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Download Thermodynamics lesson 1 Tempersture
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Thermodynamics Should take about 7 weeks Outcomes • Be able to explain thermal equilibrium • Describe the absolute scale of temperature (i.e. the thermodynamic scale) that does not depend on property of any particular substance and explain why the triple point is used. • To be able to use and convert temperature measurements both in degrees Celsius (°C) and in kelvin (K) • To recall that T(K)≈θ(°C) + 273… Energy • A can of worms – particularly at KS3 – Transfer – Transport – Transform – Stores – Pathways • A problem, but not for today • ( Some stuff on wiki, Millar) Temperature and Heat Light touch today, more another time • Temperature – a measure of hotness or coldness of an object • Heat (in parent language “we’ll see”) – Energy – Depends upon • Mass • Temperature • Nature of object (specific heat capacity) Temperature • We all know what temperature is. • So discuss. • Watch the demo • Zeroth Law. Temperature Scales • F,R,C,K • oF is for old people, like pounds and ounces BUT conversion is a skill so lets not dispose of it all together • R just for some US engineers • oC not C. Centigrade just means that, we want Celsius, and degress at that. • K is not oK as it is absolute. small point but important Lets look at heat moving (thermal transfer or energy) HEAT Work and heat Work: energy transferred to a system by the application of a force (ΔW) Heat: energy transferred not by a force and our old friend ΔT is the driving force for this (ΔQ) Now, we are nearly ready to jump into the world of thermodynamics We want to care about particles Just not yet – stay macro. Bulk properties = not particle Lets look at reality – go Macro When I heat things, they expand Thermal Expansion ΔL = k L ΔT So? Well the amount something expands when heated depends on how long it was in the first place (L), the amount it’s temperature changes (ΔT) and something to do with the material (k) called the coefficient of thermal expansion. Two important physics ideas • The coefficient: A way of making an inequality into an equals. BUT the key thing here is that it is something for a material and NOT an object. Work out k for Copper and you can do the sums for any object made of copper • The gradient: A driving force behind so much of things happening in physics. ΔT here but could be anything – ΔK is equivalent to ΔoC but best go the K way Let’s quantify ‘heating up’ E = m c ΔT c is specific heat capacity of material Units= Jkg-1K-1 What happens when state changes? Possibly not what you might expect Because of state change m c ΔT isn’t enough E=mL L is specific latent heat of fusion/vaporisation Units= Jkg-1 Change of state AT CONSTANT TEMPERATURE QE = m c ΔT QE == m m LL Q=mL Q = m c ΔT Q = m c ΔT Q=mL Q = m c ΔT Both L and c are material and not object specific quantities, much more useful. Thermal transfer of energy Conduction – Transferred directly within a material – ΔT across material is the driving force Convection – Transport by bulk movement – Density, buoyancy, currents – Free and forced, Newton, T or T5/4 Radiation – By means of electromagnetic waves – The black body – Stephan Conduction • Good conductors (metals) it’s mainly electrons • Poor conductors it’s mainly inter-atomic collisions We have idealised models Because the truth is messy Thermal conductivity We can quantify an ideal situation Q/t = k A ΔT/L Q/t = Rate of heat flow k = Thermal conductivity (Wm-1K-1) A = Cross sectional area ΔT/L = Temperature gradient An experimental value U takes into account the reality of the situation including convection at surface and a slow moving ‘trapped’ layer Radiation • The energy radiated per second – Area – Temperature – Nature of object • Why T and not ΔT? – Well, we are all at it. It is just often Qin=Qout • What comes out? – A continuous span of wavelengths, dependent upon T – At T < 1000K almost all IR – At T > 1000 Visible and UV also (1700K is white hot) Radiation The energy radiated per second – Area – Temperature – Nature of object As an equation Q/t = e σ A T4 Q/t = rate of energy emitted by radiation e = emissivity (B=1 skin=0.7) σ = SB constant 5.67 × 10-8 Js-1m-2K-4 A = Area T4= Temperature K Some more terms Internal energy: Potential energy in bonds and KE of particle motion (ΔU) Adiabatic: No heat transfer (ΔQ=0) Isothermal: You guessed it (ΔT=0) Now, lets go... 0,1 Zeroth: If Q=0 then ΔT = 0 First: ΔQ = ΔU + ΔW Signs really matter • ΔQ = Heat entering • ΔU = Change in internal energy • ΔW = Work done BY body ΔQ ΔU ΔW JPJ The mechanical equivalence of heat 2 What we normally want is ΔQ going to ΔW This is sort of the point of most engines But life isn’t like that and imperfect. The second law quantifies the imperfection η = W/Q • η = efficiency of heat engine • W = work done by engine • Q = heat provided to engine