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Chapter 29: The Electron Please remember to photocopy 4 pages onto one sheet by going A3→A4 and using back to back on the photocopier. 'The electron is a theory we use; it is so useful in understanding the way nature works that we can almost call it real.' Nobel Prize winning physicist Richard Feynman
Properties of the electron
- Orbits the nucleus
- Very small mass
- Negatively charged
- The charge on the electron is the smallest amount of charge found in nature*.
You don’t have to know the actual charge (1.6 × 10-19^ C), but you do need to know that the man responsible for first measuring this charge was Robert Millikan.
The term electron was coined by an Irishman called George Stoney.
Thermionic Emission is the giving off of electrons from the surface of a hot metal.
The Cathode Ray Tube*
Operation
- A current is passed through the heating coil, causing it to heat the cathode which in turn causes electrons to be emitted (by thermionic emission).
- Because of the high potential difference between the cathode and anode the electrons are accelerated across the tube towards the anode. As a result a current flows in the circuit.
- Electrons which pass through the hole in the middle of the anode continue on until they hit the fluorescent screen.
- The stream of electrons can be deflected by electric or magnetic fields, which are generated from the X and Y plates.
- The voltage on the Y-plates is adjusted to make the cathode rays (or stream of electrons) move up or down.
- The voltage on the X-plates is adjusted to make the cathode rays move left or right. The result is that the rays can move across the computer or television screen.
Uses of the Cathode Ray Tube Basically it can be used anywhere a small electrical signal is produced.
1. Cathode Ray Oscilloscope (CRO). This is a variation of the cathode tube; it is used today in electronics as a diagnostic tool.
- It also forms the main component in televisions and computers*.
- The ECG (electrocardiogram) is used in medicine to display electrical signals in the heart. Alternatively it can be used to display signals produced by the brain, where it is known as an EEG (electroencephalogram).
Cathode rays are streams of high-speed electrons Properties of cathode rays
- They travel from the cathode in straight lines
- They cause certain substances to fluoresce
- They can be deflected in electric and magnetic fields
- They can produce x-rays when they strike heavy metals
The Photoelectric Effect The Photoelectric Effect is the emission of electrons from a metal due to light of a suitable frequency falling upon it.
Demonstration Procedure :
- Charge the electroscope negatively.
- Shine ultraviolet light on the zinc plate. Result : The leaves fall together Observation :
Shining UV light on the zinc plate liberates electrons from the zinc and
therefore the legs become neutralised and fall back together.
Einstein’s Explanation
- The incident energy is absorbed by an electron at the surface of a metal.
- A certain amount (the work function) goes to liberating the electron.
- The remainder appears as kinetic energy of the liberated electron*.
- Shining visible light has no effect on the apparatus. This is because the packets of energy associated with visible light are too small to liberate electrons.
This “proved” that electromagnetic radiation (including light) is composed of particles (called photons).
A photon is a bundle (discrete amount) of electromagnetic radiation.
Mathematically: the photon as a packet of energy*
The energy associated with each packet of energy (photon) is given by h is known as planck’s constant and f is the frequency of the wave.
According to Einstein's equation for the photoelectric effect (also known as Einstein’s photoelectric law ):
Energy of incident photon = work function + kinetic energy of photo-electron.
φ (“phi”) is known as the work function. It represents the energy required to ‘liberate’ an electron from the surface of a metal. Because φ is a discrete amount of energy, it can in turn be represented by the formula φ = hf 0 , where f 0 is called the threshold frequency. The value of the work function is different for different metals.
The Photocell Operation
- Light of a suitable frequency shines on A (called the photocathode).
hf = φ + ½mv 2
E = hf
- This releases electrons (by the Photoelectric Effect).
- The electrons are attracted to point B (the anode), and from there they flow around the circuit, where they can be detected by a galvanometer, or alternatively they can be used to activate an electronic device.
Applications of photoelectric sensing devices
- Burglar alarms
- Automatic doors
- Control of burners in central heating
- Sound track in films
Leaving Cert Physics Syllabus
Content Depth of Treatment Activities STS
1. The electron The electron as the indivisible quantity of charge. Reference to mass and location in the atom. Units of energy: eV, keV, MeV, GeV.
