Second law of thermodynamics. Looking at energy changing processes in last chapter, quite often the energy is transferred to thermal energy in the end. For example, as a ball is dropped, there is some air resistance which heats up the ball. Think of the space shuttle as an extreme example. Second law of t.d. is the tendency of nonthermal energy to end up as thermal energy.

Conservation of energy states that the total energy of all the components is held constant. Unlike other forms of energy, thermal energy is not easily transformed into other forms. “thermal energy can be transformed into other forms with limited efficiency.” Pretend you had some kinetic energy switched to thermal energy. It’s hard to get the kinetic energy back. You won’t be able to get all of it back from the thermal energy.

Heating Thermal energy flows spontaneously from hot to cold. Touch a piece of ice. The ice gets warmer. You get colder. One way to state the 2nd Law of TD. “Thermal energy flows spontaneously from higher to lower temperature, not from lower to high temperature.”

Temperature quantitative measurement of warmth. The more thermal energy an object has, the warmer it will be. Temperature scales

TF = (9/5)TC +32

Celsius Fahrenheit or

TC = (5/9)(TF – 32)

Compare lake to coffee cup Thermal energy also depends on the amount of the material.

Take a cool lake and a hot cup of coffee. The coffee has a higher temperature even if the lake has more thermal energy. This is because the lake is so much more massive. However, a single cup of the lake water will have less thermal energy than the cup of coffee.

Heat engines Use thermal energy to do work. Convert thermal to kinetic. - Gasoline engine takes TE from burning gas to do work. - Steam engine uses hot steam to do work. heat engine – cyclic device that uses thermal energy to do work. Cyclic devices repeat a set of steps over and over again.

Notable feature is that heat engines eject lots of thermal energy. – car gets hot under the hood, also hot tailpipe. This ejected thermal energy is called the exhaust. TE input = work + exhaust energy efficiency = work/thermal energy input Because there is exhaust, efficiency < 100%

It is impossible to consume thermal energy without producing some exhaust. 2nd law of TD can be worded as “heat engines are always less than 100% efficient at using TE to do work.” Can calculate efficiency = (Tin – Tex)/Tin or efficiency = 1 – Tex/Tin Tin is the input temperature. TEX is the exhaust temperature. must put T in Kelvin. TK = TC + 273 See table on pg 139.

There’s a special type on engine called a Carnot engine. It is the most efficient engine that can run between a high temperature and a low temperature. Takes heat from hot reservoir, does work and expels TE to cold reservoir. Carnot engines are not real, just an idealization. Carnot efficiency = 1 – Tc/Th Tc and Th are the cold and hot temperature reservoirs.

Energy quality Energy is conserved yet the quality or usefulness is lost.

See rock swing from string. Thermal energy is created and that energy cannot be entirely reconverted back to kinetic energy.

When we use the Earth’s energy resources, we don’t reduce the total energy of the Earth. Instead we turn the energy from highly useful forms such as the chemical energy of oil to a lesser useful form, usually thermal energy.

Entropy

• entropy is a quantitative description of the disorganization of a system. • see picture on top of page 141 • 1 kg of water has more entropy than 1kg of ice because in the water the molecules are not organized into a regular crystal lattice. • 2nd Law of TD stated as entropy. The total entropy of a system in any physical process cannot decrease, but it can increase. (also can stay the same, but need to be very careful to do so.)

Law of entropy predicts that most processes are irreversible. Irreversible processes will not naturally occur in the opposite direction. Mix hot water with cold water. You get a mass of water at some intermediate temperature. The water will not naturally un-mix into hot and cold water again.

Entropy Deck of cards example. Start is organized deck. If you shuffle them, they get disorganized. Shuffle again, they get more disorganized. The entropy of the deck of cards is increased. Easy to disorganize things, but it takes considerable effort and lots of luck to organize them. As universe ages, its entropy increases.

Energy Use Cars are definitely an important technology. Also a huge consumer of energy. See graphs on page 144. internal combustion engine – burn a fuel-air mixture inside the cylinder of an engine external combustion – occurs in fuel that is burned. This provides thermal energy to a second substance that does the work. example: steam engine

See making estimates on page 144 See flow chart of energy for a car.

Overall energy efficiency of entire vehicle is about 14%. electric vehicles – powered entirely by batteries. no tailpipe, no emissions, no pollution short range , heavy batteries, $$$

gas-electric hybrid – small gas engine powers an electric generator. Generator provides charge to battery to drive car. Fuel cell vehicle – fueled by hydrogen, get energy from chemical energy of hydrogen gas.

See transportation efficiency tables trains good commuting cars bad

coal fueled steam plant page 150 energy flow diagram page 151 Resource use: linear growth – variable grows the same amount every time interval exponential growth – grows by a fixed percentage each time interval Exponential growth has a constant doubling time

doubling time = 70/growth rate T = 70/P or can do P = 70/T nonrenewable resource – cannot be readily replaced within a human lifetime (oil) renewable resource – can be replaced within a human lifetime. Can be sustained indefinitely as long as they are consumed at less than the replacement rate. (wood, solar, wind)