HEAT: Heat Transfer

What is heat, anyway? Heat is the flow of energy from a hotter object to a colder object.

There are 3 ways in which heat can be transferred from one object to another:

CONDUCTION: the transfer of energy through matter by direct contact of particles. This can happen in solids, liquids and gases. CONVECTION: the transfer of energy because of the movement of warm fluids. (Fluids can be liquids or gases) This can happen only in liquids and gases - not in solids. RADIATION: the transfer of energy by electromagnetic waves. Energy can move by radiation in air like the heat from your electric stove top, or in the vacuum of space the way the Sun heats the Earth. In radiation, the energy does not have to transfer through mass (particles). Heat Transfer

Conduction: the “d” means “direct contact” the teapot gets hot because it’s touching the hot burner.

Conduction Conduction is the transfer of heat within a substance, atom by atom. If you put one end of a metal rod over a fire, that end will absorb the energy from the flame. The atoms at this end of the rod will gain energy and begin to vibrate faster. As they do, their temperature increases and they begin to bump into the atoms next to them. The heat is being transferred from the warm end to the cold end.

Conductors The young woman in the pink dress is using a straightened out coat hanger wire to cook her marshmallow. Metals are good conductors of heat. The heat from the fire travelled along the thick metal wire. The wire got so hot that she had to let go of it. The young woman in the blue dress is using a wooden rod to hold her marshmallow. Wood does not conduct heat well. The wood in her hand is still cool and the marshmallows are almost done

Conduction A typical example of conduction would be the heat transferred from hot coffee, through the cup, to the hand holding the cup.

Interesting question: Which feels colder, the top of your desk or the leg of your desk? Which is colder?

Convection: the v represents the “v” in movement No fluid moving means no convection!

Convection Gases and liquids expand as they get warm, so warm fluids are less dense than cold fluids. The warm fluid will rise, creating the convection current. Warm air is less dense than cold air, making cold air heavier than warm air.

Warm air rises in your house through convection:

Radiation Heat transfer by electromagnetic radiation Examples: -anything being warmed by the sun - the heat radiating from a lightbulb, or a person who is warm from exercising

Radiation Radiation is the only form of heat transfer that doesn’t require matter. The heat from the sun travels through the vacuum of empty space, but it still gets here.

All three types of heat transfer:

Practice questions: Heat convection occurs in gases and liquids. Heat convection does not occur in solids because solids are unable to — A absorb heat by vibrating B transfer heat by fluid motion C emit radiation by reflecting light D exchange heat by direct contact

Practice Questions: In which container is the substance unable to transfer heat by convection? 

The moon’s surface becomes hot during the long lunar day because the sun transfers heat to the moon. This heat transfer is accomplished almost entirely through the process of — F convection G refraction H conduction J radiation

A man who was sleeping wakes up because he hears the smoke alarm go off in his house. Before opening the bedroom door, the man feels the door to see whether it is warm. He is assuming that heat would be transferred through the door by — A conduction B convection C radiation D compression

The transfer of heat by the movement of air currents in Earth’s atmosphere is an example of — A conduction B convection C radiation D fusion

Measuring Heat 22

Heat Energy that flows from something warm to something cooler A hotter substance gives Kinetic Energy to a cooler one When heat is transferred (lost or gained), there is a change in the energy within the substance LecturePLUS Timberlake 9923

Learning Check A. When you touch ice, 1) heat is transferred from your hand to the ice 2) coldness flows from the ice to your hand B. When you drink a hot cup of coffee, heat is transferred from 1) your mouth to the coffee 2) the coffee to your mouth LecturePLUS Timberlake 9924

Solution A. When you touch ice, heat is transferred from your hand to the ice. In science, there’s really no such thing as “coldness.” B. When you drink a hot cup of coffee, heat is transferred from 2) the coffee to your mouth LecturePLUS Timberlake 9925

Learning Check When you heat 200 g of water for 1 minute, the water temperature rises from 10°C to 18°C. If you heat 400 g of water at 10°C in the same pan with the same amount of heat for 1 minute, what would you expect the final temperature to be? 1) 10 °C2) 14°C 3) 18°C LecturePLUS Timberlake g 400 g

Solution 2)14°C Heating twice the mass of water using the same amount of heat will raise the temperature only half as much. LecturePLUS Timberlake g 400 g

Specific Heat Why do some foods stay hot longer than others? When you go to the beach, why is the sand hot, but the water is cool on the same hot day? LecturePLUS Timberlake 9928

Specific Heat Different substances have different capacities for storing energy. It may take 20 minutes to heat water to 75°C. However, the same mass of aluminum might require 5 minutes and the same amount of copper may take only 2 minutes to reach the same temperature. LecturePLUS Timberlake 9929

