Thermodynamics
Student Expectation
The student is expected to describe how the macroscopic properties of a thermodynamic system such as temperature, specific heat, and pressure are related to the molecular level of matter, including kinetic or potential energy of atoms AND contrast and give examples of different processes of thermal energy transfer, including conduction, convection, and radiation.
Key Concepts
Temperature is a measurement of the average kinetic energy of the molecules in an object or system. It does not depend on the number of molecules in the system.
Heat is a measure of the energy transferred from one object or system to another. Heat moves from a higher temperature to a lower temperature. When there is no difference in temperature, no heat flow occurs.
Conduction is when heat flows through a heated solid. Burning your feet on hot pavement is an example of heat flow through conduction. Conduction requires matter to matter contact and does not happen in a vacuum.
Convection is when heated particles transfer heat to another substance. The shimmering image produced above hot pavement is an example of air being heated and rising through heat flow due to convection. Convection requires matter to matter contact and does not happen in a vacuum.
Radiation is a transfer of thermal energy through electromagnetic waves. The heating of the pavement by the sun’s rays is an example of thermal transfer by radiation. Radiation happens in a vacuum.
The specific heat capacity of a substance is a physical property dependent upon the molecular structure and phase of the substance.
THERMODYNAMICS
The Study of Internal Energy
The kinetic energy (energy of motion) or potential energy (stored energy) of an object such as a ball, is its macroscopic energy. All substances, however, also have energy associated with their particles. This internal energy is composed of kinetic energy due to the motion of the particles and potential energy due to the forces and interactions among the particles. Another name for internal energy is thermal energy. Thermodynamics is the study of this internal energy and its effects on macroscopic properties of a substance.
The internal energy of a system can change. The first Law of Thermodynamics involving the conservation of energy says that there are two kinds of processes, heat and work, that can lead to a change in the internal energy of a system. Since both heat and work can be measured and quantified, this is the same as saying that any change in the energy of a system must result in a corresponding change in the energy of the world outside the system. The change in the internal energy of a system (U) is equal to the sum of the heat gained or lost by the system (Q) and the work done by or on the system (W):
ΔU = Q + W
In other words, energy cannot be created or destroyed. If heat flows into a system or the surroundings to do work on it, the internal energy increases and the sign of “q” or “w” is positive. Conversely, heat flow out of the system or work done by the system will be at the expense of the internal energy, and will therefore be negative.
Thermodynamics Variables: Temperature, Pressure, and Volume
Temperature is a measurement of the average kinetic energy of the molecules in an object or system. It is the terminology to express how cold or how hot a substance is. It does not depend on the number of molecules in the system, but it does depend on the motion of all the molecules. From statistical thermodynamics, motion links with temperature through Boltzmann constant. The basic unit of temperature in the International System of Units (SI) is the Kelvin (K). For everyday applications, it is often convenient to use the Celsius scale, in which 0°C corresponds very closely to the freezing point of water and 100°C is its boiling point at sea level. Absolute zero on Kelvin is equivalent to - 273.15°Celsuis.
In 1738, Daniel Bernoulli published ideas that said that gases collided with a surface, and the collisions are what could be understood as pressure. Bernoullis hypothesis laid the foundation for what is now the kinetic theory that has stood the test of time. In a gas, the kinetic energy from the microscopic collisions of the molecules give rise to macroscopic pressure which is a force per unit area on any planar slice through the gas. Pressure (P), volume (V), the number of molecules (n), the gas constant ® and temperature, (T) in Kelvin, are related in an ideal gas expressed by:
PV=nRT
The formula demonstrates that pressure is directly proportional to number of molecules and temperature. Pressure, however, is indirectly proportional to volume. This means that volume is constant and if the temperature begins to rise, the particles inside the container begin to bounce faster against the walls of the container, which in turn increases the pressure. If temperature is constant, then the pressure varies directly with volume.
Heat is the Flow of Energy
From observation and experience, we know that heat can only be transferred and moved from an object with a higher temperature to one with a lower temperature. The explanation of this simple phenomenon led to the discoveries of the Laws of Thermodynamics. Heat is a measure of how energy is transferred from one object or system to another because of a difference in temperature. When there is no difference in temperature, no heat flow occurs. This is called thermal equilibrium.
Types of Heat Transfer
In general, there are three types of heat transfer. They are conduction, convection and radiation.
Conduction: This type of heat transfer occurs when heat flows through a heated solid. Burning your feet on hot pavement is an example of heat flow through conduction. Conduction requires matter-to-matter contact and does not happen in a vacuum. When we shake hands with our friends, we feel hot or cold from their hands. This is conduction. The heat transfers from your hand to your friends hand by the type of conduction if yours is hotter.
Convection: This type of heat transfer occurs when heated particles transfer heat in a liquid or a gas. The shimmering image produced above hot pavement is an example of air being heated and rising through heat flow due to convection. Convection requires matter-to-matter contact and does not happen in a vacuum. It cannot take place in a solid, but only in liquids and gases. The atmospheric convection makes exchange of heat and steam from different layers, which lead to rain. In coastal areas such as Houston or Galveston, it is sometimes hard to predict whether it will be rainy or not. When you see cool rain on a hot day, do not forget it is because of atmospheric convection.
Radiation: The third type of heat transfer is radiation. It is a transfer of thermal energy through electromagnetic waves. The heating of the pavement by the Sun’s rays is an example of thermal transfer by radiation. Radiation happens in a vacuum. It is the radiation from the Sun that gives us warmth and food, since green plants store the radiation energy in organisms that serve as producers in the food chain.
Specific Heat
Specific heat is the amount of heat needed to raise the temperature of a mass by one degree. The units of specific heat differ depending on the quantity of mass and the degree of temperature such as joules per kilogramkelvin (J/kgK) or joules per gramCelsius (J/gC). The specific heat of a substance is a physical property dependent upon the molecular structure and phase of the substance. Students sometimes confuse heat capacity and specific heat. Heat capacity is the ratio of the amount of energy absorbed to the associated temperature rise; while specific heat is the heat capacity of a substance per unit mass. Specific heat capacity depends on the mass of the object, but heat capacity does not.
Specific heat is used to determine heat loss or gain expressed by:
Q = m Cp ΔT
“Q” is the quantity of heat transferred to or from the object, “m” is the mass of the object, “Cp” is the specific heat capacity of the material the object is composed of, and “ΔT” is the resulting temperature change of the object.