Ch. 16 Temperature & Heat and Ch. 17 Phases & Phase Changes

Thermodynamics is the study of heat and thermal energy. Thermal properties (heat and temperature) are based on the motion of individual molecules, so thermodynamics is a lot like chemistry.

Heat Energy transferred from one body to another due to a  T between them.

Once its absorbed by the 2 nd body/material it becomes internal energy.

Heat is energy in transit. Heat flows from high to low temperatures.

Heat will flow out of the body at a higher temperature and into a body at a cooler temperature.

When the heat flows, the objects are said to be in thermal contact.

Two things can happen: 1.The temperature rises. 2.The object changes state.

Thermal Equilibrium

The state in which 2 bodies in physical contact with each other have identical temperatures. No heat flows between them

Zeroeth Law of Thermodynamics: If object A is in thermal equilibrium with object B, and object C is also in thermal equilibrium with object B, than objects A and C will be in thermal equilibrium if brought into thermal contact.

Temperature A measure of the average kinetic energy of the particles in a substance.

Imagine a pail of warm water and a cup of a hot water. A 1 & 2 liter bottle of boiling water.

Temperature is NOT a measure of the total KE of molecules in the substance.

Temperature Scales 1. Fahrenheit ( o F) 2. Celsius ( o C) 3. Kelvin (K)

Boiling Point 1. Fahrenheit 212 o F 2. Celsius 100 o C 3. Kelvin 373 K

Freezing Point 1. Fahrenheit 32 o F 2. Celsius 0 o C 3. Kelvin 273 K

Rankine Temperature Scale Temperature scale having an absolute zero, below which temperatures do not exist, and using a degree of the same size as that used by the Fahrenheit temperature scale. Absolute Zero corresponds to a temperature of −459.67°F;

Absolute Zero Point at which all molecular motion has stopped. We have never reached it, but are very close. Scale is used in engineering.

Temperature Difference (  T) Is the primemover or force-like quantity in a thermal system.  T – “Delta T”

Ex: 110 o F inside and 40 o F outside. What is the  T?  T = 110 – 40 = 70 F o

Converting Temperatures Fahrenheit to Celsius T C = 5/9(T F – 32 o )

Ex: Convert 50 o F to o C T C = 5/9(T F – 32 o ) T C = 5/9(50 – 32 o ) T C = 5/9(18 o ) T C = 10 o C

Celsius to Fahrenheit T F = 9/5(T C )+ 32 o

Ex: Convert 20 o C to o F T F = 9/5(T C )+ 32 o T F = 9/5(20)+ 32 o T F = o T F = 68 o F

Convert Celsius to Kelvin T k = T c T c = T k –

Ex: Convert 72 o F to K T C = 5/9(T F – 32 o ) T C = 5/9(72 – 32 o ) T C = 22.2 o C

Thermal Expansion With a few exceptions, all substances – solids, liquids, & gases – expand when heated and contract when cooled.

Different materials expand at rates. The construction of structures and devices must take this into consideration.

Bimetallic Strip Two thin strips welded together. Usually brass and iron. Used in thermostats.

Does a hole expand of shrink when heated?

Loosening a tight nut. A nut is very tight on a screw. How shall it be loosened? By heating, or by cooling? The nut expands, the screw expands, and the space expands. Shrink-fit iron rims on wooden wheels.

calorie The amount of heat energy required to raise the temperature of 1 gram of water 1 o C.

1 kilocalorie (1000 calories) is used in rating food. Written as Calorie (capital C)

Both are units of energy. 1 calorie = J BTU – British Thermal Unit (English Unit)

Fuels are rated by how much heat is given off when a certain amount is burnt.

Mechanical Equivalent of Heat 1 calorie = J Food

Conduction Process in which heat energy is transmitted from molecule to molecule of a solid. In direct contact

Conductors A material through which heat can flow easily. ex: metals

Occurs in materials and between different materials in direct contact.

