Today, we know that atoms do not represent the smallest unit of matter. In first year we learned that atoms are made up of a positively charged nucleus containing protons and neutrons with negatively charged electrons orbiting it.
The standard model attempts to explain everything in the universe in terms of fundamental particles. A fundamental particle is one which cannot be broken down into anything else. These fundamental particles are the building blocks of matter, and the things which hold matter together.
Orders of Magnitude
Often, to help us grasp a sense of scale, newspapers compare things to everyday objects: heights are measured in double-decker buses, areas in football pitches etc. However, we do not experience the extremes of scale in everyday life so we use scientific notation to describe these. Powers of 10 are referred to as orders of magnitude, i.e. something a thousand times larger is three orders of magnitude bigger. It would be useful to get an idea of scale to better understand how sub-nuclear and astronomical dimensions compare with those in our everyday life. You can see how we fit into the grand scheme of things by carrying out the following activity.
When we get into the world of the very small or very large it is difficult to get a picture of scale in our minds. Below is a table giving some examples of scale in our world;
Human scale – the average British person is 1.69 m
The height of a house
The width of a city square
The length of an average street
The diameter of a small city like Perth
Approximate distance between Aberdeen and Dundee
Length of Great Britain
Diameter of Earth
Thinking in terms of the smaller end of the scale.
If a proton is measured as having a radius of distance roughly 10-15m, how many of these protons would fit on the point of a pencil?
Assuming the pencil point was 1mm across, there would be 1 000 000 000 000 (1012) protons.
In terms of the larger end of the scale, we have space and quasars.
The distance to a quasar is 1026 m.
This would take light, travelling at 3 x 108m/s, 10 000 000 000 (109) years to get from Earth to the quasar.
In the following table the numbers or words represented by the letters A, B, C, D, E, F and G are missing. Match each letter with the correct words from the list. try and finish this table;
What is the World Made Of? The ancient Greeks believed the world was made of 4 elements (fire, air, earth and water). Democritus used the term ‘atom’, which means “indivisible” (cannot be divided) to describe the basic building blocks of life. Other cultures including the Chinese and the Indians had similar concepts.
Elements: The Simplest Chemicals In 1789 the French chemist Lavoisier discovered through very
precise measurement that the total mass in a chemical reaction
stays the same. He defined an element as a material that could
not be broken down further by chemical means, and classified
many new elements and compounds.
The Periodic Table – Order Out of Chaos In 1803 Dalton measured very precisely the proportion of elements in various materials and reactions. He discovered that they always occurred in small integer multiples. This is considered the start of modern atomic theory. In 1869 Mendeleev noticed that certain properties of chemical elements repeat themselves periodically and he organised them into the first periodic table.
The Discovery of the Electron In 1897 J.J. Thomson discovered the electron and the concept of the atom as a single unit ended. This marked the birth of particle physics. Although we cannot see atoms using light which has too large a wavelength, we can by using an electron microscope. This fires a beam of electrons at the target and measures how they interact. By measuring the reflections and shadows, an image of individual atoms can be formed. We cannot actually see an atom using light, but we can create an image of one.
The structure of atoms
At the start of modern physics at the beginning of the 20th century, atoms were treated as semi-solid spheres with charge spread throughout them. This was called the Thomson model after the physicist who discovered the electron. This model fitted in well with experiments that had been done by then, but a new experiment by Ernest Rutherford in 1909 would soon change this. This was the first scattering experiment – an experiment to probe the structure of objects smaller than we can actually see by firing something at them and seeing how they deflect or reflect.
The Rutherford alpha scattering experiment
Rutherford directed his students Hans Geiger and Ernest Marsden to fire alpha particles at a thin gold foil. This is done in a vacuum to avoid the alpha particles being absorbed by the air.
The main results of this experiment were:
Most of the alpha particles passed straight through the foil, with little or no deflection, being detected between positions A and B.
The fact that most of the particles passed straight through the foil, which was at least 100 atoms thick, suggested that the atom must be mostly empty space!
In order to produce the large deflections at C and D, the positively charged alpha particles must be encountering something of very large mass and a positive charge
The discovery of the neutron
Physicists realised that there must be another particle in the nucleus to stop the positive protons exploding apart. This is the neutron which was discovered by Chadwick in 1932. This explained isotopes – elements with the same number of protons but different numbers of neutrons.
Science now had an elegant theory which explained the numerous elements using only three particles: the proton; neutron and electron. However this simplicity did not last long.
