Ch 21 Temperature, Heat, and Expansion

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 (oF) 2. Celsius (oC) 3. Kelvin (K)

Boiling Point 1. Fahrenheit 212 oF 2. Celsius 100 oC 3. Kelvin 373 K

Freezing Point 1. Fahrenheit 32 oF 2. Celsius 0 oC 3. Kelvin 273 K

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

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;

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

Ex: 110 oF inside and 40 oF outside. What is the DT? DT = 110 – 40 = 70 Fo

Thermometer Instrument used to measure temperature. Based upon liquid expansion in the tube with respect to temperature.

Usually mercury or an alcohol mixture.

Converting Temperatures Fahrenheit to Celsius TC = 5/9(TF – 32o)

Ex: Convert 50 oF to oC TC = 5/9(TF – 32o) TC = 5/9(50 – 32o) TC = 10 oC

Celsius to Fahrenheit TF = 9/5(TC)+ 32o

TF = 9/5(TC)+ 32o TF = 9/5(20)+ 32o TF = 36 + 32o TF = 68 oF Ex: Convert 20 oC to oF TF = 9/5(TC)+ 32o TF = 9/5(20)+ 32o TF = 36 + 32o TF = 68 oF

Convert Celsius to Kelvin Tk = Tc + 273 Tc = Tk - 273

TC = 5/9(TF – 32o) TC = 5/9(72 – 32o) TC = 22.2 oC Ex: Convert 72 oF to K TC = 5/9(TF – 32o) TC = 5/9(72 – 32o) TC = 22.2 oC

Tk = Tc + 273 Tk = 22.2 + 273 Tk = 295.2 K

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

Once its absorbed by the 2nd 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.

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

Thermal Equilibrium

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

Internal Energy The energy of a substance due to the random motions of its component particles and equal to the total energy.

Quantity of Heat When heat is absorbed it raises the temp. or when it’s lost it lowers the temperature.

Unit for heat is the calorie.

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

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

Both are units of energy. 1 calorie = 4.187 J BTU – British Thermal Unit (English Unit)

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

Heat Transfer Specific Heat (Cp) amount of energy required to raise the temp. of 1 kg of material by 1 degree Kelvin units: J/(kg·K) or J/(g·°C)

Heat Transfer 50 g Al 50 g Cu Al - It has a higher specific heat. Which sample will take longer to heat to 100°C? 50 g Al 50 g Cu Al - It has a higher specific heat. Al will also take longer to cool down.

Q = m  T  Cp Heat Transfer Q: heat (J) m: mass (kg) T: change in temperature (K or °C) Cp: specific heat (J/kg·K or J/g.oC) – Q = heat loss + Q = heat gain T = Tf - Ti

Heat Transfer heat gained = heat lost Calorimeter Coffee cup Calorimeter Calorimeter device used to measure changes in thermal energy in an insulated system, heat gained = heat lost

Specific Heat (c) Is the quantity of heat required to raise the temperature of a unit mass of that substance by 1oC.

Joules per kilogram-Celsius degree Units of Specific Heat Joules per kilogram-Celsius degree J/kg-Co

Specific heat of water is 4190 J/kg-Co On Pg. 220 is a table of Specific heat for different substances.

Heat Gain or Loss Q = mcDT

Q = quantity of heat m = mass of the substance c = specific heat of the substance DT = Temperature Difference

Ex : A 0. 5 kg cast iron skillet is heated from 20 Co to 55 Co Ex : A 0.5 kg cast iron skillet is heated from 20 Co to 55 Co. How much heat is was absorbed by the iron?

m = .5 kg c = 448 J/kg-Co DT = (55 – 20) = 35 Co Q = ?

Q = mcDT Q=(.5 kg)(448 J/kg-Co) (35Co) Q = 7840 J

Ex: A 1 kg of lead at 100 oC is dropped into a bucket containing 1 kg of water at 0 oC. What is the final temperature of lead and water when it reaches equilibrium?

