Electricity and magnetism

Chapter Seven Electricity and magnetism





Introduction


Knowledge of electricity and magnetism has been with us for centuries: the Greeks knew that, if you rubbed amber against fur, you could generate static electricity; indeed, their word for amber, ‘elektron’, was the word used by the 16th century physicist, William Gilbert, to describe the way in which amber and other objects capable of becoming electrostatically charged could attract and repel other objects. This, in turn, gave rise to the English words ‘electric’ and ‘electricity’.


Gilbert, who published his ideas in 1600, three years before his death at the age of 63, in the work De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (‘On the Magnet, Magnetic Bodies, and the Great Magnet of the Earth’), was also the first person to suggest that electricity and magnetism might be different manifestations of the same fundamental force. Magnetism had also been known to the ancient Greeks; its first practical application was the navigational compass, but it would not be until the 19th century that the laws of electricity and magnetism would be quantified and unified.


Until 1800, when Alessandro Volta succeeded in making a battery using plates of zinc and silver interspersed with damp cardboard, the study of electricity had been confined to electrostatics, the physics of static charges. Pioneering physicists such as Priestly, Coulomb, Poisson and Faraday had succeeded in showing that both static electricity and magnetism followed the same inverse square law as gravity; in this case known as Coulomb’s Law.






Electromagnetism


Electricity remained a scientific oddity until the development of the wire telegraph in the late 1830s, which started the revolution in communication – suddenly, it was possible to send messages across continents in minutes rather than weeks.



Despite the increasing practical applications of electricity, its true nature remained a mystery for another 30 years when the Scottish physicist James Clerk Maxwell described the first element of the ‘Holy Grail’ of physics, the Unified Field Theory, when he successfully demonstrated that electricity and magnetism were merely different manifestations of the same fundamental force.


Although the two forces seem quite distinct on a mundane level, and are governed by different equations, it is a remarkable fact that a changing electric field will create a magnetic field and, conversely, a changing magnetic field will generate an electric field – the principle on which the dynamo that powers bicycle lights works.





Maxwell also showed that electric and magnetic fields travel together through space as waves of electromagnetic radiation, with the changing fields mutually sustaining each other. Examples of such waves are radio and television signals, microwaves, infrared rays, visible light, ultraviolet light, x-rays, and gamma rays. In Maxwell’s time, all waves were thought to need a medium to propagate them and, in the absence of any tangible medium to support light and radio waves (which could obviously travel in the vacuum of space), the ‘ether’ was proposed as the intangible medium of propagation. It was Maxwell who showed that electromagnetic waves required no medium.


All of these waves travel at the same speed, the velocity of light (roughly 3 × 108 ms−1). They differ from each other only in the frequency at which their electric and magnetic fields oscillate; prior to Maxwell, many of these phenomena (or those that had by then been discovered) were regarded as being unrelated.


In this chapter, we will be looking at electricity and magnetism; electromagnetism is explored in depth in Chapter 8.



Electrostatics


The study of non-moving electrical charges now forms such a small part of the study of electricity as a whole that it is hard sometimes to remember that, for many centuries, it was the only form of electricity that could be studied. It was, however, a necessary step to understanding electrical phenomena as a whole and, as we have seen, has critical implications for the shape and function of many organic molecules.


The laws that govern static charges can be summarized very simply:



This is because…



The size of the attraction or repulsion is governed by…




imageCLINICAL FOCUS


Today, we take for granted electricity in every aspect of our daily – and clinical – lives: light at the flick of a switch, computerized patient records, electromagnetic treatment modalities, diagnostic imaging, electrically powered treatment benches, digital thermometers, ophthalmoscopes, dictaphones, air-conditioned and heated offices – the list could continue for pages; electricity is so much a part of our lives that we only appreciate it when a power cut robs us of access.


One hundred-and-fifty years ago, physicians had no such luxuries. I have, in the corner of my consulting room, a reminder that for our predecessors even something as simple as getting sufficient illumination to make an adequate examination could be a major challenge. The doctor’s double oil lamp (Fig. 7.1), came with two lamps, each with a double burner, mounted on pivoted arms so the full force of the light could, when required, be brought to bear on the patient. I have tried the lamp – it produced, I would estimate, the equivalent of a 5 watt bulb and had the added disadvantage of setting off the (electric) fire alarms.



At the time it was made, around 1860, neither chiropractic nor osteopathy existed and the term ‘physiotherapy’ had yet to be coined. Physicians still used leeches and surgical anaesthesia was in its infancy; manual therapy was in the hands of bonesetters.


Interestingly, the founder of chiropractic, Daniel Palmer, originally plied his trade as a ‘magnetic healer’ in the American mid-west of the 1890s. Over a century later, what has been sneered at as evidence of the ‘quack’ origins of the profession he began is now under serious investigation as a noninvasive therapy for a range of musculoskeletal and vascular conditions with proven physiological effects on the numerous polar molecules within the human body.




Conductors and insulators


Although it is tempting to think of the material world as being divided into two – those substances that do conduct electricity and those (insulators) that do not – the true picture is more complex by far. We have talked about connecting an electric potential to an electric cable in order to obtain a current but the results we get will be very different, depending on the material from which the cable is made.



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).





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.



Electrodynamics


The study of moving charges is known as electrodynamics. For our purposes, this will comprise a study of the way in which electricity and electrical components work. This will enable us to understand the operation of such devices as we encounter and rely on in our clinical practice. Clinical tools such as the ophthalmoscope; diagnostic imaging of all types, be it ultrasound, magnetic resonance imaging, conventional x-rays or cutting edge proton emission tomography; and therapeutic interventions such as interferential or TENS machines all rely upon one common factor, the flow of electricity.




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).



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).


Apr 4, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Electricity and magnetism
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