Unit 5 ~ 1

Unit 5 ~ 1

1. Radioactivity

   • The discovery of the nucleus
     • The atom's mass is concentrated in the nucleus, and it is positively charged.
     • The nucleus was discovered by Rutherford. He found the when you accelerated positively charged particles towards a the piece of gold foil, most would pass through but some would be deflected.
       • This showed that most of an atom is made up of empty space, but there are concentrated positive charges within in them
       • 1 in 2000 were deflected. 1 in 10 000 were deflected larger than 90*
   • The properties of a, b & y radiation
     • Alpha radiation, A, is the emission of a helium nucleus from the nucleus of a radioactive atom.
     • Beta radiation, B+-, is the emission of either a electron or a positron from the nucleus of a radioactive atom.
     • Gamma radiation, Y, is the emission of a gamma ray from the nucleus of a radioactive atom.
     • Marie Curie discovered the nature of radioactive materials.
     • Rutherford's investigations on radioactivity
       • He found the radiation:
         • Ionised air, making it conduct electricity.
         • Was of two types. One types which he called alpha radiation was easily absorbed. The other type which he called beta radiation was more penetrating. A third type of radiation, gamma,, even more penetrating the B, was discovered a year later.
     • Ionisation
       • A causes strong ionisation. However its effects can only be registered over a short distance.
       • B causes much weaker ionisation. It's range is much larger than A though.
       • Y has a much weaker ionising effect than either A or B. This is because photons carry no charge so they have less effect than ether A or B. but their range is still larger than A or B.
     • Cloud chamber observations
       • A particles produce straight tracks that radiate from the source and are easily visible. The tracks from a given isotope are all o the same length, indicating that the A particle have the same range.
       • B particles produce whispy tracks that are easily deflected as a result of collisions with air molecules. The tracks are not as easy to see as A track because B particles are less ionising then A particles.
     • Absorption tests
       • A radiation is absorbed by most things, including skin and paper
       • B radiation is not as easily absorbed. it can pass through paper but is stopped by foil.
       • Y radiation can pass through most things but is stopped by a few cm of lead.
     • Range in air
       • A radiation has a range of only a few cm in air. Alpha particles from the same source have the same kinetic energy, this means they roughly travel the same distance before stopping.
       • B radiation has a range in air of up to about a metre. Beta particles have a range of kinetic energy up to a maximum, and 1m is as far the max KE can go.
       • Y radiation has an unlimited range in air. It is harder to register the gamma radiation further away from its source as it spreads out spherically.
   • The dangers of radioactivity
     • Ionising radiation is harmful, it can:
       • Destroy cell membranes which cause cells to die
       • Damage vital molecules such as DNA directly or indirectly by creating 'free radical' ions which react with vital molecules. Normal cell division is affected and nuclei become damaged. Damaged DNA may cause cells to divide uncontrollably, causing a tumour which may be cancerous. Damaged DNA in a sex cell may cause a mutation which may be passed on to future generations.
     • Monitoring radiation is important, to keep people such as radiographers safe.
       • They wear badges, that were made up of different plastics, an open window, then layers of tin lead and cadmium.
     • Safe us of radiation is made up of: air (radon gas), medical, ground and building, food and drink, cosmic rays, nuclear weapons, air travel & nuclear power.
     • Background radiation occurs naturally due to cosmic radiation and from radioactive materials like rocks and radon.
     • Storage of radioactive matter: should be lead-lined containers. most radioactive sources produce Y radiation as well as A and B so the lead lining of a container must be thick enough to reduce the Y radiation from the sources in the container to about the background level.
     • When using radioactive materials, established rules and regulations must be followed. No source should be allowed to come into contact with skin.
   • The theory of radioactive decay
   • Radioactive decay
   • Radioactive isotopes in use
   • More about decay models
   • Nuclear radius
   • More about A, B and Y radiation
     • The nucleus of a radioactive atom changes during decay.
       • When an Alpha decay occurs, the nucleus of the original atom changes. It's proton number is reduced by two and the the atomic mass is reduced by four. This means that it is now a different (lighter) element.
       • When Beta + decay occurs, a proton decays into a neutron and a positron. The positron is released, and this is the Beta + particle. This reaction causes the original nucleus to change element (down by one) but remain the same (atomic) mass.
       • When Beta - decay occurs, a neutron decays into a proton and an electron. The electron is released, and this is the Beta - particle. This reaction causes the original nucleus to change element (up by one) but remain the same (atomic) mass.
     • The intensity, Y
       • For a source that emits n Y photons per second, each of energy hf, the radiation energy per second from the source = nhf.
         • At distance r from the source, all the photons emitted from the source pass through the total area = 4 Pi r^2 (the surface area of a sphere of radius r)
           • So intensity, I = (radiation energy per second)/total area = nhf/4Pir^2
       • Inversely proportional to distance squared.
     • Electron capture can also occur to make an atom's nucleus more stable. When this happens a nucleus takes in an electron. A proton turns into an electron. This makes the elemental number go down by one, the atomic mass stays constant and a neutron is released.