Matter and antimatter
In 1928, Paul Dirac found two solutions to the equations he was developing to describe electron interactions. The second solution was identical in every way apart from its charge, which was positive rather than negative. This was named the positron, and experimental proof of its existence came just four years later in 1932.
(The positron is the only antiparticle with a special name – it means ‘positive electron’.)
Almost everything we see in the universe appears to be made up of just ordinary protons, neutrons and electrons. However high-energy collisions revealed the existence of antimatter. Antimatter consists of particles that are identical to their counterparts in every way apart from charge, e.g. an antiproton has the same mass as a proton but a negative charge. It is believed that every particle of matter has a corresponding antiparticle.
When a matter particle meets an anti-matter particle they annihilate, giving off energy. Often a pair of high energy photons (gamma rays) are produced but other particles can be created from the conversion of energy into mass (using E = mc2). Anti-matter has featured in science fiction books and films such as Angels and Demons. It is also the way in which hospital PET (Positron Emission Tomography) scanners work.
The particle zoo
The discovery of anti-matter was only the beginning. From the 1930s onwards the technology of particle accelerators greatly improved and nearly 200 more particles have been discovered. Colloquially this was known as the particle zoo, with more and more new species being discovered each year. A new theory was needed to explain and try to simplify what was going on. This theory is called the Standard Model
The experimental proof for the positron came in the form of tracks left in a cloud chamber. The rather faint photograph on the right shows the first positron ever identified. The tracks of positrons were identical to those made by electrons but curved in the opposite direction.
(You will learn more about cloud chambers and other particle detectors later in this unit.)
The standard model was developed in the early 1970’s in an attempt to tidy up the number of particles being discovered and the phenomena that physicists were observing.
How do you examine a particle to see if it is actually made from more fundamental particles? You smash it up!!
In a particle accelerator a very small particle, eg an electron, can be accelerated by electric and
magnetic fields to a very high speed. Being very small, speeds near to the speed of light may be
achieved. When these particles collide with a stationary target, or other fast-moving particles,
a substantial amount of energy is released in a small space. Some of this energy may be
converted into mass (E = mc2), producing showers of nuclear particles. By passing these
particles through a magnetic field and observing the deflection their mass and charge can be
For example, an electron with low mass will be more easily deflected than its heavier cousin, the Muon. A positive particle will be deflected in the opposite direction to a negative particle. Cosmic rays from outer space also contain particles, which can be studied in a similar manner.
Most matter particles, such as protons, electrons and neutrons have corresponding antiparticles. These have the same rest mass as the particles but the opposite charge. With the exception of the antiparticle of the electron (e-), which is the positron (e+), antiparticles are given the same symbol as the particle but with a bar over the top.
When a particle and its antiparticle meet, in most cases, they will annihilate each other and their mass is converted into energy. There are far more particles than antiparticles in the Universe, so annihilation is extremely rare.
At present physicists believe that there are 12 fundamental mass particles called Fermions which are split into two groups:
Leptons and Quarks
There are also 4 force mediating particles called Bosons. The table below shows the fundamental particles [at the moment!]
In 1964 Murray Gell-Mann proposed that protons and neutrons consisted of three smaller particles which he called ‘quarks’ (pronounced kworks). There are two first generation quarks called up and down. These make up neutrons and protons. There are two 2nd generation quarks called charm and strange. Finally there are two 3rd generation quarks called top and bottom. Each quark has only a fraction (⅓ or ⅔) of the electron charge (1.6 × 10-19 C). These particles also have other properties, such as spin, colour, quantum number and even something called strangeness, which are not covered by this course.
Quarks have been observed by carrying out deep-inelastic scattering experiments which use high energy electrons to probe deep into the nucleus. However, they have never been observed on their own, only in twos or threes where they make up what are called hadrons.
Baryons are made up of 3 quarks. Examples include the proton and the neutron.
The charge of the proton (and the neutral charge of the neutron) arise out of the fractional charges of their inner quarks. This is worked out as follows:
A proton consists of 2 up quarks and a down quark. Total charge=+1 ( = 1).
A neutron is made up of 1 up quark and 2 down quarks. No charge ( = 0).
Mesons are made up of 2 quarks. They always consist of a quark and an anti-quark pair.
An example of a meson is a negative pion (Π– = ū d). It is made up of an anti-up quark and a down quark: This gives it a charge of = -1.
