3.1 thermal concepts


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All about basic Thermal concepts such as energy flows and temperatures

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3.1 thermal concepts

  1. 1. Topic 3 - Thermodynamics3.1 – Thermal Concepts
  2. 2. Energy in Substances● All objects have internal energy (U)● This is because the internal particles arevibrating randomly in the substance and thereare electrical forces between them.● Internal energy is the sum of the kinetic andpotential energies of the internal particles of asubstance.● The potential energy is due mainly to the electricfields between the particles, both bonding andintermolecular
  3. 3. Energy in a Substance● The energy in an object is internal energy● The transfer of energy between objects bynon-mechanical methods is thermal energy.● The temperature of a body is a measure of theaverage internal kinetic energy of theparticles in the substance.
  4. 4. Energy Flows● Internal Energy flows as thermal energy fromareas of high temperature to areas of lowtemperature.● The rate of flow is directly proportional to thetemperature difference between the twoobjects.● This means that the internal energy of thecolder object increases at the same rate as theinternal energy of the hot object decreases.
  5. 5. Energy FlowsHot object Cold object Warm objects
  6. 6. Energy Flows● Thermal Energy can transferred by 3 methods● It can be conducted by molecules vibrating intothe ones around it● It can be convected, where energy moves dueto the movement of particles● It can be radiated where the energy is given offas electromagnetic waves due to thetemperature of the object.
  7. 7. Energy Flows - Conduction● Conduction works best in solids than in fluids.● Conduction is due to the particles vibrating intoeach other causing the energy to be transferredfrom one to the next● Conduction worksbetter in metalsthan non-metalsdue to the additionof free electrons.
  8. 8. Energy Flows - Convection● Convection works as a method of thermal transfer influids.● As one area of fluid warms up, the particles move furtherapart from each other.● This means there the localdensity decreases.● Colder, higher density fluidflows in from thesurroundings pushing thewarm fluid upwards andtaking the energy with it.
  9. 9. Energy Flows - Radiation● All hot objects emit electromagnetic radiation of various wavelengths.● This is due to the vibration of the charges in the object causingradiation to be emitted.● The hotter the object the higher the peak intensity, and the wider thespectrum emitted● The rate of emission alsodepends on the surfacenature.● Dull black bodies are betteremitters and absorbers thanshiny silver surfaces
  10. 10. Temperature Scales● The thermal energy absorbed or emitted by abody cannot be directly measured.● The temperature, the average kinetic energyper molecule, can be measured using athermometer.● The common scales are Celsius, Fahrenheit andKelvin.
  11. 11. Temperature Scales● A temperature scale is usually defined by either twofixed points, or 1 fixed point and a fixed interval.● Celsius is defined by the fixed points of the melting andboiling point of pure water at standard pressures. (0 and100 oC)● Fahrenheit is defined by the coldest temperature thatcould be achieved in a lab at the time (0F) and theblood temperature of a race horse (100F)● The kelvin scale is a measure of the absolutetemperature of an object and is defined as 0K is thecoldest temperature possible with an interval equal to1oC.
  12. 12. Kelvin Temperatures● As temperature is proportional to the kineticenergy of the molecules, it makes no sense totalk of negative temperature.● Negative Kinetic energy doesnt exist!)● The Kelvin scale avoids this problem and mustalways be used in thermodynamics questions.● The Celsius and Kelvin temperatures are linkedby:T K = Co273.16
  13. 13. Absolute Zero● At a temperature of 0K, the average kineticenergy of the particles is zero. They are notmoving on average.● This temperature is called absolute zero and iscurrently thought to be -273.15oC.● However, this cannot be actually attained in thelaboratory.
  14. 14. Particles in Substances● When we think about substances we often thinkabout their macroscopic properties.● Mass, dimension, average temperature.● However, we sometimes need to think aboutthe properties of individual particles.● These are the microscopic properties.● Internal energy, molecular mass, shape.
  15. 15. Particles in Substances● A sample of a pure elemental substance willcontain a fixed number of atoms.● The amount of substance is known as the mole(mol)● The number of atoms in 1 mole of carbon-12 isthe same as the number of atoms in 1 mole ofuranium-238.● Clearly, these will not have the same total mass.
  16. 16. Molar MassMolar Mass=Mass of substance(g)number of molesM =mn● The mass of 1 mole of substance is given by:● By definition, 12g of carbon-12 is defined tocontain 1 mol of atoms.● Therefore 238g of uranium-238 has 1 mol ofatoms.● The molar mass of the elements can be found fromthe periodic table (atomic mass)
  17. 17. Avagadros Number● Avagadros found that equal volumes of different gases atstandard temperature and pressure contained equalnumbers of atoms.● STP = 273K & 101.3kPa● The number of particles in one mole of any gas is theAvagadro number.● It is defined as NA=6.022x1023mol-1● The actual number of entities in a sample is thus:● N is the number of particles, n is the number of moles.N = N A n
  18. 18. Molecular Mass● It is often important to work with molecules andnot elements.● e.g. SiO2, O2, CO2● To find the mass of 1 mole of this molecule,simply sum the atomic masses of eachcomponent atom.● e.g. SiO2– 28 + 2x16 = 60g mol-1
  19. 19. Questions● Express the following temperature in K● 22oC, 45oC, 37oC, 578oC, -96oC● Express the following temperatures in oC.● 6600K, 75K, 4K, 373K, 687K● Calculate the number of moles present in:● 82g of O2, 76g of H2, 97g of H2O, 350g of Fe2O3● Calculate the number of atoms present in:● 24g of SiO2, 98g of Al2O3, 45g of NH4NO3