حرارة وديناميكا حرارية المحاضرة الثانية د/ محرز لولو

What is Heat (Q)? Form of energy because it can move things - E.g: Makes a hot air balloon rise. - Steam engines Measured in JOULES (J)

The higher the temperature, the faster the particles (atoms/molecules) are moving, i.e. more Kinetic Energy. We will take heat to mean the thermal energy in a body OR the thermal energy transferred into/out of a body

Specific Heat Observational Fact: It is easy to change the temperature of some things (e.g. air) and hard to change the temperature of others (e.g. water) The amount of heat (Q) added into a body of mass m to change its temperature an amount  T is given by Q=m c  T c is called the specific heat and depends on the material and the units used. Note: since we are looking at changes in temperature, either Kelvin or Celsius will do.

Units of Heat Heat is a form of energy so we can always use Joules. More common in thermodynamics is the calorie: By definition 1 calorie is the amount of heat required to change the temperature of 1 gram of water 1  C The English unit of heat is the Btu (British Thermal Unit.) It is the amount of heat required to change the temperature of 1 lb of water 1  F. Conversions: 1 cal =4.186 J 1Btu = 252 cal

Units of Specific Heat Note that by definition, the specific heat of water is 1 cal/g  C.

Material J/kg  C cal/g  C Water41861 Ice Steam Silver Aluminum Copper Gold Iron Lead Brass Glass Wood Ethyl Alcohol Beryllium

Example Calculation Compare the amount of heat energy required to raise the temperature of 1 kg of water and 1 kg of iron 20  C?

Heat Transfer Heat always moves from a warmer place to a cooler place. Hot objects in a cooler room will cool to room temperature. e.g: tea, coffee Cold objects in a warmer room will heat up to room temperature.e.g: butter, ice

Heat Transfer Mechanisms 1. Conduction: (solids--mostly) Heat transfer without mass transfer. 2. Convection: (liquids/gas) Heat transfer with mass transfer. 3. Radiation: Takes place even in a vacuum.

Conduction Takes place in solid, liquid and gases but it is common in solid Needs physical contact Particles at the warm end vibrates faster and this then causes the next particles to vibrates faster and so on. e.g: spoon in tea In this way heat in an object travels from: the HOT end the cold end

Example

Conduction When you heat a metal strip at one end, the heat travels to the other end. As you heat the metal, the particles vibrate, these vibrations make the adjacent particles vibrate, and so on, the vibrations are passed along the metal and so is the heat. We call this? Conduction

Conduction

Convection What happens to the particles in a liquid or a gas when you heat them? The particles spread out and become less dense. A liquid or gas.

Convection It is the way in which particles in a GAS or LIQUID move upwards, carrying heat with them Think about when you boil water, the bubbles move upwards Or think of a gas heater in the room, the heat rises around the room

Convection Hot water rises Cooler water sinks Convection current Cools at the surface

Radiation How does heat energy get from the Sun to the Earth? There are no particles between the Sun and the Earth so it MUST travel by radiation ? RADIATION

Radiation The transfer of heat in rays, from a hot object, without needing a medium to pass through It travels in all directions from a hot object The hotter an object is, the more heat it will radiate out Does the surface affect the way heat is radiated?

21 The black body A black body is an ideal body which absorbs all the incident radiation within itself. The black body is an ideal absorber of incident radaition. The black body is an ideal radiator Univ. of Oregon web site

Radiation Everything that has a temperature radiates energy. The radiation transferred by radiation from a black body is governed by the fourth power low σ is Stefan’s constant =5.6 ×10 -8 wm -2 k -4, e is the emissivity (between 0,1) and A is the surface area of the of the black body

If a black body whose temperature T is in an enclosure at a temperature T o then, the net rate of loss of energy is given by:

Highly compressible. Occupy the full volume of their containers. When gas is subjected to pressure, its volume decreases. Gases always form homogeneous mixtures with other gases. Gases only occupy about 0.1 % of the volume of their containers. General Characteristics of Gases

Four Physical Quantities for Gases Phys. Qty.SymbolSI unitOther common units pressureP Pascal (Pa) atm, mm Hg, torr, psi volumeVm3m3 dm 3, L, mL, cm 3 temp.TK°C, °F molesnmol

Gas: Equation of State It is useful to know how the volume, pressure, and temperature of the gas of mass m are related. The equation that interrelates these quantities is called the equation of state. These are generally quite complicated. If the gas is maintained at a low pressure, the equation of state becomes much easier. The equation of state can be determined from experimental results. This type of a low density gas is commonly referred to as an ideal gas. Section 19.5

Ideal Gas Model The ideal gas model can be used to make predictions about the behavior of gases. If the gases are at low pressures, this model adequately describes the behavior of real gases. Section 19.5

The Mole The amount of gas in a given volume is conveniently expressed in terms of the number of moles, n. One mole of any substance is that amount of the substance that contains Avogadro’s number of constituent particles. Avogadro’s number is N A = x The constituent particles can be atoms or molecules. Section 19.5

Moles, cont The number of moles can be determined from the mass of the substance: M is the molar mass of the substance. Can be obtained from the periodic table Is the atomic mass expressed in grams/mole Example: He has mass of 4.00 u so M = 4.00 g/mol m is the mass of the sample. n is the number of moles. Section 19.5

Calculating the Number of Moles The number of moles, n, of a gas can be can be calculated using:- Where N is the total number of molecules and N A is Avogadro’s constant (=6.02 × )

(3) Avogadro's Law or V  n (T, p at constant)

Gas Laws When a gas is kept at a constant temperature, its pressure is inversely proportional to its volume (Boyle’s law). When a gas is kept at a constant pressure, its volume is directly proportional to its temperature (Charles and Gay-Lussac’s law). When the volume of the gas is kept constant, the pressure is directly proportional to the temperature (Guy-Lussac’s law). Section 19.5

(1) Boyle's Law

Robert Boyle ( ),English

(2) Charles' Law V  T (n, p constant) Jacques Charles ( ), French

We can combine these into a general gas law: The Ideal Gas Equation Boyle’s Law: Charles’s Law: Avogadro’s Law:

R = gas constant, then The ideal gas equation is: R = L·atm/mol·K = J/mol·K Real Gases behave ideally at low P and high T. The Ideal Gas Equation

Equation of State Recall that each phase can exist in a variety of states e.g. the temperature and pressure Thus the Ideal Gas Equation of State pV = nRT summarises the physically possible combinations of p, V and T for n moles of the ideal gas.