2. Nuclear energy

   • Energy and mass
   • Binding energy
   • Fission and fusion
   • The thermal nuclear reactor

3. Thermal physics

   • Internal energy
     • Definition: of an object is the sum of random distribution of kinetic and potential energies of its molecules.
     • The lowest possible temperature is absolute zero.
       • This is 0 K
       • Which is equal to -273.15*C
     • A gas thermometer is a thermometer consisting of a dry gas in a sealed container. The pressure of the gas it proportional to the absolute temperature of the gas. All thermometers are calibrated by these.
     • Heat transfer occurs when:
       • There is a difference of temperature.
       • When one object does work on another one.
     • A temperature scale is defined in terms of fixed points which are standard degrees of hotness which can be accurately reproduced.
       • The Celsius scale:
         • Ice point, 0 *C, which is the temperature of melting ice.
         • Steam point, 100*C , which is the temperature of steam at standard atmospheric pressure.
       • The absolute scale (kelvins K)
         • Absolute zero, 0K, which is the lowest possible temperature
         • The triple point of water, 273.16K, which is the temperature at which ice, water and water vapour coexist in thermodynamic equilibium.
   • Change of state
     • Latent heat
       • This is released when a gas condenses, or a liquid solidifies.
       • Latent heat of fusion, is the energy need to melt a solid.
       • Latent heat of vaporisation, is the energy needed to vaporise a liquid.
     • Energy = ml, where l is latent heat of vaporisation or fusion.
     • When a substance is changing state, its temperature remains constant. This is because the energy that was previously been supplied or removed, is being used to change state.
   • Specific heat capacity
     • The specific heat capacity, c, of a substance is the energy needed to raise the temperature of unit mass of the substance by 1K without change of state. (Jkg^-1K^-1)
       • The specific heat capacity of water is 4200 Jkg^-1K^-1
     • The higher the specific heat capacity, the more energy need to change the temperature.
     • The temperature change of a object when heated depends on
       • The mass of the object.
       • The amount of energy supplied to it.
       • The substance from which the object is made.
     • Energy to change temperature = mc(T2-T1)

4. Gases

   • The experimental gas laws
     • Boyle's law
       • Boyle's law states that for a fixed mass of gas at constant temperature, pV = constant
         • A change at constant temperature is called an isothermal change
     • Charles' law
       • For a fixed mass of gas at constant pressure, V/T = constant
     • The pressure law
       • For a fixed ,ass of gas at constant volume, P/T = constant
     • The pressure of a gas is the force per unit area that the gas exerts normally (i.e. at right angles to) on a surface. It is measured in pascals (Pa), where 1 Pa = 1Nm^-2. The pressure of a gas depends on its temperature, the volume of the container and the mass of gas in the container.
   • The ideal gas law
     • Definition ; a gas under conditions such that it obeys Boyles law.
     • The Avogadro constant, Na, is defined as the number of atoms in exactly 12g of the carbon isotope 12/6C. The value of Na ( to 4 significant figures) is 6.023 x 10^23. Therefore the mass of an atom of 12/6C is 1.993 x 10^-23 (= 12g/6.02 x10^23)
     • One mole of a substance consisting of identical particles is defined as the quantity of substance that contains Na particles. The number of moles in a certain quantity of substance is its molarity. The unit of molarity is mol.
       • The molar mass of a substance is the mass of 1 mol of the substance. the unit for molar mass is kgmol^-1. For example, the molar mass of oxygen gas is 0.032kgmol^-1 of oxygen gas contains Na oxygen moleules.
       • 1 The number of moles in mass Ms of a substance = Ms/M, where M is the molar mass of the substance.
       • 2 the number of molecules in the mass Ms of a substance = NaMs/M
     • The three experimental gas laws can be combined to give the equation: pV/T = constant, for a fixed mass of ideal gass
       • It can also be written as pVm = RT
         • pV=nRT
       • Molar gas constant, R
     • In gas calculations, the temperature must be in kelvins; be clear about molar mass and molecular mass
     • The mass Ms of a substance is equal to its molar mass, M X the number of moles, n.
       • Ms = M X n
     • The density of an ideal gas of molar mass M:
       • q = mass, Ms/ volume V
         • =nM/V
           • =pM/RT
     • In the equation pV=nRT, substituting the number of moles n=N/Na (N=total number of molecules) gives:
       • pV=Nkt
         • Where k, is the Boltzman constant: k = R/Na
   • The kinetic theory of gases
     • Boyle's law ex.: the pressure of a gas at constant temperature is increased be reducing its volume because the gas molecules travel less distance between impacts at the walls due to reduced volume.
     • Pressure lay ex.: the pressure of a gas at constant volume is increased by raising its temperature. The average speed of the molecules is increased by raising the gas temperature so the impacts of the molecules on the container walls are harder and more frequent.
     • The root mean square speed, is used as despite when a molecules speed changes, the distribution of molecules' speed does not change, therefore the appears to be no change the overall speed.
       • Grms = [(G1^2+G2^2+...+Gn^2)/N]^(1/2)
       • If the temperature increases, its molecules move faster on average.
     • pV=(1/3)Nm(Grms)^2
       • Check p216 for derivation (its long but makes sense)
     • The mean kinetic energy of a molecule of the gas = (1/2)*m(Grms)^2
       • = (3/2)kT
     • The total kinetic energy of n moles of an ideal gas = (3/2)nRT

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