Discovery: Part 1 - Electricity and Magnetism: Electric Charge

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


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.



Wow…cannot compete with that Phil. I was lucky to work for Ford Motor Company and lived in Michigan for a few years. The Henry Ford Museum was just wonderful…a collection of U.S memories before they would be destroyed.
They have the laboratories that Edison used…and made his life changing discoveries for the rest of us. I post a picture of Henry Ford, Edison, and Harvey Firestone( rubber tires).
Who knows what these guys could have done with computers in their day…they were geniuses in their day. Hope not too far off topic Phil.

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No problem Gazza. I want people to do a bit of reading and thinking because in many ways physics is fairly easy to understand and we can all make this journey. I’d go as far as to say they probably cover some of this at school for GCSE physics these days. The Secrets of Quantum Physics should be available on iPlayer.

Could you like the initial post please?


Yes your post is very thought provoking, just i am not able to add too much other than my wonderment of the guys in my post and so many of their time inc Madam Curie. So few tools to use, but so many achievements that lead to your post.

Thanks Phil, a very interesting read. Apologies for a moment of pedantry, but the original Special Relativity paper did come out in 1905, Einstein’s Annus Mirabilis. Not just Special Relativity and the photo-electric effect, but also mass-energy equivalence and explanation of Brownian motion.


Yes also in 1905 (The electrodynamics of moving bodies). The curious title is because following the Michelson Morley experiment that established that the speed of light is constant for all observers and Maxwell’s formulation of the laws of electromagnetism there was a need for a better understanding. Maxwell explained that light is an electromagnetic phenomenon and related the speed of light to the electric and magnetic permittivity constants in the process and gave us the theory to understand the generation of radio waves using a simple dipole. Maxwell’s equations were known to be invariant under what was known as the Lorentz transformation (its got the funny stuff involving velocity of the moving body divided by the velocity of light stuff).

The brilliance of Einstein was to find a different way of looking at the known phenomena (invariance of the velocity of light) as an invariance of the laws of physics (Mechanics) for observers in constant relative motion to one another. Invariance then started to be a fundamental way of looking at physical phenomena which got further developed to understand quantum phenomena in terms of symmetries under rotation as opposed to translation (moving the coordinate axes). Roger Penrose’s book The Road to Reality contains lots more about symmetry which is relatively readable for a few hundred pages at least!

When I mentioned Dirac and the positron I was hinting that it arose to make the mathematical theory of quantum mechanics consistent with special relativity but more properly invariant under coordinate transformation. The result was equations that had positive and negative solutions!



In answer to your questions:

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.

Robert Millikan; using an electric field to suspend charged oil droplets and proving that their charge was always an integer multiple of the charge on what we now call an electron. Though it must be said he did some quite appalling cherry-picking of his data that has proved a little embarrassing to Physics apologists in recent decades. It’s arguable that he mainly got away with this for so long because he just happened to be right!

The electron itself had only been discovered in 1897 (who and how?).

I presume you’re referring to JJ Thomson, who didn’t really ‘discover the electron’, though that is almost always how it’s described. He actually used a cathode ray tube to show that the ratio of mass to charge of these ‘cathode rays’ was always the same, independent of the cathode material used and therefore proposed that they (i.e. what we now call electrons) were a fundamental part of all matter.

If you’ll forgive me a couple of other corrections:

  • Planck had derived an expression for black body radiation in 1900, not after 1905.
  • ‘[a]nnihilation destroys electric charge’ - well, not really, it just conserves electric charge. When a particle of charge +1 annihilates with its antiparticle, you do end up with zero charge, but that’s only because +1 + (-1) = 0!
  • Gravity is the weakest force of nature but operates over the scale of the universe’ - true but the electromagnetic interaction is also infinitely-ranged.
  • ‘the scaling constant of gravity G by ε’ -well, it’s usually given in SI units as 1/4(pi)(epsilon-0), which is a lot easier to say than to type without an equation editor.
  • ‘In the case of gravity inside the earth, the weird thing is that the gravitational force increases linearly’ - true, but only if the density of the object is constant. If it’s not - as indeed it isn’t with the Earth - then it’s a bit more complex than just linear. Google ‘preliminary reference Earth model graph’ to see how.

In answer to your other question, there is no electric field inside a hollow earthed sphere. A Faraday cage is any hollow metallic object with sufficiently small holes in it (think chain-link fence without any gates or big gaps), within which you can’t get (almost) any EM radiation effects, though static magnetic fields will still penetrate it.



Excelled work, Mark! Are you a fellow physicist? If you are I would be happy to exchange material. It gets more tricky soon. I will correct my Apple Notes. I am trying to convey enough to get over ideas. I liked your critique of Millikan.


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It is worth emphasising the wonder of our universe is that gravity is purely attractive while electric charge comes in two types with opposites attracting and sames repelling in our classical way of thinking. When we come to colour charge (strong nuclear) it becomes even more complicated.

Just think also about that metal sphere surrounding an electric charge I mentioned at the beginning, and the weird change in the ‘electric field’ it brings about - it ‘disappears’. Think also about Millikan’s experiment being able to determine that electric charge comes in integral multiples of a fundamental unit that is so small that 1 amp of current is the equivalent of 10 to the power nineteen electrons approximately per seconds! And he used tiny oil droplets charged by ionising xrays suspended by an electric field to resist gravity.

