Q Is For Heat
THE LAWS OF THERMODYNAMICS fundamentals that can not be negotiated
The Fundamental Concept What is heat and how is it different from temperature? Kinetic Theory Of Matter: All atoms are in motion all the time. Heat is a measure of the energy an object has. Temperature is the average kinetic energy of the particles in the object.
The Kinetic Theory & Ludwig Boltzman Particles in a container are colliding all the time. These collisions create the pressure inside the container. Boyles Law relates pressure to volume and the Ideal Gas Law relates pressure, temperature, volume and amount of gas as: PV=nRT. Ludwig Boltzman combined this with the idea of KE to derive KE = 3kT/2 where k is Boltzman’s constant. This is how temperature is defined.
Work – A Review We know 2 things about work…. Work is Force x distance W = F ∙ Δd For work to be done an object must change position (move).
Work – A Review The rock experiences no change in position W = F ∙ (d2 – d1) = 0 Also, the energy of the rock before the event = energy after the event. U2 - U1 = 0
Work – A Review Here work is done on the box, but because it will not return to its starting point, no energy is stored in it. W = F ∙ (d2 – d1) U2 - U1 = 0 d1 d2
Work – A Review Work is done against gravity and energy is now stored in the box. The energy of the system has increased. W = F ∙ (d2 – d1) = mgh U2 - U1 = P.E. d2 d1
Work – A Review In both cases the work done is the area under the F vs. d curve F W d
Work – A Review Work done on an elastic spring is now energy stored in the spring due to the mechanical deformation of the material W = F ∙ (d2 – d1) = 1/2kx2 U2 - U1 = E.P.E. d1 d2
Work – A Review Again, work done is the area under the curve, in this case a direct linear relation. F W d
Work Done On A Gas Gases tend to act like springs. If we look at an “ideal” atomic gas like Argon, work can be done in compressing it and like a spring, energy can be stored in it. d1 d2
Work Done On A Gas W = F ∙ d Although we can measure force, pressure is easier to deal with. P = F/A and F = PA Volume of the cylinder = Area x d and d = V/A W = PA x V/A W = P(V1-V2) ; W = PΔV d1 d2
Work Done On A Gas The work done is the area between 1 and 2. Boyles Law keeps T constant so the curve is a one of constant temperature called an isotherm. This is an isothermic process and is reversible. 1 P1 2 P2 W V1 V2
Work Done On A Gas If we go back to 9th grade and think about Boyle’s Law…. We remember that it’s an inverse relationship. Just like the rock vs. box, WORK IS ONLY DONE WHEN VOLUME CHANGES – not pressure (force). But wait, we have a problem. 1 P1 2 P2 V1 V2
Work Done On A Gas With the rock, box and spring we only had one pathway to travel. Slide the box, lift the box and stretch the spring. We CAN NOT deviate from the path defined by the mechanism involved. Gasses have no such constraints since temperature can vary.
Work Done On A Gas Nobody said we had to follow the isotherm. The work done via 1-3-2 is clearly greater than via 1-4-2 since the area under each is different. P1 3 1 P2 2 4 V2
Work Done On A Gas The path is defined by controlling the temp which is associated with heat (Q) flow. P1 3 P1 1 1 W P2 P2 2 2 4 V2 V2 V1 V1
Work Done On A Gas But lets stick to the isotherm and add some heat to a gas. This is an OTTO cycle, which is your car engine. c Q in b c W Q out a V2 V1
COE Still Applies The basic law of the Universe states: Energy Can Not Be Created or Destroyed, Only Changed In From. The 1st Law of Thermodynamics When heat is transformed into any other form of energy, or when other forms of energy are transformed into heat, the total amount of energy (heat plus other forms) in the system is constant.Energy is conserved.
To Say It Another Way The 1st Law of Thermodynamics Heat added to a system transforms into other forms of energy. The total energy in a system is constant. You can not get more energy out than is put in. Ex: Heat added to water causes greater motion of the water molecules which is seen as an increase in temperature. Also, ice does not freeze at room temperature, you have to remove energy from it.