Specific Heat Values Specific heat is the a mount of heat needed to raise the temperature of 1 g of a substance by 1°C J/g°C water 4.18 aluminum 0.90 copper 0.39 silver 0.24 gold 0.13 LecturePLUS Timberlake 9930

Learning Check A. A substance with a large specific heat 1) heats up quickly2) heats up slowly B. When ocean water cools, the surrounding air 1) cools 2) warms3) stays the same C. Sand in the desert is hot in the day, and cool at night. Sand must have a 1) high specific heat 2) low specific heat LecturePLUS Timberlake 9931

Solution A. A substance with a large specific heat 2) heats up slowly B. When ocean water cools, the surrounding air 2) warms C. Sand in the desert is hot in the day, and cool at night. Sand must have a 2) low specific heat LecturePLUS Timberlake 9932

Measuring Heat Required to warm something up: You need: Grams of substance Temperature change  T Specific heat of the substance LecturePLUS Timberlake 9933

Calculating Heat Q = mass(grams) x specific heat x temp. change Q = M x C x  T LecturePLUS Timberlake 9934

Heat Calculations A hot-water bottle contains 750 g of water at 65°C. If the water cools to body temperature (37°C), how many calories of heat could be transferred to sore muscles? heat = mass x  T x Sp. Ht. (H 2 O) 750 g x 28°C x 4.18 J g°C = J LecturePLUS Timberlake 9935

Laws of Thermodynamics

Zeroth Law If two thermodynamic systems are in thermal equilibrium with a third system they are in thermal equilibrium with each other. Heat goes from hot to cold until thermal equilibrium

First Law of thermodynamics Energy can not be created or destroyed only change forms. In any isolated system the total energy remains the same. Heat applied to the system equals net work done by the system. U(internal energy)=Q+ Work done on system U(internal energy)= Q- Work done by the system

Entropy Entropy- how we measure the randomness or disorder of a system. The natural progression of objects is to form disorder. So could be said to be the number of arrangements available to a system.

Second Law of thermodynamics In any spontaneous process there is in increase in the entropy of the universe. In other words the entropy of the universe is increasing.

Third Law of Thermodynamics As temperature approaches absolute zero the entropy of the system is also zero.

Phase Change

A Little Reminder... Laws of Thermodynamics 0 th 0 th Heat floes from hot to cold until equilibrium is reached 1 st 1 st There is conservation of heat energy 2 nd 2 nd Entropy, or disorder, tends to increase

A Little Reminder... Types Of Heat Transfer Convection Heat Transfer BY a material (usually a fluid) Conduction Heat Transfer THROUGH a material Radiation Heat Transfer not requiring any matter

Boiling and Melting point Every substance has a melting/freezing point – Melting is going from ______ to _______ – Freezing is going from _______ to _______ Every substance has a boiling/condensing point – Boiling is going from _______ to ________ – Condensing is going from _______ to ______

Flow Chart of Phase Changes Flow Chart of Phase Changes Adding Thermal Energy sublimation meltingvaporization ionization solid liquid gas plasma freezing condensation neutralization Deposition Removing Thermal Energy

Phase Change

Latent Heat Latent heat is applied when a substance is transitioning from one state of matter to another (i.e. melting, evaporating, condensing) Represented by a flat line on the phase change diagram Heat Input is required to raise the temperature of matter, and Heat Input is also required to change the state of matter.

Heat of Fusion and Vaporization required to melt a substancegiven off while freezing Heat of Fusion is the heat required to melt a substance, and given off (taken out) while freezing the substance Q =mH f heat required to vaporize given off (taken out) during condensation Heat of Vaporization is the heat required to vaporize (evaporate) a substance, and given off (taken out) during condensation of a substance. Q=mH v

An Everyday Example... When water is boiled, heat is put into the water. The steam will hold onto that heat until is condenses. That is Latent Heat. When the water condenses, the Latent Heat is given off, or taken out of the water. That is why, when water condenses on a cold drink, the drink warms up.

Phase change of water 1/data/ch04/anim/anim0405.htm 1/data/ch04/anim/anim0405.htm

Steps to solving a problem... Identify (on the phase change graph) where you start and where you end. Use Q =mH f or Q=mH v if the substance undergoes a phase change. Use Q=mc∆T when a substance is heating up.

Example How much heat is needed to raise a 2 kg piece of copper from 400º C to 1200º C? Information needed – Melting point 1083º C – Boiling point 2567º C – Specific heat.39 kJ/kg-ºC – Heat of Fusion 205 kJ/kg – Heat of Vaporization 4790 kJ/kg

To Solve... We start at 400* and go to 1200* so we cross the melting point at 1083*. We will then use the heat of fusion. From 400* to 1083* (melting point).. Q = 2kg x 390J/kg*c x 683* = J To melt 2 kg... Q = 2kg x J/kg = J Finally from 1083* to 1200* Q = 2kg x 390J/kg*c x 117* = J