Electron named by G. J. Stoney. Quantity of charge measured by Millikan.
2. Thermionic emission
Principle of thermionic emission and its application to the production of a beam of electrons.
Cathode ray tube consisting of heated filament, cathode, anode, and screen.
Deflection of cathode rays in electric and magnetic fields.
Use of cathode ray tube to demonstrate the production of a beam of electrons – deflection in electric and magnetic fields.
Applications
- cathode ray oscilloscope
- television. Use of CRO to display signals:
- ECG and EEG. 3. Photoelectric emission
Photoelectric effect. The photon as a packet of energy; E = hf Effect of intensity and frequency of incident light.
Photocell (vacuum tube):
structure and operation. Threshold frequency. Einstein's photoelectric law.
Demonstration, e.g. using zinc plate, electroscope, and different light sources.
Demonstration of a photocell.
Applications of photoelectric sensing devices:
- burglar alarms
- automatic doors
- control of burners in central heating
- sound track in films. 4. X-rays X-rays produced when high- energy electrons collide with target. Principles of the hot-cathode X-ray tube. X-ray production as inverse of photoelectric effect. Mention of properties of X- rays:
- electromagnetic waves
- ionisation
- penetration.
Uses of X-rays in
medicine and industry.
Hazards.
Extra Credit About one hundred years ago, several remarkable and highly-important discoveries were crowded into the short space of ten years: X-rays in 1895, radioactivity the following year, the electron in 1897, quantum theory in 1900, and special relativity in 1905. Individually, each had enormous significance and collectively they heralded what is now known as ‘modern physics’.
*The charge on the electron is the smallest amount of charge found in nature Physicists still don’t know why an electron has the same charge as a proton! Later on we will come across particles called quarks, some of which have a charge of 1/3rd^ that of the electron. But these are not found isolated in nature.
*The Cathode Ray Tube This phenomenon was noticed before people were familiar with the concept of the electron. What was known was that if a fluorescent screen was placed on the inside of the tube, fluorescence would occur. Therefore something seemed to be coming from the cathode. Because the electrons could not be seen, this ‘something’ was called cathode rays , and the apparatus was called a cathode ray tube.
Incidentally, the cathode ray tube was first developed by Sir William Crookes, of Crooke’s Radiometer fame (remember what that is?). Crooke was a major player in the world of physics in the late 19th^ century. Crooke later lost all respectability among his peers due to his willingness to get involved in physic research (being able to read someone else’s mind and all that). Crookes' final report so outraged the scientific establishment that there was talk of depriving him of his Fellowship of the Royal Society.
*The Cathode Ray Tube forms the main component in televisions and computers. For this the screen is divided up into little squares called ‘pixels’ (from ‘picture elements’). The ‘electron-gun’ scans across each row of pixels, starting at the top left hand corner and working down, changing intensity as it travels. Each pixel stays bright for a fraction of a second, but before it starts to fade the gun has gone back over it and ‘refreshed’ it. The quality of a television screen is therefore determined by, among other things, the speed at which the gun travels, and the number of pixels on the screen. For example, a 640-by-480 pixel screen is capable of displaying 640 distinct dots on each of 480 lines, or about 300,000 pixels. For colour televisions there are actually three ‘guns’ involved, each having one of the three primary colours, and the product of their relative intensities determine the colour seen on a given part of the screen. ‘Course by the time you read this the cathode ray tube will probably be obsolete.
*Energy associated with an electron When an electron gets accelerated across a potential difference, it gains kinetic energy (½ mv^2 ). But just like an apple falling from a tree which similarly gains kinetic energy, this energy had to come from somewhere. In the case of the falling apple, the energy it gained came at the expense of (Gravitational) Potential Energy (mgh) which it lost. In the case of the electron the kinetic energy gained comes at the expense of Electrical Potential Energy, the formula for which is W = QV. Therefore we end up with the expression eV = ½ mv^2. If any of this seems vaguely familiar it’s because we studied it in chapter 20.