Note: A bar above a quark represents an antiquark e.g. ū is the anti-up quark (this is not the same as the down quark.) The negative pion only has a lifetime of around 2.6 x 10-8 s
Particles which are made up of quarks are called hadrons (the word hadron meant heavy particle). The Large Hadron Collider at CERN collides these particles.
There are two different types of hadron, called baryons and mesons which depend on how many quarks make up the particle.
(National 5 only)
The Three Generations of leptons Leptons are a different type of particle which include the familiar electron which is a first generation particle. In addition, there is a second (middle) generation electron called the muon and a third (heaviest) generation electron called the tau particle. (The word lepton meant a light particle but the tau particle is actually heavier than the proton!)
Neutrinos All 3 leptons have a “ghostly” partner associated with it called the neutrino. This has no charge (its name means little neutral one). There is an electron neutrino, a muon neutrino and a tau neutrino.
Neutrinos were first discovered in radioactive beta decay experiments. In beta decay, a neutron in the atomic nucleus decays into a proton and an electron. When physicists were investigating beta decay they came up with a possible problem, the law of conservation of momentum appeared to be being violated.
To solve this problem, it was proposed that there must be another particle emitted in the decay which carried away with it the missing energy and momentum. Since this had not been detected, the experimenters concluded that it must be neutral and highly penetrating.
This was the first evidence for the existence of the neutrino. (In fact, in beta-decay an anti-neutrino is emitted along with the electron as lepton number is conserved in particle reactions).
Interesting facts More than 50 trillion (50x 1012) solar neutrinos pass through an average human body every second while having no measurable effect. They interact so rarely with matter that massive tanks of water, deep underground are required to detect them
Fundamental particles The 6 quarks and 6 leptons are all believed to be fundamental particles. That is physicists believe that they are not made out of even smaller particles. It is possible that future experiments may prove this statement to be wrong (just as early 20th Century scientists thought that the proton was a fundamental particle.)
Forces and Bosons In the nucleus of every element other than hydrogen there is more than one proton. The charge on each proton is positive, so why don’t the protons fly apart, breaking up the nucleus?
The particle responsible for carrying the strong force is called the gluon.
The weak nuclear force is involved in radioactive beta decay. It is called the weak nuclear force to distinguish it from the strong nuclear force, but it is not actually the weakest of all the fundamental forces. It is also an extremely short-range force.
The electromagnetic force stops the electron from flying out of the atom. The theory of the electromagnetic force and electromagnetic waves was created by the Scottish Physicist James Clerk Maxwell in the 19th Century.
The final force is gravity. Although it is one of the most familiar forces to us it is also one of the least understood.
It may appear surprising that gravity is, in fact, the weakest of all the fundamental forces when we are so aware of its affect on us in everyday life. However, if the electromagnetic and strong nuclear forces were not so strong then all matter would easily be broken apart and our universe would not exist in the form it does today.
There is a short range force that exists that holds particles of the same charge together. This force is stronger than the electrostatic repulsion that tries to force the particles apart. We call it the strong force.
This force acts over an extremely short range [approx 10-15 m], of the order of magnitude of a nucleus. Outside of this range the strong force has no effect whatsoever. If a proton was placed close to a nucleus it would be repelled and forced away.
Force particles – The bosons Each force has a particle associated with it which transmits the effects of that force. The table below summarizes the current understanding of the fundamental forces.
At an everyday level we are familiar with contact forces when two objects are touching each other. Later in this unit you will consider electric fields as a description of how forces act over a distance. At a microscopic level we use a different mechanism to explain the action of forces; this uses something called exchange particles. Each force is mediated through an exchange particle or boson.
Many theories postulate the existence of a further boson, called the Higgs boson (sometimes referred to as the ‘God particle’), which isn’t involved in forces but is what gives particles mass. Attempts to verify its existence experimentally using the Large Hadron Collider at CERN and the Tevatron at Fermilab were rewarded on the 4th July 2012 when the announcement was made that the Higgs boson had been discovered
The Higgs boson plays a unique role in the Standard Model, by explaining why the other elementary particles, except the photon and gluon, are massive. In particular, the Higgs boson would explain why the photon has no mass, while the W and Z bosons are very heavy. The Higgs itself is incredibly massive with a mass equivalent to that of 133 protons (10-25 kg).
Practical Uses of Antimatter
Positron Emission Tomography (PET) Scanning Positron emission tomography (PET) scanners use antimatter annihilation to obtain detailed 3-D scans of body function. Other imaging techniques called CT and MRI scans can give detailed pictures of the bone and tissue within the body but PET scans give a much clearer picture of how body processes are actually working.