Heat Transfer – A Basic Idea Two objects of different temperature will transfer heat until the pair is in thermal equilibrium. Hot Cold Cooling Warming Time 1 Time 0 Heat Flow Same Temp Time 2
But It Gets Worse More of the 1st Law Heat (Energy) spontaneously flows from a region of high energy (SOURCE) to a region of lower energy (SINK). Heat will not move from sink to source unless work is done. No process is possible where heat can be converted entirely into work since the mechanism loses heat to its surroundings.
Which in English Is The 1st Law of Thermodynamics Heat will never by itself flow from cold to hot. When heat energy flows some is lost to the environment and can never by recovered. Heat flow is an irreversible process. No cyclic machine, process or engine can be 100% efficient. Everything in the Universe flows downhill. More importantly it MUST FLOW DOWN HILL and “FRICTION” is ever present.
Heat Transfer – A Basic Idea But what if I have three different objects? What then? A B B C Are In Equilibrium Are In Equilibrium A C Must Be In Equilibrium
The 0th Law If A is in equilibrium with B, and B with C then A must be in equilibrium with C. To change this condition work must be done. A B B C Are In Equilibrium Are In Equilibrium A C Must Be In Equilibrium
Look at Water Check out figure 12-14 in the book Heat of vaporization Hv Heat of fusion Hf T Boiling Melting Q
Look at Water Check out figure 12-14 in the book Entropy is a measure of disorder in a system, process or working fluid in system or process. s = Q/T (J/°K) Increasing disorder ENTROPY T Boiling Melting Q
Entropy With real gasses and processes, temperature is a function of pressure and volume, so entropy can be derived in terms of all three gas laws. Which we won’t do (save it for college). P1 Increasing disorder ENTROPY P2 V1 V2
And Worse Yet The 2nd Law All systems in the Universe will increase in entropy unless work is done on the system to maintain order.
And The English Version Is? The 2nd Law of Thermodynamics All systems will tend towards increasing disorder unless work is done to keep that from happening. Ex: Ice melts unless it’s in a freezer where a machine does work to remove energy from the ice to keep it from melting. Also, a puddle of water will evaporate to vapor even though the water is well below boiling.
Work Done On A Gas If we look at Charles’ Law….. Because we have ΔV work is done on the gas. Yet something subtle is wrong with the diagram which will cause great trouble in the real world T2 W T1 V1 V2
Work Done On A Gas The work done is relative to absolute zero, which we can never get to. If we look at a real world process…… T2 W T1 T = 0 V1 V2
Work Done On A Gas d T2 The farther apart T2 and T1 are the more efficient the engine. But since T1 can never of T=0, we can never be 100% efficient. Which is a tenant of the 1st Law. c W a T1 b T = 0 V1 V2
Work Done On A Gas d T2 In the real world T1 must be greater than the environment to remove heat from the engine to keep the process going. Which is why your car has a radiator and can’t be 100% efficient. c W a b T1 TE T = 0 V1 V2
And Yet Even Worse Still The 3rd Law It is not possible to get to absolute zero and as T approaches 0 entropy asymptotically approaches a constant value.
Which Means That no matter what you do, no system can ever be perfectly ordered because you must continue to do work on the system, increasing it exponentially as you approach T=0
The Nerd Joke Version 0th Law: Everybody’s in the same game. 1st Law: You Can’t Win. 2nd Law: You Can’t Break Even. 3rd Law: You Can’t Get Out Of The Game.
Water As A Working Fluid The 3 gas laws limit one variable at a time to explain behavior. In the real world this is not possible, P, V & T all change over time. The implication is that thermo is 3 dimensional exercise.
Process Vocabulary Isochoric – constant volume Isothermal – constant temperature Isentropic – constant entropy Isobaric – constant pressure Adiabatic – no heat is lost Enthalpy – heat content
The Carnot Cycle Here’s an example of the “most efficient” engine possible. Volume changes isothermically and pressure changes adiabatically
Scared? Don’t Be. It’s Easy Thermo is like accounting and geometry mixed together. When a process is graphed as Pressure vs. Volume (PV), PT, Ts, VT the area of the curve is the amount of work done by or on the process. The area under curve is the work or energy added to or removed by the process. It’s that easy.
Specific Heat All materials have intensive properties such as conductivity, density, malleability, etc. Like density, materials have a property of heat conductivity or lack of it called the R value. They also have a property called SPECIFIC HEAT which can be thought of as the heat storage capacity. Q = mcΔT