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Question: Why do superconductors allow electric current to pass through them?
First, you should know that all materials allow electrical current to pass through them to some extent. Electrical insulators such as glass and some plastics are poor electrical conductors but they are useful because they prevent large electrical currents from flowing where we don't want it to flow.
On the other hand, metals are very good conductors of electricity and they are useful in bringing electrical power to our homes. Semiconductors have an intermediate level of electrical conductivity and they are useful because their conductivity can be easily modified. This property makes them useful in transistors and computer chips.
But even the best "normal" (non-superconductive) metals generate some heat when electrical current flows through them. Materials conduct electricity by means of charged particles called electrons. Electrons move through the material when we apply an electric field. But the electrons will collide into vibrating atoms and loose energy when these collisions occur. This is a type of friction. The lost energy appears as heat.
To appreciate how even a little friction prevents perfect motion think about ice-skating. Even though there is hardly any friction that could slow you down, the little friction that still remains will eventually stop unless you supply more energy with your legs.
Superconductors are something different. Superconductivity was discovered in 1911 but not fully explained until 1957, which demonstrates how profound your question is.
A superconductor is a material through which a direct current can flow without any resistance. How can this frictionless flow occur? It is because high frequency sound waves, far beyond what we can hear, tend to bind two electrons together. These are called "copper pairs" named after Leon Copper, one the physicists who first explained superconductivity. Copper pairs tend to align themselves in a common "march" through the superconductor. The energy required to stop the army of pairs is so great that vibrations of the atoms cannot do this. Therefore unimpeded, frictionless current can flow.
Above the critical temperature, the highest temperature at which a material becomes superconducting, the density of copper pairs is zero. As the temperature is lowered towards absolute zero, all of the electrons form copper pairs.
"High temperature superconductors" were discovered in 1986. These materials can have critical temperatures above 100 kelvins (above minus 173 degrees Celsius). But physicists have yet to completely understand why these strange nonmetallic materials are superconductors. This is another example of the difficulty of the question you asked.
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