In this case Q represents the charge on an electron, for which we use the letter e. So we get W = eV.
*A beam of electrons moving at right angles to a magnetic field will move in a circular path. A beam of electrons constitutes an electric current but remember that if a current flows in one direction the electrons are actually moving in the opposite direction (and vice versa). Now if an electron is moving towards the right (meaning current is to the left ) in a magnetic field where the direction of flux density is into the page , applying Fleming’s Left Hand Rule tells us that the force on the electron must be downward, and so the electron changes direction. But because the flux density is constant and is always perpendicular to the direction of the electron, the electron will continually change direction, while moving at the same speed. The result is that it travels in a circular path. We’ve come across this concept before, i.e. something which is travelling at constant speed, but changing direction is accelerating.
Why is the flux density always perpendicular to the direction of the electron? Because the flux density is always into the page, while the electron is always travelling along the page, even though it’s changing direction on the page. This is one of those concepts that I would be afraid of tackling if it came up on an exam question. You never know how much detail is required, and the chances are that the marks are only awarded for two or three key phrases, all of which may or may not have been included in the notes above.
*The photon as a packet of energy; In 1905 Einstein published a famous paper that suggested that light could be considered to consist of packets, which he called photons. Max Planck was the dude who kick-started all this Quantum Theory stuff ten years previously by suggesting that heat energy could be quantised. Funnily enough, he could never accept Einstein’s findings (which followed on from Planck’s work) that all energy could be considered to consist of discrete packets. But this was only the beginning of Planck’s misfortune. To get an idea of how poor a hand he got dealt in life, read Bryson’s account of it in his book A Short History of Nearly Everything. One of the consequences of Einstein’s work was that it was realised that there was a randomness inherent in the laws of physics at the quantum level. Ironically Einstein could never accept this. His famous phrase was “God does not play dice”.
For what it’s worth, it is a common misconception that work-function is the same as ionisation energy. Ionisation energy involves bumping electrons out of single standalone atoms (or perhaps molecules) in a gas. In a metal there is a ‘sea of electrons’ on the surface, and it is one of these electrons which leaves the surface when light of a suitable frequency falls on it. Because the electron arrangement is different in both cases, the energy involved will be different.
Coining of the term ‘Photon’. " I therefore take the liberty of proposing for this hypothetical new atom, which is not light but plays an essential part in every process of radiation, the name photon ." Gilbert N. Lewis, 1926 Incidentally Einstein's description of the quanta of energy of EM radiation first used 100 years ago still serves as an excellent definition of a photon to this day: “ Energy during the propagation of a ray of light is not continuously distributed over steadily increasing spaces, but it consists of a finite number of energy quanta localised at points in space, moving without dividing and capable of being absorbed or generated only as entities.”
*The Photoelectric Effect
The incident energy is absorbed by an electron at the surface of a metal. A certain amount (the work function) goes to liberating the electron. The remainder appears as kinetic energy of the liberated electron.
The strange thing is that if the incident light hasn’t got enough energy to liberate an electron, no amount of time (or increase in intensity) will make a difference. An increase in intensity simply means that more packets of light are used per second, but if they don’t have sufficient energy, increasing the number will simply have no effect. However if each photon has enough energy to liberalise an atom, increasing the intensity (i.e. the number of photons) will result in more atoms being liberalised, and therefore an increase in current. Remember that the energy of the incident light is determined by its frequency. So for example for the metal zinc, visible light is of too low a frequency, and therefore has too little energy, to liberalise electrons form the surface of the zinc. However Ultra Violet light has a higher frequency and therefore is able to liberalise electrons. This can be detected as a small current using an ammeter or galvanometer. This proves that light is like a particle. It was Einstein who realised this.