A β+ tracer with a short half-life is introduced into the body attached to compounds normally used by the body, such as glucose, water or oxygen.
When this tracer emits a positron it will annihilate nearly instantaneously with an electron. This produces a pair of gamma-ray photons of specific frequency moving in approximately opposite directions to each other.
(The reason it is only an approximately opposite direction is that the positron and electron are moving before the annihilation event takes place.)
The gamma rays are detected by a ring of scintillators, each producing a burst of light that can be detected by photomultiplier tubes or photodiodes. Complex computer analysis traces tens of thousands of possible events each second and the positions of the original emissions are calculated. A 3-D image can then be constructed, often along with a CT or MRI scan to obtain a more accurate picture of the anatomy alongside the body function being investigated.
The detecting equipment in PET scanners has much in common with particle detectors and the latest developments in particle accelerators can be used to improve this field of medical physics.
Tracing the use of glucose in the body can be used in oncology (the treatment of cancer) since cancer cells take up more glucose than healthy ones. This means that tumours appear bright on the PET image. Glucose is also extremely important in brain cells, which makes PET scans very useful for investigation into Alzheimer’s and other neurological disorders. If oxygen is used as the tracking molecule, PET scans can be used to look at blood flow in the heart to detect coronary heart disease and other heart problems.
Forces on Charged Particles Introduction
You may wonder why it is important to study charged particles? What use are they to us in everyday life? Well, without charged particles we wouldn’t have an electric current – unthinkable in our technological age. But there are more applications you may not have thought of. Laser printers and photocopiers use charged particles to get the toner to stick to the paper and car companies use charged particles to ensure that spray guns paint cars evenly. The Large Hadron Collider accelerates positively charged protons to 99% the speed of light, then collides them head on to try and recreate the conditions that existed when the Universe was 1/100th of a billionth of a second old. Scientists then study these collisions to try and explain what mass is and what 96% of the universe is made of.
In this section we will study how charged particles move in electric and magnetic fields. We will then study the different types of particle accelerator and their applications.
Forces on Charged Particles
Force Fields The idea of a field should be familiar to you. In Physics, a field means a region where an object experiences a force without being touched. For example, there is a gravitational field around the Earth. This attracts masses towards the Earth’s centre. Magnets cause magnetic fields and electric charges have electric fields around them.
Electric Field Patterns
These are called radial fields. The lines are like the radii of a circle. The strength of the field decreases as we move away from the charge.
The field lines are equally spaced between the parallel plates. This means the field strength is constant. This is called a uniform field.
In an electric field, a charged particle will experience a force. We use lines of force to show the strength and direction of the force. The closer the field lines the stronger the force. Field lines are continuous they start on positive charge and finish on negative charge. The direction is taken as the same as the force on a positive “test” charge placed in the field.
Electric fields have a number of applications and play an important role in everyday life. For example,
• the cathode ray tube (the basis for traditional television and monitor systems)
• paint spraying, e.g. for cars
• photocopying and laser printing
• pollution control.
Stray electric fields can also cause problems, for example during lightning storms there is a risk of damage to microchips within electronic devices caused by static electricity.
If an electric field is applied to a conductor it will cause the free electrons in the conductor to move
We have seen already that electric fields are similar to gravitational fields. Consider the following:
If a mass is lifted or dropped through a height then work is done i.e. energy is changed.
If the mass is dropped then the energy will change to kinetic energy.
If the mass is lifted again then the energy will change to gravitational potential energy.
Change in gravitational potential energy = work done
Now consider a negative charge moved through a distance in an electric field
If the charge moves in the direction of the electric force, the energy will appear as kinetic energy. If a positive charge is moved against the direction of the force, as shown in the diagram, the energy will be stored as electric potential energy
Potential difference (p.d.) is defined to be a measure of the work done in moving one coulomb of charge between two points in an electric field. Potential difference (p.d.) is often called voltage. This gives the definition of the volt.
There is a potential difference of 1 volt between two points if 1 joule of energy is required to move 1 coulomb of charge between the two points, 1 V = 1 J C−1
This relationship can be written mathematically: Ew = QV
Where Ew is energy (work done) in joules (J), Q is the charge in coulombs (C) and V is the potential difference (p.d.) in volts (V).
If the small positive charge, above, is released there is a transfer of energy to kinetic energy, i.e. the charge moves. Again, using the conservation of energy means that;
Ew = EK
QV = ½mv2
Example: A positive charge of 3.0 µC is moved from A to B. The potential difference between A and B is 2.0 kV.
