Thermodynamics: Energy & Heat Transfer

• Thermodynamics covers the relationship between heat and other forms of energy.

Conservation of Energy

• Energy cannot be created or destroyed.
• Energy can only be converted from one form to another.
• Thermal energy can't be created or destroyed, but it can be converted to and from other energy forms.
• Thermal energy can originate from electrical, chemical, mechanical, or nuclear energy.
• Thermal energy can be converted to mechanical or kinetic energy.
• Thermal processes can add energy to chemical reactions.

Heat Transfer

• Heat is the energy concerning the application, loss or transfer of heat.
• Heat transfers from a body having a high level of heat energy and a body having a lower level of heat energy
• Energy transfers to the molecules in the colder material when a cold object contacts a hotter object.
• Temperatures eventually equalize, and heat is lost to surroundings.
• Cooling fins on an engine cylinder exemplify heat transfer by conduction.

Convection

• Convection is the process by which heat transfers through the bulk movement of a fluid.
• A heated fluid becomes less dense, rises, and is displaced by cooler fluid.
• Heating water in a kettle and the circulation of atmospheric heat are examples of convection.

Radiation

• Electromagnetic radiation refers to the emission of energy from the surface of most objects.
• Emission of energy is related to the acceleration of charged particles.
• Most of the sun's energy we receive is radiant heat.
• Radiation occurs at the speed of light, faster than conduction and convection, which are comparatively slow.

Kinetic Theory of Matter

• Molecules are in constant, random motion.
• Increased heat energy accelerates molecular movement, causing changes in the substance.
• Dissimilar substances joined and heated will expand at different rates, creating stress.

Change of State

• With added heat, molecules separate, and a substance changes state to a liquid.

Units of Heat

• Calorie (cal) is the heat required to raise the temperature of 1 gram of water by 1°C.
• British thermal unit (Btu) is the quantity of heat required to raise the temperature of one pound of water by 1°F.
• Joule (J) is the SI unit for all forms of energy, denoting the capacity for work.

Temperature

• Temperature signifies the degree of heat one mass possesses over another.
• The hotter mass is at a higher temperature when heat flows from one body to another.
• Specific heat and heat capacity define two properties.
• Specific heat is the calories needed to raise 1 gram of a substance by 1°C.
• Specific heat is also defined as the number of BTU’s required to raise 1 pound of a substance by 1°F.
• Water takes 1 calorie to raise 1 gram by 1°C so water is the benchmark
• Water's high specific heat is why ocean temperature is more stable than land temperature.
• Heat capacity (C) is the heat required to change the temperature of a substance by one degree.

Temperature Scales

• Four temperature scales: Celsius (°C), Fahrenheit (°F), Rankine (°R), and Kelvin (K).
• Celsius scale sets the melting point of pure ice at 0° and boiling point at 100°.
• The Kelvin scale has 100 increments between the freezing and boiling point of water.
• Zero on the Kelvin scale represents absolute zero or -273°C.
• K = °C + 273 and °C = K - 273
• Fahrenheit scale has 180 increments between the freezing point and boiling point of water.
• Freezing point of water is 32°F and the boiling point is 212°F.
• Rankine scale uses absolute zero as its zero point, corresponding to -460°F.
• °R = °F + 460 and °F = °R - 460

Latent and Sensible Heat

• Sensible heat changes an object's temperature when added.
• Latent heat changes the state of a substance without changing its temperature.
• Sensible heat changes the temperature of a substance without changing its phase.
• Latent heat is required to change the state of a substance without altering its temperature.
• At 100°C, the added heat to turn water into vapor is latent heat.
• Latent heat breaks intermolecular bonds
• Sensible heat is stored in intermolecular forces, increasing kinetic energy.

Gas Laws

• Gases can be compressed.

Boyle's Law

• A gas is easily compressed, increasing pressure and decreasing volume if temperature is constant.
• With a halved volume, pressure doubles as the same molecules bombard a smaller area.

Charles’ Law

• The volume of a gas varies in direct proportion to its temperature when pressure remains constant.
• Heating a gas in a flexible container will cause the container to increase in size.
• Doubling the temperature doubles the volume, and vice versa.
• Temperature change without external heat is an adiabatic process.

General Gas Law

• The general gas law combines Boyle's and Charles' laws.
• Formula: P1V1/T1 = P2V2/T2.
• Temperatures and pressures must be in absolute values.

Thermal Energy and Laws of Thermodynamics

• The first law of thermodynamics mirrors the law of conservation of energy.
• Heat energy cannot be destroyed; it can only be changed from one form to another.
• Combustion heat energy in an engine is transformed into mechanical energy.
• Energy can be transformed into sound energy, resulting in lost or inefficiencies.
• The second law of thermodynamics states that heat flows from a warmer body to a cooler body.
• Theory applied in car radiators, heat exchangers, and oil coolers.

Heat of Combustion

• Heat is produced when fuel is burned (combustion).
• Heat is a by-product of combustion; it can be useful or unwanted.
• Combustion ranges from lighting a match to a coal-fired power station.
• Combustion can use liquid, solid, or gaseous fuel.

Work Done by Expanding Gases

• The expansion of gas created by heat is the prime purpose for the combustion.
• Heat produced by igniting a pyrotechnic charge increases gas volume to push a bullet in a gun.
• A gas turbine engine relies on heat to expand gas, thus gas drives the engine turbines.
• Expanding gases do similar work to other mechanical processes.
• W = Fs: the greater the force applied to an object or the greater the distance an object moves, the more work is done.