Electric Charge
I thought it might be fun to start a Discovery thread on Electricity and Magnetism that promotes the understanding of HiFi equipment in terms of the fundamental physics. On the journey starting at the beginning with the Electric Charge we can share knowledge and ask questions. I see no reason not to delve into modern physics as well. I only ask that everyone be patient because it will be more fun to develop the whole subject slowly and avoid thread drift. We can cover everything eventually but let’s keep each thread fairly tightly scoped. If you are interested I would ask you to like this post and others as we go.
It was not until 1909 (who and how?) that it was discovered that there is a fundamental unit of electric charge that just happened to be carried by the electron. The electron itself had only been discovered in 1897 (who and how?). Moving on quite a bit the proton carries a unit of positive charge. Through the 20th century many more discoveries were made including the strange world of quantum mechanics when Einstein (1905) explained the photo electric effect and was awarded the Nobel prize (he had not applied himself to special relativity and general relativity yet). Later Max Planck explained black body radiation in terms of quantum effects, thereby avoiding the ultraviolet catastrophe predicted by classical physic.
In 1928 Paul Dirac successfully applied special relativity to quantum mechanics and predicted the existence of the antiparticle to the electron, which he called the positron. For the purposes here it has the positive charge and basically other similar properties. It does not exist naturally because when it encounters an electron it and the electron annihilate with the emission of a pair of gamma rays. Some of you may have undergone a medical diagnostic investigation using a PET scanner (positron emission tomography). So annihilation destroys electric charge. Quite amazing!
The field of elementary particle physics gained momentum with the invention of the cloud chamber by Anderson which he used to discover the positron (1932) and muon (1936). At this point physicists’ curiosity was really aroused and they went on to create particle accelerator based on the laws of electricity and magnetism that accelerated electrons and protons to ever increasing energies and smashed them together to create a bizarre array of exotic particles. By 1970 the beginnings of a theory for the strong nuclear force was emerging based on the very strange particles called a quarks. These had -1/3 and +2/3 of the charge of the positron but held together by the strong nuclear force they made all the known particles in the hadron family. The electrons formed a different family that was known collectively as leptons.
The reason for delving this far into modern physics is that charge is a concept associated with what used to be called force. So just as we had the electric force to begin with which is discarded in quantum electrodynamics (QED) in favour of a field (the force is described by the exchange of virtual photons (light, radio waves , gamma rays etc are just photons of differing energies)) with many more properties, so the nuclear force is described as a field involving gluons. Quarks have intrinsic properties of mass, electric charge, colour charge (for the strong nuclear force) and spin (likened to angular momentum in quantum mechanics because it is quantised - a quantum is a discrete amount of something).
In our macroscopic everyday world it was Newton who discovered the the gravitational force and used it to model the motion of the planets. Although we have first hand experience of this force which is based on the intrinsic property of particles we call mass and the force between point masses obeys an inverse square law, modern physics has not discovered the graviton although we have recently detected gravitational waves. Gravity is the weakest force of nature but operates over the scale of the universe.
Long before the discovery of the electron electric charge was known to obey an equivalent inverse square law named after Coulomb (1785) with mass replaced by electric charge and the scaling constant of gravity G by
ε
the dielectric constant or relative permittivity of free space. Gravity in some respects is simple because even in the strange world of antimatter (positrons etc) mass is always positive in value and never quantised although particular particles always have a fixed rest (stationary) mass and get heavier in accordance with Einstein’s Special Theory of Relativity as they move faster towards their maximum of the speed of light. Gluons are also limited to the speed of light as are gravitational waves.
The existence of positive and electric charge in equal amounts saves us from being exposed to the incredible strength of the electric field. Before we understood the fundamental nature of electric charge physicists had discovered that some materials called dielectrics as opposed to conductors or insulators had the property of reducing the strength of the electric force. Today materials can be understood at a quantum mechanical level.
Conductors are a bit easier because although they contain equal amounts of positively charged atoms (worth understanding what an atom is) the conducting electrons (interesting physics here) are only loosely attached to the atom. So what happens if we surrounded a quantity of negative electric charge with a conducting metal sphere? Well the free electrons in the conductor are repelled by the negative charge in the middle - a new property of electric charge that like charge repels and opposite attracts (unlike gravity). So what happens is that the inside of the sphere has a quantity of positive charge spread over it equal to the quantity it surrounds and charge conservation demands that the outside has a matching quantity of negative charge. So the physics is such that it can be modelled by completely ignoring the metal sphere. The mathematics is attributed to Gauss. In the case of gravity inside the earth, the weird thing is that the gravitational force increases linearly before decreasing according to Newton’s Law. You might like to search for details but the clue is that the amount of matter enclosed by a sphere of same radius is a function of the volume.
With electric charge everything changes if the sphere is earthed. Q: Describe what happens and what is a Faraday cage?
So we have covered some of the basics of charge right down to elementary particles. Please ask questions or post on additional basic properties. I think we will start a new thread to cover dielectrics (and insulators) along with material properties. Also in a while we can move on to the idea of electric currents and start exploring DC (magnetism), AC (electromagnetic field), electric fields, power and energy. In time I hope to get to a better understanding with your help of why the mains supply to our HiFi may not be as simple as we might imagine.
Your involvement in this can make it a fun exercise where we all learn. I am far from being an expert in cables, networks, electronics and HiFi circuit design. What I have written so far is just from memory with a few checks online. There may be errors and omissions so don’t be afraid to ask.
I was partly inspired by seeing Jim Al Khalili’s BBC4 series on Quantum mechanical explanations of how the robin navigates, quantum tunnelling and DNA etc.
Phil