(a) Calculate the electric potential energy gained by the charge–field system.
(b) The charge is released. Describe the motion of the charge.
(c) Determine the kinetic energy when the charge is at point A.
(d) The mass of the charge is 5.0 mg. Calculate the speed of the charge
(a) Q = 3.0 C = 3.0 × 10−6 C
V = 2.0 kV = 2.0 × 103 V
Ew = ?
(b) The electric field is uniform so the charge experiences a constant unbalanced force. The charge accelerates uniformly towards the negative plate A
(c) By conservation of energy,
EK = Ew = 6.0 × 10−3 J
(d) m = 5.0 mg = 5.0 × 10−6 kg
EK = 6.0 × 10−3 J
v = ?
Ew = QV
Ew = 3.0 × 10−6 × 2.0 × 103
Ew = 6.0 × 10−3 J
Ew = QV
Ew = 3.0 × 10−6 × 2.0 × 103
Ew = 6.0 × 10−3 J
EK = ½mv2
6.0 × 10−3 = 0.5 × 5.0 × 10−6 × v2
v2 = 2.4 × 10−3
v = 49 m s−1
Charged Particles in Magnetic Fields
The discovery of the interaction between electricity and magnetism, and the resultant ability to produce movement, must rank as one of the most significant developments in physics in terms of the impact on everyday life.
This work was first carried out by Michael Faraday whose work on electromagnetic rotation in 1821 gave us the electric motor. He was also involved in the work which brought electricity into everyday life, with the discovery of the principle of the transformer and generator in 1831. Not everyone could see its potential. William Gladstone (1809–1898), the then Chancellor of the Exchequer and subsequently four-time Prime Minister of Great Britain, challenged Faraday on the practical worth of this new discovery – electricity. Faraday’s response was ‘Why, sir, there is every probability that you will soon be able to tax it!’ The Scottish physicist, James Clark Maxwell (1831–1879), built upon the work of Faraday and wrote down mathematical equations describing the interaction between electric and magnetic fields. The computing revolution of the 20th century could not have happened without an understanding of electromagnetism.
Magnetic Field Around A Current Carrying Wire
In 1820 the Danish physicist Oersted discovered that a magnetic compass was deflected when an electrical current flowed through a nearby wire. This was explained by saying that when a charged particle moves a magnetic field is generated. In other words, a wire with a current flowing through it (a current-carrying wire) creates a magnetic field.
Moving charges experience a force in a magnetic field
A magnetic field surrounds a magnet. When two magnets interact, they attract or repel each other due to the interaction between the magnetic fields surrounding each magnet.
A moving electric charge behaves like a mini-magnet as it creates its own magnetic field. This means it experiences a force if it moves through an external magnetic field (in the same way that a mass experiences a force in a gravitational field or a charge experiences a force in an electric field.)
Simple rules can be used to determine the direction of force on a charged particle in a magnetic field.
Movement of a negative charge in a magnetic field
One common method is known as the right-hand motor rule. This is shown in the figure on the right. The thumb gives the motion (M), the first finger gives the field (F) and the second finger is the direction of electron current (I).
Movement of a positive charge in a magnetic field For a positive charge, the direction of movement is opposite to the direction worked out above. It is easiest to work out which way a negative charge would move using the right hand rule and then simply reverse this.
If a charge travels parallel to the magnetic field, it will not experience an additional force. The direction of the force is determined using the same right hand rule. The speed of the charge will not change, only the direction of motion changes.
Electron curves out of page Electron curves to the right No change in direction
The Electric Motor
When a current-carrying wire is placed between the poles of a permanent magnet, it experiences a force. The direction of the force is at right-angles to:
the direction of the current in the wire;
the direction of the magnetic field of the permanent magnet
We can utilize this principle in the electric motor;
An electric motor must spin continuously in the same direction. Whichever side of the coil is nearest the north pole of the field magnets above must always experience an upwards force if the coil is to turn clockwise.
That side of the coil must therefore always be connected to the negative terminal of the power supply. Once the coil reaches the vertical position the ends of the coil must be connected to the opposite terminals of the power supply to keep the coil turning. This is done by split ring commutator.
In order for the coil to spin freely there cannot be permanent fixed connections between the supply and the split ring commutator. Brushes rub against the split ring commutator ensuring that a good conducting path exists between the power supply and the coil regardless of the position of the coil