Heat and Temperature PDF

Heat and Temperature Definition of Heat and Temperature Heat is a measure of the internal energy of the molecules that make up a substance. Heat may be transferred from one substance to another. We say that substances can absorb or radiate heat. Temperature is a measure of the average kinetic ene...

Heat and Temperature Definition of Heat and Temperature Heat is a measure of the internal energy of the molecules that make up a substance. Heat may be transferred from one substance to another. We say that substances can absorb or radiate heat. Temperature is a measure of the average kinetic energy of the molecules in a substance. Energy, heat, and temperature are all related. Thermodynamic Equilibrium and Temperature The quantities which describes a system as a body are called state variables. (Durum değişkenleri) ⚫ T, temperature ⚫ P, pressure ⚫ V, Volume, ⚫ M, mass. No matter how complex the internal structure of a macroscopic system, with a limited number of the state variable, the system can be defined as a whole/ body. If a system's state variables do not change over time and if it is the same value everywhere, this situation is called thermodynamic equilibrium. The Zeroth Law of Thermodynamics If the two separate objects are in thermal equilibrium with a third object, then their heat with each other is in balance. Temperature Temperature, which is the most fundamental thermodynamic quantity, is the quantitative measurement balance of thermodynamic equilibrium. Temperature When the two bodies with different temperatures such as A and B contact each other but isolating, eventually their temperatures will be equal. It is called as thermodynamic equilibrium. Here we describe the temperature, macroscopicly. However, the temperature is a measure of the average kinetic energy of molecules which form a system. (Microscopicly) When the temperature of an object changes, another quantity must be changed. For example, a heated piece of iron expands, a heated gas pressure increases or the electrical resistance of a heated wire increases. The temperature measure is defined over the properties. An instrument that measures temperature is called a thermometer. The most popular thermometer type is the mercury thermometer. Liquid mercury expands with temperature and rises. However, mercury is a poisonous substance, it is now replaced by dyed alcohol. It has a sensor and resistor. The sensor that detects the change in temperature, causes a resistance change and is read by the software inside. temperature amount is displayed as digital output. Optical pyrometer measures by comparing the intensity of light emitted from a hot object and temperature of a luminous filament. The thermal camera detects infrared wavelengths in the electromagnetic spectrum and converts it to the visible image. There are 3 types of temperature scales ⚫ Fahrenheit ⚫ Celsius ⚫ Kelvin Celsius; Since 1743 the Celsius scale has been based on 0 °C for the freezing point of water and 100 °C for the boiling point of water at 1 atm pressure. Fahrenheit; In 1724 by the physicist Daniel Gabriel Fahrenheit described The Fahrenheit scale defined by two fixed points with a 180 °F separation: the temperature at which pure water ice freezes is defined as 32 °F and the boiling point of water is defined to be 212 °F, both at sea level and under standard atmospheric pressure. Celsius and Fahrenheit scales needs two fixed points, however... Kelvin, the absolute zero! Heat As Energy Transfer We often speak of heat as though it were a material that flows from one object to another; it is not. Rather, it is a form of energy. Unit of heat: calorie (cal) 1 cal is the amount of heat necessary to raise the temperature of 1 g of water by 1 Celsius degree. 4.186 J = 1 cal 4.186 kJ = 1 kcal Definition of Heat: Heat is energy transferred from one object to another because of a difference in temperature. • Remember that the temperature of a gas is a measure of the kinetic energy of its molecules. Internal Energy The sum total of all the energy of all the molecules in a substance is its internal (or thermal) energy. Temperature: measures molecules’ average kinetic energy Internal energy: total energy of all molecules Heat: transfer of energy due to difference in temperature Internal Energy Internal energy of an ideal (atomic) gas is equal to the average kinetic energy per molecule multiplied by the number of molecules. But since we know the average kinetic energy in terms of the temperature, we can write: Specific Heat The amount of heat required to change the temperature of a material is proportional to the mass and to the temperature change: The specific heat, c, is characteristic of the material. Some values are listed at left. Specific Heat Specific heats of gases are more complicated and are generally measured at constant pressure (cP) or constant volume (cV). Some sample values: How much heat is required to raise the temperature of 250g of iron from 20°C to 80°C? How much heat is required to raise the temperature of 250g of iron from 20°C to 80°C? Calorimetry Closed system: no mass enters or leaves, but energy may be exchanged Open system: mass may transfer as well Isolated system: closed system where no energy in any form is transferred For an isolated system, Energy out of one part = energy into another part Or: heat lost = heat gained Calorimetry The instrument to the left is a calorimeter, which makes quantitative measurements of heat exchange. A sample is heated to a wellmeasured high temperature, plunged into the water, and the equilibrium temperature measured. This gives the specific heat of the sample. Calorimetry What is a bomb calorimeter? Calorimetry Another type of calorimeter is called a bomb calorimeter; it measures the thermal energy released when a substance burns. This is the way the caloric content of foods is measured. Heat Transfer: Conduction Heat conduction can be visualized as occurring through molecular collisions. The heat flow per unit time is given by: Heat Transfer: Conduction The constant k is called the thermal conductivity. Materials with large k are called conductors; those with small k are called insulators. Heat Transfer: Conduction Building materials are measured using R−values rather than thermal conductivity: Here, l is the thickness of the material. Heat Transfer: Convection Convection occurs when heat flows by the mass movement of molecules from one place to another. It may be natural or forced; both these examples are natural convection. Heat Transfer: Convection Many home heating systems are forced hot-air systems; these have a fan that blows the air out of registers, rather than relying completely on natural convection. Our body temperature is regulated by the blood; it runs close to the surface of the skin and transfers heat. Once it reaches the surface of the skin, the heat is released through convection, evaporation, and radiation. Heat Transfer: Radiation The most familiar example of radiation is our own Sun, which radiates at a temperature of almost 6000 K. Heat Transfer: Radiation The energy radiated has been found to be proportional to the fourth power of the temperature: The constant σ is called the Stefan-Boltzmann constant: σ = 5.67 × 10−8 W/m2∙K4 The emissivity e is a number between zero and one characterizing the surface; black objects have an emissivity near one, while shiny ones have an emissivity near zero. Heat Transfer: Radiation If you are sitting in a place that is too cold, your body radiates more heat than it can produce. You will start shivering and your metabolic rate will increase unless you put on warmer clothing.