Thermodynamics Section 1

Overview This chapter relates internal energy (energy of particle motion and particle distances), heat, and work

Thermodynamics The study of heat and its transformation into mechanical energy

Absolute zero No more energy can be extracted from a substance and its temperature cannot be lowered anymore

Heat, Work, and Internal energy Heat and work are always energy transferred to and from a system Substance can only contain internal energy I.E. is used to do work

Heat, Work, and internal energy System- a substance or combination of substances Ex: Balloon, flask, and burner Environment- conditions and influences that affect the behavior of a system

Work done by a gas Work done on or by a gas is pressure multiplied by volume change W = PΔV Work = pressure x volume change *SI unit: Joules

Work done by a gas cont’d If a gas expands, ΔV and the work done by the gas are positive Expansion = “+” work and volume change

Work done by a gas If a gas is compressed, ΔV and the work done by the gas are negative Compression = “-” work and volume change

Work done by a gas If the gas volume stays constant there is no displacement and no work done

Example An engine cylinder has a cross-sectional area of 0.010 m². How much work can be done by a gas in the cylinder if the gas exerts a constant pressure of 7.5 x 10⁵ (750,000) Pa on the piston and moves the piston a distance of 0.040 m?

Isovolumetric process Constant-volume process Occurs when a gas experiences a change in temperature but not in volume, no work is done Example: Water heated in a concrete container

Isothermal Process Constant temperature process Occurs when the temperature remains constant and the internal energy does not change when heat or work is transferred Example: Expansion of a balloon when barometric pressure drops (before a storm)

ADIABATIC PROCESS Process where changes occur but no energy is transferred to or from the system as heat This can be done by performing the process rapidly so the heat has little time to enter or leave (bicycle pump) or thermally insulate it Ex: Thermally insulated balloon and tank containing compressed gas or wrapping something in Styrofoam

When a gas is adiabatically compressed it gets warmer b/c it gains heat When a gas adiabatically expands it loses heat and gets cooler You try it! Place your hand in front of your both and blow out warm air with your mouth wide-open. Repeat this but only open your mouth a little. What do you notice?

Chapter 10 Thermodynamics Section 2

First Law of thermodynamics Principle of energy conservation that takes into account a system’s internal energy as well as work and heat Sum it up: Whenever heat is added to a system it transforms to an equal amount of another type of energy.

You put an airtight can full of air on a hot stove and heat it up You put an airtight can full of air on a hot stove and heat it up. The can has a fixed volume and will not move, so no work is done. The internal energy of the can is so the temp. However, if you attached a piston to the can so it could do work the internal energy of the can would be less b/c some of the energy is transformed into work.

Equation ΔU = Q – W Change in internal energy = heat – work All units in Joules

Example A total of 135 J of work is done on a gaseous refrigerant as it undergoes compression. If the internal energy of the gas increases by 114 J during the process, what is the total amount of energy transferred as heat? Has energy been added to or removed from the refrigerant as heat?

Cyclic Processes Thermodynamic process where a system returns to the same conditions it started in Ex: Refrigerators and Heat Engines

Heat Engine Changes internal energy  mechanical work Basic Idea: Heat flows from high temp.  low temp.

Chapter 10 Thermodynamics Section 3

If we place a hot brick next to a cold brick what do we expect to happen? What if the hot brick absorbed all of the heat from the cold brick and became even hotter… does this violate the first law of thermodynamics?

No it will not. The total amount of energy will remain the same No it will not. The total amount of energy will remain the same. It will however violate the second law of thermodynamics.

Second Law of Thermodynamics Basically: Heat will never, of itself, flow from a cold object to a hot object. Meaning… If you want heat to flow from cold to hot you have to impose external effort (you have to do some work on it!)

Examples Heat pumps that increase the air temperature. Air conditioners that reduce air temperature.

It’s easy to change work completely into heat, try it… Rub your hands together quickly and they will warm up. It is impossible to completely convert heat into work… The best you can do is convert SOME heat into mechanical work.

Heat Engine That is where the heat engine comes into play Changes internal energy into mechanical work

Heat engine process Absorbs heat from a reservoir of higher temperature (increases the internal energy) Convert some of this energy into mechanical work Expel the remaining energy as heat to some lower-temperature reservoir

When discussing heat engines, the 2nd Law of Thermodynamics may be stated: When work is done by a heat engine running between two temperatures, Thot and T cold, only some of the input heat at Thot can be converted to work, and the rest is expelled as heat at Tcold

The expelled heat could be in the form of hot steam When expelled heat is undesirable (hot steam on a hot day) it is called thermal pollution.

Before the development of the 2nd Law of Thermodynamics it was thought that a LOW FRICTION heat engine could convert nearly all input energy into mechanical work. Sadi Carnot, French engineer, said he didn’t think so in 1824.

Carnot efficiency AKA the ideal efficiency equation. Ideal eff. = Thot - Tcold Thot He said that under ideal conditions only 25% of the internal energy is converted into work and the other 75% is expelled as waste.

Order tends to disorder Organized energy degenerates into disorganized energy Ex: Gasoline  exhaust fumes

Application of 2nd Law Could be restated to apply here as the following: Natural systems tend to proceed toward a state of greater disorder

Entropy Measure of the amount of disorder in a system Disorder increases, Entropy increases

Entropy The second law says that when things are left to themselves, over time, they will run down and become disorganized.

The laws of Thermodynamics have been sometimes put this way: You can’t win (because you can’t get any more energy out of a system than you put in), you can’t break even (because you can’t even get as much energy out as you put in), and you can’t get out of the game (entropy in the universe is always increasing).