Is the result of collisions on an atomic & molecular level.

Materials that conduct heat poorly are called insulators. Ex: straw, wood, paper, cork, Styrofoam, etc.

Liquids and gases, especially air, are good insulators.

No insulator can totally prevent heat from getting through it.

It can only reduce the rate at which heat penetrates or escapes.

Heat Conduction is Slowed by Insulation

Convection Process in which heat energy is transferred through a liquid or a gas by means of currents.

Occurs in all fluids. Fluid is heated, expands, becomes less dense, and rises.

Heated Water Rises

Hot water rises, cools, and falls. Heated air rises, cools, then falls. Air near heater is replaced by cooler air, and the cycle repeats.

Convection currents produce the winds.

Inversion layer. Air near ground is more dense than air higher up; no convection currents to lift pollutants.

Radiation Process by which heat energy is transferred by electromagnetic waves. Ex: UV rays, infrared rays, etc.

Radiation

Any energy, including heat, that is transmitted by radiation is called radiant energy.

All objects continually emit radiant energy in a mixture of wavelength.

High temperature emit waves of shorter wavelength. Low temperatures emit waves longer length.

If the temperature is high enough, it emits waves of length of visible 500 o C  red 1200 o C  white light

Examples of Radiation Burning embers, light filament, & the Sun.

Ideal Gas Equation # 1 (moleculess) PV = Nk B T N - # of gas particles

T – must be in Kelvin T k = T c k B - Boltzmann’s constant 1.38 x J/K

Sample Problem: What is the # of gas particles for a gas that is at 27 o C, a pressure of 400,000 Pa, and a volume of 1.2 m 3 ?

G: V = 1.2 m 3, P = 400,000 Pa k B = 1.38 x J/K T = 27 o C

G: V = 1.2 m 3, P = 400,000 Pa k B = 1.38 x J/K T = 27 o C = 300 K

U: N = ? E: PV = Nk B T N = PV/k B T S: N = (400,000)(1.2) / (1.38 x )(300) S: N = 1.16 x 10 26

If a gas undergoes a change in volume, pressure, or temperature when the # of particles in the gas are constant.

N 1 = N 2

Ideal Gas Law

Sample Problem: Suppose an ideal gas occupies 4.0 liters at 23 o C and 2.3 atm. What will be the volume of the gas if the temperature is lowered to 0 o C and the pressure is increased to 3.1 atm?

Ideal Gas Equation #2 P V = n R T P: pressure (in Pa) V: volume (in m 3 ) n: number of moles R: gas law constant J/(mol K) (Derived from N A K) T: temperature (in K)

Sample problem: Determine the number of moles of an ideal gas that occupy 10.0 m 3 at atmospheric pressure and 25 o C.

Boyle’s Law The product of the pressure and volume of a given mass of gas is constant as long as temperature is constant.

P 1 V 1 = P 2 V 2

Sample Problem: A piston chamber has a volume of 0.75 m 3 when a pressure of 250,000 Pa is applied. What is the pressure applied, when the volume is decreased to 0.3 m 3 ?

G: V 1 = 0.75 m 3, P 1 = 250,000 Pa, V 2 = 0.3 m 3 U: P 2 = ? E: P 1 V 1 = P 2 V 2 P 2 = P 1 V 1 /V 2

S: P 2 = (250,000)(0.75)/(0.3) S: P 2 = 625,000 Pa

Charles’s Law The volume of a gas divided by its temperature is constant as long as the pressure and the number of molecules is constant.

The kinetic theory of gases uses mechanics to describe the motion of each single molecule in a sample of an ideal gas. When a very large number of molecules are considered, the mechanical properties of the individual molecules are summed in a statistical way to predict the behavior of the gas sample. Kinetic Theory of Gases

Sample Problem: What is the average kinetic energy and the average speed of oxygen molecules in a gas sample at 0 o C?