It’s worth mentioning that atoms (I didn’t say much about them) being composed of protons (with positive charge) and neutrons (no electric charge) would break apart without the strong nuclear force.

In yet another twist both protons and neutrons can flip from one to the other with the emission of positron/electron in exceptional (in terms of normal life on earth) circumstances. Some so called radioactive materials exhibit this characteristic referred to at the time as beta (another name for the electron) decay. One has to remember one can’t easily look at these ‘particles’ and say ahah an electron…

In time we could talk a bit about why we don’t talk about force so much in modern physics. The word force gets it meaning from mechanics which is about the movement of objects.


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Correction. I didn’t think properly about the question I set, which was about concentric hollow spheres with different charges essentially.

I should perhaps give you the simple fact that Guass’s Law essentially says in the case of a conducting sphere that all the charge on it appears to act as a point charge at its centre. The conducting nature of the sphere means the charge is only on the surface and not inside whether solid or hollow.

The concentric sphere problem is solved by the supposition of the two problems so if the inner sphere is charged its electric field is unaffected irrespective of being surrounded by another conducting sphere. If the outer has the opposite amount of charge the fields would cancel outside the outer one!

Here is a link


Next we have a big step to make by talking about the other laws of electromagnetism collectively referred to as Maxwell’s Laws. Like Einstein his genius was to pull together the laws of Coulomb, Ampere and Faraday into a coherent mathematical framework with the added benefit of showing that light is an electromagnetic wave. It then paved the wave for the invention of the radio and understanding the whole electromagnetic spectrum.

Long before the discovery of the electron electricity was known about through the work of Alessandro Volta (among others) who discovered (1800) that the differing electric potential of different metals could be used to used to cause an electric current to flow. The volt is named after him. Electric potential and field are related in the sense that if the positive and negative terminals of a battery are connected to two parallel conducting plates an electric field is created with the strength equal to the differences of electrical potential of the plates.

Ampère in 1825 and Gauss in 1833 then established that two current carrying parallel wires experience a force between them through the magnetic field each creates. The force is proportional to the length of the wires and reduces linearly with separation. With further study the simplistic early law was generalised culminating in the work of Maxwell describing the full relationship between electric charge in motion (current) and the magnetic field it produces and associated force experienced by electric charges. Unlike the electric field flowing from positive to negative charges (by convention) there are no magnetic charges because it is purely a consequence of moving electric charge! This is one of the 4 Maxwell laws. The magnetic field instead goes round straight wires or mostly through the middle of a toroid.

Faraday’s law is based on his simple experiment illustrated by the picture of the equipment he used.

Faraday’s experiment showing induction between coils of wire: The liquid battery (right) provides a current which flows through the small coil (A), creating a magnetic field. When the coils are stationary, no current is induced. But when the small coil is moved in or out of the large coil (B), the magnetic flux through the large coil changes, inducing a current which is detected by the galvanometer (G).

In integral (as in integral calculus) form, it states that the work per unit charge required to move a charge around a closed loop equals the rate of decrease of the magnetic flux through the enclosed surface. The amazing thing about this is that it says a time varying electric field can be produced by time varying magnetic field. Essentially it is not necessary to have electric charges to produce electric fields.

We will continue in a later post with the basics of the mathematical formulation of the Maxwell Laws of Electromagnetism.


Consideration of the Linear Power Supply


Doing my best to explain this to you - please help.

Let us consider the behaviour of the input (mains) side of a simple unregulated power supply. The waveforms at the bottom of the above image show the idealised input voltage, the rectified output and the smoothed output.

The application of the laws of electromagnetism needs some simplification to get an understanding of what happens. Firstly let us suppose that the output of the transformer is open circuit (infinite resistance) and that the input coil has no resistance. The self inductance of the primary coil when an AC voltage is applied creates an equal and opposite emf (voltage). Therefore, no current flows. Otherwise it would blow the fuse! Linear power supplies are very efficient at transferring energy irrespective of output demand.

Now let us consider just the transformer with a simple resistor across the output. A current now flows in the secondary coil to oppose the magnetic field created by the primary coil. If 1 denotes primary and n (number of turns), I (current) and V (voltage) then in ideal conditions

I2/I1 = V1/V2 = n1/n2

Power is lost through

  1. Generation of heat in the coils
  2. Generation of heat in the core by eddy currents
  3. Hysteresis loss in the core
  4. Loss of magnetic flux.

The effect of the rectifier bridge and simple voltage regulator (capacitor and resistor) is to only allow a current to flow in the secondary when the voltage exceeds voltage across the capacitor. So the current in the primary only flows when the output capacitor needs charging.

The net effect is that a power supply behaves as a very complex electrical device that is very dependent on the current being drawn by the device it is powering. Add the fact that each Naim Power Supply has multiple outputs and there are multiple power supplies and you will appreciate the interaction at the mains is very complex.

The reason we have a dedicated mains supply is to reduce the interaction of the domestic devices with the HiFi.

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