CHAPTER 4
MATTER AND ENERGY
 
Temperature
•      A physical quantity which gives rise to sensations of hot and cold.  A thermometer measures temperature.
 

–   Liquid-in-glass thermometer
–   Bimetallic strip thermometer-used in thermostats; steel and copper are often used as the metals.
 
Heat Vs. Temperature
•      Temperature and heat are easy to confuse.
 
•      It cannot be said that an object at one temperature contains more heat than another object at a lower temperature just because of the temperature difference.
 
Temperature Scales
•      Fahrenheit scale (T_F)–
–   freezing point of water is 32°
–   boiling point of water is 212°  
 
•      Celsius scale (T_C)–
–   freezing point of water is 0°
–   boiling point of water is 100°
 
Equations for converting between TF and TC
 
•    TF = 9/5 T_C + 32°
•    TC = 5/9 (T_F – 32°)
 
Heat
•     In a body of matter heat is the sum of the kinetic energies of all the separate particles that make up the body.  
 
•      The more energy these particles have, the more heat the body contains, and the higher the temperature.  
 
•      The SI unit of measurement for heat is the joule (J).  
 
 
•      For a given temperature change, liquid water must have more heat added to or taken away from it per kilogram than nearly all other materials.  
 
•      It takes 4.2kJ of heat to change the temperature of 1kg of water by 1°C.
 
Heat Transfer
•     Heat can be transferred from one place to another in three ways:
 
1.     Conduction – heat is carried from one place to another by molecular collisions.  
•   One end of a poker is placed in a fire the other end becomes warm as heat flows through the poker
 
2.    Convection – the transport of heat is by the motion of a volume of hot fluid.
 
3.     Radiation – heat is transferred by means of electromagnetic waves, which requires no material medium for their purpose.  
·        Earth receives heat from the sun in the form of radiation which consists of electromagnetic waves (light and radio waves are examples of such radiation).
 
Metabolic Energy
•      The energy of people and animals.  
 
•      Metabolism refers to the biochemical processes by which the energy content of the food an animal eats is liberated.  
 
•      The unit is the kilocalorie (kcal), which is the amount of heat required to change the temperature of 1 kg of water by 1°C.  
 
•      1 kcal = 4.2kJ.
 
Fluids
•    Liquids and gases together are called fluids because they flow readily.
 
•     A liquid’s particles are about as far apart as those in a solid, but they are able to move about.
 
•     A liquid has a definite volume but flows to fit its container.
 
•     Particles in a gas can move about freely
 
•     Gases have neither a definite volume nor shape, but fill whatever container they are in.  
 
Density
•    Density of a material is its mass per unit volume.  
 
•    The SI unit of density is kg/m₃ or g/cm₃.


In equation form:
•            d = m/V   Where
 

d=density
m=mass
V=volume (length (l) x width (w) x depth (d))
 
Pressure

 

·        When a force, F, acts perpendicular to a surface whose area is A, the pressure acting on the surface is the ratio between the force and the area.
 
In Equation Form:
•     p = F/A   Where
 

p = pressure
F = Force
A = Area =  length (l) x width (w)
 
•      The SI unit of pressure is the Pascal (Pa) where 1 Pa = 1N/ m2
o       Since the Pa is a very small unit, the kilopascal (kPa) is often used
o       1kPa = 1000 Pa = 103 Pa.  
 
•      The SI unit for pressure was named after the French scientist and philosopher Blaise Pascal (1623-1662).  
 
•      Atmospheric pressures are measured with instruments called barometers.
 
 

Buoyancy
•      Occurs when the upward force on the bottom of an object immersed in a fluid is greater than the downward force on its top.  
 
•      The difference between these two forces is the buoyancy force.
 
•      Archimedes’ Principle – states the buoyant force on an object in a fluid = weight of the fluid displaced by the object.
 
In Equation Form:
·        Fb = dVg  where
 

Fb = Buoyancy force
d = density of fluid
V = volume of displaced fluid
g = acceleration of gravity
 
Archimedes’ Principle
·    Archimedes – (287? – 212 B.C.) – scientist/mathematician of the ancient world in which the above principle was named for.
 
Find the Buoyant Force of the Atmosphere on a 60 kg Person (example in the book)
 
•      Assume density of person is same as water at 1000 kg/m3.
 

•      Step 1:  Find person’s volume
V=m/dwater
 
•      Step 2:  The buoyant force is equal to the weight of air displaced by the person, so
Fb=d_airVg
 
The Gas Laws
•      Boyle’s Law – the volume of a given amount of gas at constant temperature is inversely proportional to the pressure applied to it.
 
•     In equation form, Boyle’s Law:
 =   At constant temperature
p₁V₁ = p₂V₂  At constant temperature
 
·    Charles’s Law – at constant pressure, the volume of a gas sample is directly proportional to its absolute temperature Tk, where Tk = Tc + 273°
·    In equation form:
 =    At constant pressure
 
Absolute Temperatures
·    Absolute zero is a temperature of -273°C.
 
·   For many scientific purposes it is convenient to begin the temperature scale at absolute zero.  
 
·   Temperatures on such a scale, given as degrees Celsius above absolute zero, are called absolute temperature.
 
