FAMILIAR FORCES Probably the most familiar force is the gravitational force. It does not only pull objects towards the centre of the Earth. The Earth is actually pulled up by the man! With the same force! Gravity is a force between any two objects with mass. For small amounts of mass the force is tiny

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It is useful to see the formula The size of the gravitational force between two objects is given by Newton's Universal Law of Gravitation It is useful to see the formula M and m are the masses of the two objects k is a fixed number- (Newton's gravitational constant) r is the distance between the objects F F M m r

Familiar forces 2 The second familiar force is the electromagnetic force Every force that we are familiar with (that is not due to gravity) is due to electromagnetism e.g. friction contact forces magnetism, electrostatic force upthrust

Electromagnetism For instance all contact forces are due to “electromagnetic repulsion” of this kind

+ + r The electrostatic force like the gravitational force also obeys an inverse square law. The force depends on each charge and it gets weaker as the distance r increases. The force formula is: Where k = 9 x 109

10-15m The charge on each proton is 1.6 x10-19C In an atomic nucleus protons can be separated by as little as 1 femtometre (1 x 10-15m) Using the electrostatic force formula

230N Now we know that the mass of nucleons is tiny. We could use the formula : Force = mass x acceleration (F=ma) to calculate how fast they must accelerate away from each other and we get ridiculously large answers!!

The problem with the existence of the atomic nucleus The atomic nucleus consists of protons and neutrons. It has (for its size) a massive positive charge. How can it hold together against this repulsive electromagnetic force? There has to be some previously unknown force doing it. This force must be incredibly strong because it is not easy to break up nuclei. The force holding the nucleus together became known as the STRONG NUCLEAR FORCE (or Strong Interaction).

The Strong Force The strong force must only operate at very short distances because two nuclei approaching each other are generally repelled electromagnetically

The strong force Repulsive force The strong force has no effect at distances of greater than 5fm At distances below 2 fm the strong force is very attractive and binds protons together 20 10 -10 -20 Force/ kN 1 2 3 4 5 Separation fm At very small distances the strong force is repulsive and so protons are not drawn in to collapse to a single point. Attractive force The blue line represents the electromagnetic force

The force between protons Repulsive force This graph shows the combined effect of the strong force and electromagnetic force between protons. 20 10 -10 -20 Force/ kN 1 2 3 4 5 Separation fm Protons are trapped at separations of approximately 1 to 2 fm. Large nuclei contain many protons and they are too far apart for the strong nuclear force to overcome the electromagnetic force. Lots of neutrons are need to “glue” them together. Large nuclei can still fall apart much more easily. Attractive force

Hadrons The particles in the nucleus of an atom are affected by the strong force. Particles which are affected by the strong force are referred to as HADRONS.

Leptons Protons and neutrons are Hadrons. Electrons are NOT hadrons because they are not affected by the strong force. Electrons belong to a class of particle called the LEPTONS ( from the Greek word for light)

Natural radioactive decay Alpha emission

Alpha particles Alpha particles are emitted naturally from some large nuclei. They are identical to the nucleus of a helium atom and consist of 2 neutrons and two protons. A typical alpha emitter is When alpha particles are emitted the new nucleus will have reduced proton number(Z) and nucleon number (A)

Alpha decay Example U-238

DAUGHTER PRODUCT PARENT

The NZ plot Alpha and beta emission are often represented on a graph of neutron number against proton number. This is known as an NZ plot

147 146 145 144 143 142 141 140 139 Alpha decay represents a loss of 2 neutrons and 2 protons from the nucleus N 85 86 87 88 89 90 91 92 93 94 95 Z

Where does the alpha particle get its kinetic energy? In 1905 Einstein showed that mass and energy are intimately related by the formula E = mc2. This really means that mass is a sort of super concentrated energy. When an alpha particle is emitted by a nucleus a negligible part of the mass of the nucleus is lost and converted directly into the kinetic energy of the alpha particle.

The velocity of the alpha particles. In the decay of uranium-238 all of the alpha particles have the same amount of kinetic energy. This is because each decay involves the same amount of energy. Every time this happens the same amount of mass is lost as KE The amount of mass converted into energy in this process is so tiny that we can always ignore it in OUR calculations

They all travel the same distance in air before absorption. Alpha particles from the same source are MONOENERGETIC. They are produced with the same initial energy They all travel the same distance in air before absorption. Number of alpha particles reaching detector Alpha source Alpha particles travel a few cm in air. range

The Conservation of Energy ` This is a fundamental principle in physics “energy can never be destroyed or created from nothing in any process”. The importance of this principle cannot be overstated Another way of saying this is that in any process “energy is a conserved quantity”.

Beta emission β- Beta emission is the emission of an electron from the NUCLEUS of an atom. This is very peculiar because the nucleus of an atom does not contain any electrons!

Beta emission What is actually happening in the atomic nucleus is that a neutron is changing into a proton at the same time: p n

Beta Emission When beta emission occurs in the atomic nucleus, it has the effect of increasing the proton number. The daughter nucleus has a proton number which is larger than the parent by 1 unit. There is no effect on the nucleon number One of the neutrons here has turned into a proton. Note that this equation is incomplete

Beta emission Beta particles have a continuous range of energies and therefore travel different distances in air This gave theorists a big problem in the 1920’s! How could beta particles have different energies if they are all produced in an identical process?? beta source Some theorists thought of abandoning the conservation of energy!!

Energy Beta Particles have a range of energies The number of beta particles with a particular energy Beta Particles have a range of energies Energy

The Neutrino To get over this problem, physicist Wolfgang Pauli proposed that there might be another particle of tiny mass that is produced in every beta decay at the same time as the electron is produced.

Intensity Energy Electrons have a Range of energies The energy of the missing particle always adds back the missing energy so that each B decay event does release the same total energy Energy

The antineutrino This new particle was eventually discovered in 1956 and for technical reasons became known as the antineutrino The symbol is made up of the Greek letter “nu”. The bar tells you its an antineutrino and the little e suffix tells you that it is associated with the electron.

Notice that on the NZ plot of beta decay the proton number increases by 1 and the neutron number N decreases by 1 N 9 8 7 6 5 4 3 2 1 1 2 3 4 5 6 7 8 9 10 11 Z

Be careful with these plots as sometimes nucleon number (A) can appear on the Y axis instead of neutron (N) number A 15 14 13 12 11 10 9 8 7 1 2 3 4 5 6 7 8 9 10 11 Z