We know the heat lost by the lead is gained by the water. Qlead = Qlost = ? m = 1 kg c = 128 J/kg- oC DT = (100 oC – TF)

Qlost = (1 kg) (128 J/kg- Co) (100 oC - TF) Qlost = (12800 - 128TF) J

Qwater = Qgained = ? m = 1 kg c = 4190 J/kg- oC DT = (TF – 0 oC)

Qgained = (1 kg)(4190 J/kg-Co) (TF - 0 oC) Qgained = (4190 TF )J

Qlost = Qgained (12800 - 128TF)J = (4190TF )J 12800 J = (4318 TF) J

12800 J / 4318 J = TF TF = 2.964 oC

Water has a very high specific heat capacity: 4190 J/kg-Co Very useful in cooling agent.

A very small amount of water absorbs a great deal of heat. Ex: radiator.

Water also takes longer to cool. This resistance to change temp. improves weather conditions/climates in many places.

Specific Heat Applications Water has a high specific heat capacity, and therefore has several important applications.

1. Car Radiators Water is used as a coolant in car radiators. Water can absorb a large amount of heat before it boils because water has a high specific heat capacity. An engine produces a lot of heat when running, so the heat must be removed. Water is circulated throughout the engine where it absorbs the heat. This water is then pumped to a radiator where the heat is released to the metal core of the radiator, which then releases the heat to the surrounding air.

2. Ocean Breeze Water has a much higher specific heat capacity than sand, therefore it takes more energy to heat the water than the sand. The air above the sand heats up faster and rises while cool air above the ocean comes in to take its place. Thus a breeze coming from the ocean toward the sand beach occurs on a hot day. What direction do you think the breeze is at night when the ocean water is warmer than the cool sand?

Because of high specific heat capacity (ability to accept heat without a large temperature increase) water during the day is cooler than land.  Rising air above warm land is replaced by cooler air pushed in from the lake.  The reverse happens at night, when the land's temperature has fallen below that of the lake; the lake's temperature drops, too, at night, but not as much as the land's.                                                                 

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?

Does a hole expand of shrink when heated?

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.

Pyrex glass – designed not to expand with increase of temperature. Gasoline

Expansion of Water 0 – 4 oC  Water actually contracts. > 4 oC  Water expands. Water is densest at 4 oC

Remember ice floats on water, so it is less dense. This has to do with the structure of the ice crystals. They form a hexagonal structure.

Remember ice floats on water, so it is less dense. This has to do with the structure of the ice crystals. They form a hexagonal structure.

Water's Physical Properties Water is unique in that it is the only natural substance that is found in all three states -- liquid, solid (ice), and gas (steam) -- at the temperatures normally found on Earth. Earth's water is constantly interacting, changing, and in movement.

Water has a high specific heat index Water has a high specific heat index. This means that water can absorb a lot of heat before it begins to get hot. This is why water is valuable to industries and in your car's radiator as a coolant. The high specific heat index of water also helps regulate the rate at which air changes temperature, which is why the temperature change between seasons is gradual rather than sudden, especially near the oceans.

Water has a very high surface tension Water has a very high surface tension. In other words, water is sticky and elastic, and tends to clump together in drops rather than spread out in a thin film. Surface tension is responsible for capillary action, which allows water (and its dissolved substances) to move through the roots of plants and through the tiny blood vessels in our bodies.

Ch. 22 Heat Transfer

Conduction In direct contact 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.                         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 light. @ 500 oC  red light @ 1200 oC white light

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

Absorption of Radiant Energy Absorption and reflection are opposite processes.

Good absorbers reflect little radiant energy, so they appear dark. (A perfect absorber reflects no energy & appears perfectly black.)

Examples: pupils, bird house, & door opens for distant houses.

Appears black because the energy is reflected many times inside and is partly absorbed with each reflections.

Emission of Radiant Energy Good absorbers are also good emitters.

All objects emit as much as they absorb.

Newton’s Law of Cooling The rate of cooling is approximate proportional to the

temperature difference (DT) between the object and its surroundings.

The earth gains energy by absorbing energy from the sun. In turn the earth emits radiation called “terrestrial radiation”.

Greenhouse Effect The warming effect whose cause is that short wavelength radiant energy from

the sun can enter the atmosphere and be absorbed by the earth more easily than the long wavelength energy from the earth can leave.

Greenhouse effect animation