·    For example, the freezing point of water is 273° absolute, written as 273 K (Kelvin)
 
·    Any Celsius temperature can be changed to an absolute temperature by adding 273
 
·    Named after Lord Kelvin (1824-1907)
 
Ideal Gas Law
·   Boyle’s and Charles’s laws can be combined in a single formula:
=  
•     At constant temperature, T1 = T2 and we have Boyle’s law.
 
•     At constant pressure, p1 = p2 and we have Charles’s law.
 
•     The simplified or generic way to write the ideal gas law is:
 
                        pV / T = constant
 
 

 

 

 

Kinetic Theory of Matter
•     Accounts for a wide variety of physical and chemical properties of matter in terms of a simple model.
 
•      According to this model, all matter is composed of tiny particles called molecules that are in constant motion.  
 
•      Molecules are the basic particles characteristic of a substance.
 
•      The three basic assumptions of the kinetic theory for gas molecules, which have been verified by experiment, are as follows:
 
Three Basic Assumptions of the Kinetic Theory for Gas Molecules:
 

1.    Gas molecules are small compared with the average distance between them.
 

2.    Gas molecules collide without loss of kinetic energy.
 

3.     Gas molecules exert almost no forces on one another, except when they collide.
 
Molecular Motion and Temperature
•      To account for the effect of a temperature change on a gas, the kinetic theory requires one further concept.
 
4.    The absolute temperature of a gas is proportional to the average kinetic energy of its molecules.
 
Changes of State
•     The kinetic theory of matter successfully explains the behavior of gases.
 
•      But…..What does this theory say about liquids and solids?  
 
•      Specifically, what does the theory say about changes of state between gas and liquid and liquid and solid?
 
Changes of State
·    Liquids – flow because their molecules slide past one another easily, but not as easily as gases because of intermolecular attractions that act only over short distances.
 
·    Solids – The forces between the molecules of a solid are stronger than those in a liquid, so strong that the molecules are not free to move about.  
o       The molecules are not at rest, however.  Each molecule vibrates back and forth rapidly as if connected by springs to one another.
 
Evaporation and Boiling
•      Liquid into a Gas
·   At any instant some molecules in a liquid are moving fast enough upward to escape into the air in spite of the attractions of their slower neighbors.  
 
·   By this loss of the faster molecules liquids gradually evaporate.  
 
·   Since the remaining molecules are the slower ones, a cool liquid is left behind.
 
·   When heat is added to a liquid, a temperature is eventually reached where even molecules of average speed can overcome the forces binding them together.  
 

·   Bubbles of gas form throughout the liquid, and boiling begins.  
 
·   This temperature is called the boiling point of the liquid.
 
·    Evaporation and Boiling differ in two ways:
–   Evaporation occurs only at a liquid surface
–   Boiling occurs in the entire volume of liquid
–   Evaporation occurs at all temperatures
–   Boiling occurs only at the boiling point or higher temperatures
 
·    Heat of Vaporization – Forming a gas from a liquid requires energy, whether or not evaporation occurs by itself or due to heating.  
·    If evaporation occurs by itself, energy is supplied from the heat content of the liquid itself (since the liquid grows cooler)
 
·    If heated, the source of energy is an outside source of heat.  
 
·    For water at its boiling point of 100°C, 2260 kJ (the heat of vaporization) is needed to change each kg of liquid into gas.
 
 

Melting
•      Solid into liquid
·   The heat required to change 1 kg of a solid at its melting point into a liquid is called the heat of fusion of the substance.  
 
·   The heat of fusion of water is 335 kJ/kg.
 
·   The heat of fusion of a substance is always much smaller than its heat of vaporization.
 
·    Most substances change directly from the solid to the vapor state, a process called sublimation, under the right conditions of temperature and pressure.  
 
·    Usually pressures well under atmospheric are needed for sublimation.  
 
 
Energy Transformations
•      Any form of energy, including heat, can be converted to any other form.
 
•      Heat, however, is unusual in that it cannot be converted efficiently.
 
Heat Engines
•      A device that turns heat into mechanical energy.
 
•      Examples include
–   gasoline and diesel engines of cars
–   jet engines of aircraft
–   the steam turbines of ships and power stations
 
•      To make an engine perform a net amount of work in each cycle, we must first cool the gas so that less work is needed to compress it.  
 
•      It is in this cooling process that heat is lost.  
 
•      A complete cycle includes heat flowing in and out of the engine, and during the flow some of the heat is changed into mechanical energy.  
 
Thus we have the discipline of thermodynamics and the following laws:

 

 

 

Two Laws of Thermodynamics
 

1.     Energy cannot be created or destroyed, but it can be converted from one form to another.
 

o       This is the same as the law of conservation of energy
 
2.    It is impossible to take heat from a source and change all of it to mechanical energy or work; some heat must be wasted.  
 
Heat Engine Efficiency
•      Thermodynamics is able to specify the maximum efficiency of a heat engine, ignoring losses to friction and other practical difficulties.  
 
•      The maximum efficiency depends on the absolute temperatures Thot and Tcold of the hot and cold reservoirs between which the engine operates.
 
Maximum efficiency = work output/(energy input)_maximum which is equal to                                                = 1 -  T_cold/T_hot         -    Engine Efficiency
 
•      The greater the ratio between the two temperatures, the less heat is wasted and the more efficient the engine.