Superconductors

There are actually not two but four classes of conductive substance. The first group comprises superconductors. Early in the 20th century, physicists discovered that when certain metals are cooled below their transition temperatures (typically less than 20 K), they lose all resistance to electron flow. This means that a current can flow indefinitely without any electrical potential to drive it – it was, and remains, the closest thing to perpetual motion; imagine having a car that, once started, would continue without ever decelerating (the catch, of course, is the amount of energy required to cool to and maintain a temperature of 20 K).

In the 1980s, a breakthrough occurred when it was found that certain ceramic compound materials which, at normal temperatures are extremely poor conductors of electricity, when cooled sufficiently would also act as superconductors. Unfortunately, nobody knows why, which makes the search for high-temperature superconductors a matter of trial and error with increasingly complicated ‘designer’ molecules.

By 1986, a molecule comprising yttrium, barium, copper and oxygen was exhibiting superconductivity at temperatures of 92 K, above the liquification point of nitrogen. The current record of 138 K is held by a thallium-doped, mercuric-cuprate consisting of the elements mercury, thallium, barium, calcium, copper and oxygen, although recent claims have been made for a lead-doped composite of tin, indium and thulium, which has been reportedly observed superconducting at 181 K, only −92°C. By comparison, the lowest climatic temperature ever recorded is −89.2°C at Vostok in Antarctica in July 1983.

The discovery of a ‘room temperature’ superconductor would not only have huge implications for power generation and storage but could also revolutionize diagnostic imagine in medicine.

Conductors

Conventional electrical conductors are those materials that allow the easy transmission of an electrical current. Most conductors are metals and the charge carriers are the outer electrons of the individual atoms, which can move about the atomic lattice formed by the metal without ‘belonging’ to any one atom. These free-moving, conductive electrons are sometimes referred to as the ‘electron gas’. Ionized gases and electrolytic solutions can also act as conductors; here, the charge carriers are ions.

Typically, electric cables are made from copper or aluminium; however, unlike superconductors, these metals do not conduct perfectly – there is resistance (R) to the electron flow. Using the previous analogy to our perpetually moving car, in the real world the car is slowed by the resistance of the air molecules through which it must move.

Electrical resistance is measured in ohms (Ω). Typically, copper wire offers a resistance of 0.15 Ω per metre length, although this will depend, amongst other things, on the thickness of the wire. Because of these variable factors, a material will be referred to in terms of its resistivity (ρ), calculated by taking the resistance of the wire, multiplied by its cross-sectional area and divided by its length. Conversely, conductance, G, is measured in siemens (S) or, occasionally, the ‘mho’ (‘Ohm’ spelt backwards).

Ohm’s law

The man after whom the unit of resistance is named, Georg Simon Ohm, was the first to experimentally observe the relationship between voltage, current and resistance. This law states that, for a conductor, the amount of current flowing in a material is directly proportional to the potential difference across the material. The constant of proportionality is the resistance. Thus:

Graphically, this can be demonstrated by plotting current against voltage (Fig. 7.3).

Figure 7.3 • Graphical representation of Ohm’s law demonstrating Georg Ohm’s original observations. By altering the potential difference and measuring the current, he demonstrated that the two were directly proportional. The gradient of the resultant curve was the resistance of the material.

Insulators

Materials that do not readily conduct electricity are termed insulators. In reality, they may be regarded as being at the opposite end of a spectrum that starts with superconductors: insulators are in fact merely conductors with extremely high resistivity. Typical examples of such materials are glass, rubber and the plastics that coat electrical cables so that they may be handled without the inadvertent transfer of electricity to the electrician – the classic electric shock! With insulators, the outer electrons are tightly bound to the atoms and are therefore not available to carry charge: the higher the binding energy, the better the insulator.

Semiconductors

Halfway down our spectrum of conductivity lie materials that have both conductive and insulating capabilities. Therein lies the field of solid state physics, a subject whose scope and complexity lie well beyond the limitations of this text; however, this branch of physics underpins the working of nearly every electronic device: computers, mobile phones, televisions, radios, MP3 players and, increasingly, domestic appliances. Anything with a ‘chip’ or transistor is reliant upon the atomic and molecular properties of semiconductors and it is therefore worth understanding the basic principles behind semi-conductors.

