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The movement of icebergs is another example of convection accompanied by a phase change. Suppose an iceberg drifts from Greenland into warmer Atlantic waters. Heat is removed from the warm ocean water when the ice melts and heat is released to the land mass when the iceberg forms on Greenland.

Explain why using a fan in the summer feels refreshing!

Using a fan increases the flow of air: warm air near your body is replaced by cooler air from elsewhere. Convection increases the rate of heat transfer so that moving air “feels” cooler than still air.

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Test prep for ap courses

Under which two conditions would convection in a fluid be greatest?

  1. The gravitational acceleration is large, and the fluid density varies greatly for a given temperature change.
  2. The gravitational acceleration is small, and the fluid density varies greatly for a given temperature change.
  3. The gravitational acceleration is large, and the fluid density varies slightly for a given temperature change.
  4. The gravitational acceleration is small, and the fluid density varies slightly for a given temperature change.

(a)

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Sea breezes occur along coastlines, and consist of cool air moving toward the shore from the ocean. However, this only occurs during the day, and is a stronger effect when the air temperature on the land is greatest and the air temperature above the water is coldest. At night, the breezes are reversed, moving from the land toward the ocean. Taking into consideration the specific heat capacities of water and sand (which is about the same as that of concrete), explain how sea breezes form during the day and change direction at night.

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Summary

  • Convection is heat transfer by the macroscopic movement of mass. Convection can be natural or forced and generally transfers thermal energy faster than conduction. [link] gives wind-chill factors, indicating that moving air has the same chilling effect of much colder stationary air. Convection that occurs along with a phase change can transfer energy from cold regions to warm ones.

Conceptual questions

One way to make a fireplace more energy efficient is to have an external air supply for the combustion of its fuel. Another is to have room air circulate around the outside of the fire box and back into the room. Detail the methods of heat transfer involved in each.

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On cold, clear nights horses will sleep under the cover of large trees. How does this help them keep warm?

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Problems&Exercises

At what wind speed does 10º C air cause the same chill factor as still air at 29º C size 12{ - "29"°C} {} ?

10 m/s

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At what temperature does still air cause the same chill factor as C size 12{ - 5°C} {} air moving at 15 m/s?

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The “steam” above a freshly made cup of instant coffee is really water vapor droplets condensing after evaporating from the hot coffee. What is the final temperature of 250 g of hot coffee initially at 90 . 0ºC size 12{"90" "." "0°C"} {} if 2.00 g evaporates from it? The coffee is in a Styrofoam cup, so other methods of heat transfer can be neglected.

85 . C size 12{"85" "." 7°C} {}

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(a) How many kilograms of water must evaporate from a 60.0-kg woman to lower her body temperature by 0 . 750ºC size 12{0 "." "750°C"} {} ?

(b) Is this a reasonable amount of water to evaporate in the form of perspiration, assuming the relative humidity of the surrounding air is low?

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On a hot dry day, evaporation from a lake has just enough heat transfer to balance the 1 . 00  kW/m 2 size 12{1 "." "00"`"kW/m" rSup { size 8{2} } } {} of incoming heat from the Sun. What mass of water evaporates in 1.00 h from each square meter? Explicitly show how you follow the steps in the [link] .

1.48 kg

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One winter day, the climate control system of a large university classroom building malfunctions. As a result, 500  m 3 size 12{"500"`m rSup { size 8{3} } } {} of excess cold air is brought in each minute. At what rate in kilowatts must heat transfer occur to warm this air by 10 . 0ºC size 12{"10" "." "0°C"} {} (that is, to bring the air to room temperature)?

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The Kilauea volcano in Hawaii is the world’s most active, disgorging about 5 × 10 5 m 3 of 1200ºC lava per day. What is the rate of heat transfer out of Earth by convection if this lava has a density of 2700 kg/m 3 and eventually cools to 30ºC ? Assume that the specific heat of lava is the same as that of granite.

A view of a lava flow on the Kilauea volcano of Hawaii.
Lava flow on Kilauea volcano in Hawaii. (credit: J. P. Eaton, U.S. Geological Survey)

2 × 10 4  MW size 12{2 times "10" rSup { size 8{4} } `"MW"} {}

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During heavy exercise, the body pumps 2.00 L of blood per minute to the surface, where it is cooled by 2 . 00ºC size 12{2 "." "00°C"} {} . What is the rate of heat transfer from this forced convection alone, assuming blood has the same specific heat as water and its density is 1050  kg/m 3 size 12{"1050"`"kg/m" rSup { size 8{3} } } {} ?

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A person inhales and exhales 2.00 L of 37 . 0ºC air, evaporating 4 . 00 × 10 2  g of water from the lungs and breathing passages with each breath.

(a) How much heat transfer occurs due to evaporation in each breath?

(b) What is the rate of heat transfer in watts if the person is breathing at a moderate rate of 18.0 breaths per minute?

(c) If the inhaled air had a temperature of 20 . C size 12{"20" "." 0°C} {} , what is the rate of heat transfer for warming the air?

(d) Discuss the total rate of heat transfer as it relates to typical metabolic rates. Will this breathing be a major form of heat transfer for this person?

(a) 97.2 J

(b) 29.2 W

(c) 9.49 W

(d) The total rate of heat loss would be 29.2 W + 9.49 W = 38.7 W . While sleeping, our body consumes 83 W of power, while sitting it consumes 120 to 210 W. Therefore, the total rate of heat loss from breathing will not be a major form of heat loss for this person.

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A glass coffee pot has a circular bottom with a 9.00-cm diameter in contact with a heating element that keeps the coffee warm with a continuous heat transfer rate of 50.0 W

(a) What is the temperature of the bottom of the pot, if it is 3.00 mm thick and the inside temperature is 60 . 0ºC size 12{"60" "." "0°C"} {} ?

(b) If the temperature of the coffee remains constant and all of the heat transfer is removed by evaporation, how many grams per minute evaporate? Take the heat of vaporization to be 2340 kJ/kg.

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Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
Aislinn Reply
cm
tijani
what is titration
John Reply
what is physics
Siyaka Reply
A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
Jude Reply
Can you compute that for me. Ty
Jude
what is the dimension formula of energy?
David Reply
what is viscosity?
David
what is inorganic
emma Reply
what is chemistry
Youesf Reply
what is inorganic
emma
Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
Adjei
please, I'm a physics student and I need help in physics
Adjanou
chemistry could also be understood like the sexual attraction/repulsion of the male and female elements. the reaction varies depending on the energy differences of each given gender. + masculine -female.
Pedro
A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
Krampah Reply
2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
Sahid Reply
you have been hired as an espert witness in a court case involving an automobile accident. the accident involved car A of mass 1500kg which crashed into stationary car B of mass 1100kg. the driver of car A applied his brakes 15 m before he skidded and crashed into car B. after the collision, car A s
Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
Ryan
what's motion
Maurice Reply
what are the types of wave
Maurice
answer
Magreth
progressive wave
Magreth
hello friend how are you
Muhammad Reply
fine, how about you?
Mohammed
hi
Mujahid
A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
yasuo Reply
Who can show me the full solution in this problem?
Reofrir Reply
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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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