Part 2: Atomic theory, explanation of electrostatics and what really is charge I’m going straight in.
Atomic theory, a quick tour: The essential atomic theory is that solids, liquids and gases are made up of either molecules or atoms. If they’re elements such as gold they consist of atoms; while if they’re compounds such as carbon dioxide, they consist of molecules—which are themselves made up of atoms, so we get to atoms in the end. Atoms consist of a central nucleus, made up of neutrons and protons, which is surrounded by electrons. The number of protons in an atom determines its type: e.g. hydrogen has one, carbon has six and lead has 82. An atom of one type can have a different number of neutrons. For example, carbon-12, with six neutrons (which is by far the most common type), differs from carbon-14 with eight, and these are called isotopes of carbon. Carbon-14 radioactively decays at a fixed rate and measuring the ratio of carbon-12 to carbon-14 is the principle behind carbon dating of organic matter. The size of an atom is from about 1 x 10-10 to 5 x 10-10 m or 0.00000001 to 0.00000005 mm, which is round about a millionth the width of a human hair. Although the heaviest atoms are 200-odd times heavier than the lightest, the size discrepancy is much less (no more than five times larger, from biggest to smallest).
Atoms are mostly empty space. The nucleus is about 1/100,000 the size of the atom, though it varies with the atom—if the nucleus was a tennis ball, the edge of the atom would be roughly 30 km away. This is why structures such as neutron stars or black holes can be so incredibly dense, but that’s another story. Protons are positively charged, neutrons are neutral, and electrons are negatively charged. Each atom contains the same number of electrons as protons and is therefore electrically neutral, However, electrons can be stripped from an atom (the atom becomes a positive ion) or added to atoms (it becomes a negative ion). We’re not sure of the size of an electron but it’s mass is about 1/1,836 the mass of a proton, and protons and neutrons have similar masses. A number of other fundamental particles have been detected or created (they’re often short-lived), but as far as understanding the structure of matter is concerned we can stick to electrons, protons and neutrons.
The negative charge on an electron has the identical magnitude to the positive charge on a proton, which is a value called e for protons (and -e for electrons). The value of e is approximately 1.602 x 10-19 Coulombs. The charge on any object, when measured accurately, is always found to be an exact multiple of e—you can’t have, for example, a charge of 2.5e, and no charge can be smaller than e. I’ll briefly mention quarks and say the last comment isn’t completely true. It turns out protons and neutrons aren’t completely fundamental and, like some other particles, are made up of quarks. Quarks are never directly observed or found in isolation, and are only found as the constituent parts of particles such as protons. When they were first proposed (by Murray Gell-Man and George Zweig, independently, in 1964), physicists weren’t sure whether they were real or a just a device used to explain concepts that weren’t fully understood. However, scattering experiments showed that protons contained smaller particles (because occasionally electrons fired into protons were scattered at large angles as if smaller point-like particles existed within the protons), which provided evidence for quarks. Further evidence has shown the existence of all the difference types of quarks proposed (there are six in total). The point of this diversion is that quarks have charges of either -1/3e or 2/3e, but when combined in protons or other particles, the total always adds up to e or -e, and these fractional charges are never seen because quarks don’t exist on their own. The theory and classification of fundamental particles, including quarks, is called the Standard Model of particle physics and has been very successful, including predicting the discovery of particles such as the Higgs boson. The Standard Model also explains the mechanisms of three of the four fundamental forces of nature, but we’ll come to that later.
Atomic theory, how do we know? We’ll take a brief wander to highlight the major steps towards deriving the atomic theory, but this will be brief and won’t do justice to all the science and scientists involved. The concept of the atom can be traced back to ancient Greece (in particular, Democritus, Leucippus and Lucretius in about 440 BC), where philosophical debate discussed whether matter could be divided forever. Democritus suggested it couldn’t and proposed that eventually you could split matter no further and would be left with an indivisible, solid ‘atom’; and there were many different types of atoms with empty space between them. He used rational arguments, roughly saying that materials decay but can also be recreated. It was a good effort for the times, but the view didn’t prevail, and the opposing concept of continuous matter was supported by Aristotle and remained the consensus view for a long time (also supported by the Catholic church in medieval times).
