diffuse, and some reach a junction where they are accelerated into a different material by a built-in potential (Galvani potential). This generates an electromotive force, and thus some of the light energy is converted into electric energy.

Source: Wikipedia - Photovoltaic Effect

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Wikipedia definition of the device. http://en.wikipedia.org/wiki/Rotary_encoder

So, what can we do with an optical shaft encoder (optical rotary encoders)? Sound like a fancy name for a position detector. The number of applications can be enormous, but narrowing down for anything Ham related they can be used for detecting the:

1. Position of a VFO knob
2. Position of an rotary antenna
3. Position of a volume control
4. etc...

Memory Trick: A wheel rotates around a shaft.

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Photoconductive devices are made from materials that change their electrical conductivity when exposed to light. Crystalline semiconductors, such as silicon and gallium arsenide, are the most commonly used for this purpose. When light photons hit the semiconductor, they excite electrons, increasing the material’s conductivity.

Other materials listed in the question, such as polyphenol acetate and argon, are not suitable for photoconductive devices. Polyphenol acetate is a type of plastic used in adhesives and films, and argon is an inert gas often used for shielding in welding—not for creating photoconductive elements.

In summary, crystalline semiconductors are the best answer because their electrical properties can be precisely controlled, making them ideal for devices like light sensors and photodiodes.

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The key is the term "solid state" meaning no moving parts. Otherwise it performs very similar to a mechanical relay, except for much faster and with greater reliability, so long as it is operated within its design specifications.

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Follow this link for a good explanation of the optoisolator:

https://en.wikipedia.org/wiki/Opto-isolator

Hint: Correct answer has "isolation" and "switched," reflecting verbiage in question stem ("optoISOLATORS" and "switching.")

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The best (and correct) answer is that the efficiency of a photovoltaic cell is the relative fraction of light that is converted to current.

Efficiency is expressed as a ratio or percentage. It can range from 0 to 1, or 0% to 100%. It does not have a unit. You can eliminate the open-circuit voltage divided by short-circuit current option because this gives you a number expressed in ohms, which is not unitless.

You can eliminate RF output power divided by input DC power because the question does not ask about the efficiency of a transmitter or any other radio, it asks about the efficiency of a photovoltaic cell (which doesn't involve RF power at all).

The effective payback period is not a term used to describe any kind of electrical phenomena.

Hint: Only the answer has the word light in it.

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Memory Trick: Silicon Solar Cell

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This is one of those question/answer pairs that just has to be memorized. Fully-illuminated silicon photovoltaic cells produce about half a volt.

Test tip: It is currently theorized that someday we may see solar cells approaching 50% efficiency in converting light to electricity. While voltage and efficiency are not actually directly related in this way, as a memory aide it serves well enough to say that "0.5" is our goal for such cells.

Test Tip: Photovoltaic cells (solar panels) produce power only about half (0.5) of the time (during the day, not during the night).

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