Analogy of the Photoelectric Effect The energy of the incident photon is like the money in your pocket at the start of a night out The work function energy is analogous to the admission charge at the nightclub. It's an all or nothing thing, even if you're only just one penny short, you will be refused entrance. Different clubs may charge different entrance fees, which is analogous to the differing work functions of different metals. The maximum Kinetic Energy bit represents the money left over after paying for entrance, which can then be spent on lemonade
A nice analogy for work function, but as with all analogies it's worth discussing the limitations. For instance if one of the group does not have quite enough money, but the rest have some to spare, they can all get in. Photons, on the other hand, cannot re-distribute energy between themselves. Even if a photon with insufficient energy arrives at the same time as others with excess energy, it still cannot eject an electron.
- X-Rays X-Rays will prove to be a hoax Lord Kelvin Remember Kelvin’s remark on alternating current also being a mistake in the last chapter?
*Hazards X-rays Radiation can cause the ionization of atoms in a persons’ DNA. This means the atom is now charged, where before it was neutral. This can affect the function of the DNA. The DNA can be damaged and then repaired in such a way that the cell continues to divide but with an altered message in the DNA. This altered message may eventually result in a cell turning into a cancer. Thus any dose of radiation increases the risk of cancer.
Believe it or not, X-Rays used to be used to help in ascertaining the size and shape of a person’s foot; The customer put his or her foot in the machine for up to 45 seconds, while the salesperson had to use this machine all day.
Quantum Theory “ Curiouser and curiouser,” cried Alice Alice’s Adventures in Wonderland
“The more successful quantum physics gets, the sillier it looks” Albert Einstein
“No-one understands quantum physics” Richard Feynman
Introduction to Quantum Theory Planck started the whole thing off, saying that heat was given off as little packets as opposed to continuous waves. Einstein took this a step further and suggested that perhaps light was also given off in little packets (called photons) instead of as a continuous wave. Planck, however, thought this was preposterous and could never accept Einstein’s idea. But Einstein also suffered a similar fate. Predictability is one of the corner-stones of physics. Resolving controversies in science usually involves successfully predicting the outcome of a specially- designed experiment. But it turns out that in nuclear physics an atomic nucleus breaks down into other, smaller nuclei, and predicting when this will occur is impossible. The best we can do is to give the probability of a break-down occurring. Einstein, whose own work formed the foundation for these ideas, could never accept these conclusions. In fact one of his most famous quotes was on this subject: “God does not play dice”. What he meant by this is that the universe was not built around laws of probability (the reference to God was not meant to be taken literally – Einstein did not believe in a personal God). For the record, all scientists now accept that probability does indeed lie at the heart of physics.
It gets better. When formulating the rules for General Relativity – probably his greatest work of all – Einstein had assumed that the universe was static. i.e. it was not expanding. This was in keeping with the belief at the time. But one of the outcomes of General Relativity was that the universe was expanding. Believing that he had made a mistake somehow, he introduced a ‘fudge factor’ into his equations which stabilised the universe. When the deliberate error was later discovered (by a postgraduate student) Einstein described it as ‘the greatest mistake of my life’. By this stage the inflationary universe was becoming accepted as the correct description of the universe, which in turn implied that the ‘Big Bang’ did indeed occur. One of the many ironies in all this is that it is now thought that the universe may not go on expanding forever, but may indeed either stabilise or begin to contract again, resulting in the universe finishing in a ‘Big Crunch’. Either way, Einstein’s fudge factor may not have been such a mistake after all.
Of course this was just when Einstein acknowledged he was wrong. He also made a bit of a serious boo-boo when it came to trying to unify the four forces of Nature. As a result of this he lost almost all respect among his peers, and spent the last decades of his career on a lone crusade which yielded nothing but the patronising sympathy of his fellow physicists. Interestingly, Einstein seems to have paid very little attention to developments in physics (including the discovery of all the new particles) during these latter years
We will come across the concept of the four forces in more detail in the chapter on ‘Particle Physics’.
By the way, one of the biggest advocates of the ‘steady universe’ scenario was a scientist called Fred Hoyle. Hoyle died recently, but in yet another irony, actually coined the term ‘Big Bang’ when he used it in a derogatory fashion. The nerds among you may claim that this isn’t actually an example of irony, but you know what I mean.
Boom Boom
