Thermodynamics And Relationships between heat and work.

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Thermodynamics And Relationships between heat and work

What is Internal Energy? Internal energy is defined as the energy associated with the random, disordered motion of molecules. It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale. (Hyperphysics - University of Georgia)

For example, a room temperature glass of water sitting on a table has no apparent energy, either potential or kinetic. But on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters per second. If the water were tossed across the room, this microscopic energy would not necessarily be changed when we superimpose an ordered large scale motion on the water as a whole. (Hyperphysics -University of Georgia)

Heat, Work and Internal Energy Internal energy can be used for work. Example #1: Friction forces generated through pulling a nail from wood, increase the nail’s temperature. The energy can be transferred to the surrounding air. (The work is done by the friction forces.) Serway/Faughn Physics – pg 332

Heat, Work and Internal Energy Internal energy can be used for work. Example #2: Consider a flask of water with a balloon placed over the opening. Heating the water cause it to boil. The water vapor expands the balloon. The balloon expansion provides a force that does work on the atmosphere. The steam does the work.

Heat & Work are Energy Transferred to or from a System Objects contain internal energy, but are not said to have heat or work. The heat or work is transferred to or from a substance. Serway/Faughn Physics – pg 332 The coffee cup feels hot as it is transferring heat energy to your hand.

Heat energy is transferred from a warmer object to a cooler object.

Energy transfer to or from a system A balloon, flask, water, steam can be thought of as a system. A burner transfers energy to this system. The system internal energy is increased. When the expanding balloon does work on the surroundings, the system’s internal energy is decreased. Some of the energy is transferred into the system as heat is transferred to the surroundings. Serway/Faughn Physics – pg 332

For Thermodynamic systems work is defined in terms of pressure and volume change. Thermo – thermal energy – heat Dynamic – changing Therefore thermodynamic involves changes in heat/energy.

Example – Gas expanding and pushing a piston within a cylinder does positive work the piston. As the gas is compressed, the work done on the piston is negative. Serway/Faughn Physics – pg 332

First Law of Thermodynamics Energy cannot be created or destroyed, but transferred or converted from one form to another.

Another way to look at the first law:

Sample 1 st Law Calculation 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? W = -135J U = 114J Q = ? U = Q – W so Q = U + W Q = 114J + (-135J) = -21J

A cyclical process. In a cyclical process, the system’s properties at the end of the process are identical to the system’s properties before the process took place. (The final and initial values of internal energy are the same, and the change in internal energy is zero.)

Refrigerators and Heat engines (Cyclical processes) A refrigerator performs mechanical work to create temperature difference between its closed interior and its environment (the air in the room). This is accomplished in a cyclical process of compression and expansion or refrigerant, and transferring thermal energy.

Heat engines A heat engine is a device that uses heat to do mechanical work. A heat engine does work by transferring energy from a high-temperature substance to a lower- temperature substance. A sterling engine is driven by thermal energy transfer

Internal Combustion Engines Internal combustion engines are examples of heat engines. Potential energy of chemical bonds in fuel is converted to kinetic energy of particle products from combustion. These gaseous products push against a piston to do work. Only part of the internal energy leaves the engine as work done on the environment (pistons). Most of the energy is removed as heat.

Four-Cycle Gasoline Engine Intake stroke – An air-fuel mixture is drawn into the cylinder through the intake valve as the piston moves downward. (The exhaust valve stays closed.)

Compression stroke – work is done by the piston as the air-fuel mixture is compressed in the cylinder. (Both valves are closed at this time.)

Power stroke – The compressed, hot gases are ignited and combustion takes place. The combustion of gases cause the piston to move downward in the cylinder. A great deal of this energy, in the form of heat, is transferred to the surrounding environment. (Both valves remain closed)

Exhaust Stroke – The piston moves up through the cylinder and pushes the combustion products back out the cylinder through an exhaust valve.

The Second Law of Thermodynamics With regards to a heat engine: “No cyclic process that converts heat entirely into work is possible. Some energy is always transferred as heat into the surroundings.” Serway/Faugh Physics pg 348 How much heat energy is converted to work, instead of being lost to the surroundings refers to the efficiency of that engine.

Perpetual motion machines do not exist because energy must be supplied to the system continually. Since energy is always lost, or transferred out of the system, energy must be continually supplied to the system to keep it going.

Entropy – tendency toward disorder “In thermodynamics, a system left to itself tends to go from a state with a very ordered set of energies to one where there is less order.” Serway/Faugh Physics pg 351 All systems tend toward more disorder and randomness.

The measure of a system’s disorder is called the entropy of the system. The greater the entropy, the greater the disorder. Greater disorder means there is less energy to do work.

New Calculations Suggest Universe May be Closer to Heat Death Universe has more entropy than once thought. An analysis by Chas Egan of the Australian National University in Canberra and Charles Lineweaver of the University of New South Wales in Sydney indicates that the collective entropy of all the supermassive black holes at the centers of galaxies is about 100 times higher than previously calculated. Because supermassive black holes are the largest contributor to cosmic entropy, the finding suggests that the entropy of the universe is also about 100 times larger than previous estimates, the researchers reported online September 23 at arXiv.org.

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