In intrinsic semiconductors, such as germanium and silicon, the outer electrons, which are loosely bound, form covalent bonds with neighbouring atoms. They can, therefore, be released quite easily if energy is imparted to the system by increasing its temperature. A free electron will leave behind it a ‘hole’, which may then be filled by another free electron. This ‘hole’ will therefore move from electron to electron, in effect becoming a positive charge carrier. This arrangement is known as a hole–electron pair.

In an extrinsic semiconductor, small traces of impurities are deliberately added in a process known as ‘doping’. If the impurity is from Group III of the periodic table, such as indium or gallium, when it joins with the semiconductor material there will only be three electrons free to fill four valence bonds (silicon and gallium are both from Group IV). An electron is therefore ‘borrowed’ from a neighbouring Group IV atom, creating a ‘hole’ in the same way that a non-doped sample works, albeit more efficiently. This type of extrinsic semiconductor is known as a p-type semiconductor.

By contrast, an n-type semiconductor is obtained when Group V impurities, such as antimony or arsenic, are added. Once the four valence bonds required by silicon or germanium are satisfied, there is an electron left over, which can then act as a charge carrier. In n-type semiconductors, conduction is therefore mainly due to free electrons, although intrinsic ‘holes’ will also still carry positive charge in the opposite direction.

Unlike conventional conductors, the resistance of a semiconductor will drop as its temperature rises as the increase in energy creates more charge carriers.

By sandwiching together layers of p-type and n-type semiconductor material, the transistor was invented to replace the large, cumbersome and unreliable valve components of electronic circuits prior to the 1960s. No longer did radios – which could suddenly fit in your pocket – take half a minute to ‘warm up’. Finally, it was possible to build a computer that didn’t require a room the size of a house to contain it and wouldn’t require a component replacement every few hours.

Within a generation, the transistor was replaced by the integrated circuit, which itself became miniaturized, with p-n-p or n-p-n junctions of atomic widths now appearing in the microchips that allow our radios to store thousands of MP3s whilst fitting in our credit card holders, and our computers to fit into hand luggage. A single, modern chip can contain millions of components packed in to a few square millimetres.

Circuit diagrams

This flow of electricity through a conductive device or apparatus can be best described using a circuit diagram. This is the electrical equivalent of a map and, like a map, it utilizes symbols to describe the topography through which the electricity flows. The most common symbols – and certainly all those you are likely to encounter in the course of your studies and subsequent clinical career – are detailed in Figure 7.4.

Figure 7.4 • Circuit diagram symbols.

Current

This flow of electrons is called a current, I, and is measured in amperes (amps; A), one of the seven basic SI units that, as you may recall from Table 1.1, is defined as:

It is undoubtedly much easier to visualize an amp as being the number of electrons flowing past a point each second; a device that measures this is called an ammeter.

The direction in which electric current flows is important; unfortunately, early physicists assumed that positive charges were moving when electricity flowed and so, on a circuit diagram, the current moves from positive to negative. This is known as conventional current but takes place in the opposite direction to actual electron flow; electrons are of course attracted towards the positive terminal and repelled by the negative. Conventional current is used by electricians and electron flow by physicists. The circuit diagrams you encounter in this book, and most others that you (as clinicians) are likely to read, will be based on conventional current.

In order for a current to flow, there must be a continuous circuit running out from and back to the source of the electric potential: electrons will then flow down the potential gradient (Fig. 7.5). If there is a break in the circuit then no current will flow (this is how a switch works). Current flow can also be disrupted by a short circuit (where two wires within the circuit touch in such a way that the continuous flow of electrons is broken).

Figure 7.5 • In order for a current to flow, there must be a continuous circuit between the two terminals of a battery (B). If there is a break in the circuit (such as that provided by an open switch), no current can flow (A). The same effect is achieved if there is a ‘short’ in the circuit (C).

The type of current provided by a battery gives a continuous flow of electrons (Fig. 7.6A), at least until the battery becomes ‘flat’ when its charge is exhausted. Unfortunately, this direct current is limited in its ability to be generated and transmitted over long distances, which is why domestic electricity comes in a different form, known as alternating current (Fig. 7.6B).

Figure 7.6 • With direct current, there is a steady, unbroken flow of electrons (A). With alternating current (B), the current first flows in one direction AB reaching a maximum at B, then diminishing until it reaches zero at C, at which point it reverses direction and repeats the same pattern, CD, until it again reaches zero at E, completing one cycle. With three-phase electricity (C), three loads are transmitted, each 120° out of phase with the other. This form of current is particularly useful in industrial applications for running electric motors; it is also the standard supply for x-ray machines.

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