Scientists separate substances into many different categories according to their behavior towards different conditions for better understanding and easily remembering them. One of the ways to categorize substances is through their behavior on electricity, that is, how well they pass electricity through themselves. But how electricity passes through a substance? Of course that depends on what substance we are talking about.
In general, all atoms consist of a number of electrons that revolve around the atom core consisting of protons along with other particles such as neutrons. Atoms are bonded together to create individual molecules or a crystal network. Electricity is usually passed through material by the electrons that carry negative charge. They are extremely smaller than the atom core and the atom core itself is about 1/2000 of the atom size. So you may wonder what fills the space around the atom core? It is filled with electrons equal in number to the protons in the core! They move so fast, almost close to the speed of light around the core in random paths so that no one can tell where the electron is. It is like electrons are everywhere in the empty space around the core, although their size is close to zero compared to this space surrounding the core. Actually each one of them orbits around the core in a specific range of distance from the core, called Orbital. Every electron falls in a different orbital depending on the energy it carries. That's why orbital is also referred to as Energy Band. The higher energy an electron has, the outer orbital it falls in. The lower energy electrons will be closer to the core and are much more stable, meaning that it is much harder to move them around and disconnect them from the core, unless with injecting a huge energy into the atom. But outer electrons already have a much higher energy level and disconnecting them from the atom requires much less energy. When they are disconnected from the atom, they would be able to get away from it, which results in an electrical current.
Yet in some substances it is not as easy as it sounds, and in some it is very easy. This is how material are categorized into different divisions of electrical properties. There are four categories for electrical material: conductors, insulators or nonconductors, semi-conductors and super conductors. As the science goes further ahead, new categories maybe added, like the almost recently added super conductor category. These categories are discussed below.
Conductors, such as metals like copper, iron or gold are substances that very easily can pass the electricity. The electrons in their outer orbital have a high energy and are not firmly held by their atom cores. With only a very small energy they start moving out of the atom to shape an electrical current. Every substance can pass some amount of electricity through it. But only substances with a current flow in a noticeable or useful range fall in the conductor category.
Insulators or Nonconductors
These material such as wood or plastic show a huge resistance against electricity. As mentioned before electricity can pass through everything, but in insulators it is so small that sometimes it is not measurable. Even if there is an electrical current through them, it is called a leakage current, as normally it is assumed that there is no current passing through them. The electrons in these material are strongly bonded in their orbital around atom cores and it is not possible to break them apart, unless under a huge potential energy that as explained in the Electrical Charge section, it is called the Break Down potential. In this case the molecules will break into ions and usually their structure falls apart while they allow the electricity to pass and the original material will be damaged or burnt.
Semi-conductors are what made the technology advance with a fast speed. Their interesting behavior made the design of many sophisticated devices possible. Semi-conductors are the material used in components such as transistors, diodes or integrated circuit (ICs). Their molecular structure and sophisticated behavior falls within the Electronic Engineering and Systems interest. For a more complete discussion on semiconductors please refer to Semi-Conductors section in electrical systems.
Semi-conductors can have different behavior towards passing the electricity. Depending on how they are used, they can act as a total insulator or a very good conductor. Even sometimes their properties can be adjusted to a suitable point. They are also sensitive to light and that makes them good light sensors. Their excellent features has opened a door to a non stopping development of technology.
As mentioned before, all substances pass some electricity through them and each shows a different resistance against passing the electricity. Even the best conductors such as gold shows a very small resistance against the electricity. But super conductors are material that under some conditions, that is temperatures very close to zero degree Kelvin or -273 degree Celsius, show no resistance towards passing the electricity. That's why they are called super conductors.
But why so cold? One of the sources of resistance in substances is the molecular or atomic vibration. As the atoms vibrate, it becomes harder for the electrons to pass these atoms without a friction between the two. The more energy a substance has, the more vibration its atoms will have and therefore the more resistance it shows. In contrast, when the substance gets colder, there will be less vibration and lower resistance. That's why some substances which make super conductors show no resistance in extremely low temperatures.
But it is not easy or practical to make a device using super conductors, as it always has to be in a very low temperature. That's why scientists are trying hard to find new material that won't require low temperatures to show super conductive features, and they have achieve some success in finding super conductors at higher temperatures. Yet it still needs to be very cold to be freely used in devices outside laboratories.
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