Atomic views remained on the fringes for a long time, but gradually experimental results pushed them to the fore. Pierre Gassendi put atoms on a more acceptable basis in 1649, by publishing a manuscript separating the existence of atoms from the suggestion that this rejects God. Chemists provided the initial support of atomic theory by discovering that elements always combine in definite proportions, and John Dalton proposed an experimentally-based atomic theory in 1805. An example of his evidence is given by the law of multiple proportions: when elements combine to create more than one compound (e.g. carbon and oxygen can create carbon monoxide or carbon dioxide) then the ratios of the second element in each compound are whole numbers (e.g. 100 grams of carbon combine with 133 grams of oxygen to create carbon monoxide, and with 266 grams of oxygen to create carbon dioxide). Dalton interpreted this to mean carbon monoxide (CO) has one oxygen atom and carbon dioxide two (CO2), which is correct. Dalton built up a table of relative atomic weights for some elements, which also made use of the law of definite proportions (chemical compounds have a fixed mass ratio of their elements, e.g. water (H2O) has about 8/9 oxygen and 1/9 hydrogen, by mass). Hydrogen was defined to have an atomic weight of 1, and Dalton could then establish, for example, that an atom of carbon had a relative weight of 12 (was 12 times heavier); he didn’t get them all right, but his atomic theory was a major step forward. Since 1912, we’ve been able to measure absolute atomic mass using mass spectrometers, invented by the English physicist, J.J. Thomson. They use electric and magnetic fields to accelerate the atoms (which have been ionized—had electrons added or removed—so they are attracted by electric and magnetic fields); the acceleration is measured and Newtons second law (Force = mass x acceleration) is used to work out the mass of the atoms. This allows us to work out how many atoms there are in, for example, 12 grams of carbon-12, which is a standard chemical quantity called a mole. The answer is called Avogadro’s constant and is a rather large 602 sextillion, or 602 thousand million million million, or 6.02 x 1023. We’re going to leave chemistry behind now, but the chemists stormed ahead and derived the periodic table of elements, which now contains 118 elements, and did a whole bunch more.
After Dalton’s theory, atoms were still assumed indivisible. J.J Thomson discovered the electron in 1897, in experiments in Cambridge involving cathode ray tubes, and measured the charge to mass ratio of the electron. The American physicist, Robert Millikan measured the charge on an electron using what is called the ‘oil drop experiment’, and could therefore work out the mass of an electron, which was almost 2000 times smaller than the mass of a hydrogen atom. Ernest Rutherford, a New Zealand-born British physicist published a model of the atom as having a small positively-charged nucleus and orbiting electrons. He was led to this by the result of the Geiger-Marsden experiments he managed at Cambridge. The experiment fired alpha particles (positively charged particles created from radioactive decay) at a thin gold foil and found most particles went straight through the foil, but a few deflected by large angles. Rutherford deduced the atom was mostly empty space, with the positive charge concentrated at the centre. Rutherford’s contributions to science are impressive— he had been awarded the Nobel prize in 1908 for his work on radioactivity, before his atomic theory was published. James Chadwick, an English physicist (working under Rutherford at Cambridge) discovered the neutron in 1932 and won the Nobel prize for this in 1935. The discovery was based on showing that “radiation” emitted when alpha particles were fired at certain light elements was actually a neutral particle of about the mass of a proton. From here on, nuclear and atomic physics (and quantum physics which is needed for deeper analysis of atomic structure) continued apace, and we’ll head back to electricity!
Static electricity explanation A key point of atomic structure related to static electricity is that electrons carry a negative charge and are not bound to the atomic nucleus. In electrical conductors, such as metals, the electrons are free to move. In insulators (also called dielectrics), the electrons are bound to atoms and, although they do move within the atom itself, they don’t ordinarily move much within the material—but they can be moved under some circumstances, e.g. vigorous rubbing. Semi-conductors are somewhere in the middle and conduct electricity a little (and are crucial for electronics, which we’ll leave until later).
The explanation for static electricity goes like this: contact-induced static electricity is caused by one material rubbing electrons off another material. The material with the extra electrons is negatively charged and the one that’s lost electrons is positively charged. Two oppositely charged bodies are then attracted towards each other and two similarly charged bodies are repelled because of the Coulomb force between charged particles. The total force is the result of the sum of trillions of individual forces between electrons and protons. It’s possible to do some maths and work out the force between different shapes of charged bodies (e.g. two spheres or two parallel plates), if you know the charges on each—this kind of maths involves an infinite sum (integration) and is easier if the bodies have a simple shape. Once a charged body touches a conductor, the electrons will flow to or from the conductor to neutralize the charge. Induced static electricity is the result of either, in an insulator, atoms lining up so the electrons face towards a positive charge (like the balloon on the wall); or, in a conductor, electrons flowing towards a positive charge (like rubber shoes kicking on carpets). Piezoelectric static properties are related to their non-symmetric crystal structures. Individual cells of the crystal are polarised because of this structure (charges point in a particular direction), although this cancels out over the crystal and it is electrically neutral. Stress on the material changes the polarisations, such that they don’t cancel out and a charge is created which permeates through the material, leaving opposite sides charged. The size of the effect depends on which way the material is stressed (relative to the crystal structure). The explanation for pyroelectricity is similar, except temperature changes cause the change in polarisation.
So what really is charge? We’ve concluded that a body is negatively charged because it has an excess of electrons and each electron has a negative charge of –e; or it’s positively charged because it has an excess of protons, which each have a positive charge of +e. But what is charge and why does each electron have the exact negative of the charge on a proton and why does it have the value it does? I’d love to answer that question, but…we don’t know. Charge is considered to be a fundamental property of a particle (just like mass is). For example, an electron’s charge is –e, a proton’s is e, a neutron has charge 0, and a positron (an antielectron) has a charge of e. And charged particles are mutually attracted or repelled by other charged particles according to Coulomb’s Law (that is, the force between them is proportional to the size of the charges and inversely proportional to the square of the distance they are apart). An advanced field of physics called quantum electrodynamics (QED) can explain the mechanism by which charged particles interact (it’s about photons being exchanged between the particles), but as to what charge is and why it’s there—that’s a deeper question. I hope you’re not too disappointed.
