<< Chapter < Page Chapter >> Page >

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.

Got questions? Get instant answers now!

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.

Got questions? Get instant answers now!

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

Got questions? Get instant answers now!

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

Got questions? Get instant answers now!

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

Got questions? Get instant answers now!

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} {}

Got questions? Get instant answers now!

(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?

Got questions? Get instant answers now!

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 Problem-Solving Strategies for the Effects of Heat Transfer .

1.48 kg

Got questions? Get instant answers now!

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)?

Got questions? Get instant answers now!

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"} {}

Got questions? Get instant answers now!

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} } } {} ?

Got questions? Get instant answers now!

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.

Got questions? Get instant answers now!

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.

Got questions? Get instant answers now!

Questions & Answers

how does a model differ from a theory
Friday Reply
what is vector quantity
Ridwan Reply
Vector quality have both direction and magnitude, such as Force, displacement, acceleration and etc.
Besmellah
Is the force attractive or repulsive between the hot and neutral lines hung from power poles? Why?
Jack Reply
what's electromagnetic induction
Chinaza Reply
electromagnetic induction is a process in which conductor is put in a particular position and magnetic field keeps varying.
Lukman
wow great
Salaudeen
what is mutual induction?
je
mutual induction can be define as the current flowing in one coil that induces a voltage in an adjacent coil.
Johnson
how to undergo polarization
Ajayi Reply
show that a particle moving under the influence of an attractive force mu/y³ towards the axis x. show that if it be projected from the point (0,k) with the component velocities U and V parallel to the axis of x and y, it will not strike the axis of x unless u>v²k² and distance uk²/√u-vk as origin
Gabriel Reply
show that a particle moving under the influence of an attractive force mu/y^3 towards the axis x. show that if it be projected from the point (0,k) with the component velocities U and V parallel to the axis of x and y, it will not strike the axis of x unless u>v^2k^2 and distance uk^2/√u-k as origin
Gabriel Reply
No idea.... Are you even sure this question exist?
Mavis
I can't even understand the question
Ademiye
yes it was an assignment question "^"represent raise to power pls
Gabriel
mu/y³ u>v²k² uk²/√u-vk please help me out
Gabriel
An engineer builds two simple pendula. Both are suspended from small wires secured to the ceiling of a room. Each pendulum hovers 2 cm above the floor. Pendulum 1 has a bob with a mass of 10kg . Pendulum 2 has a bob with a mass of 100 kg . Describe how the motion of the pendula will differ if the bobs are both displaced by 12º .
Imtiaz Reply
no ideas
Augstine
if u at an angle of 12 degrees their period will be same so as their velocity, that means they both move simultaneously since both both hovers at same length meaning they have the same length
Ademiye
Modern cars are made of materials that make them collapsible upon collision. Explain using physics concept (Force and impulse), how these car designs help with the safety of passengers.
Isaac Reply
calculate the force due to surface tension required to support a column liquid in a capillary tube 5mm. If the capillary tube is dipped into a beaker of water
Mildred Reply
find the time required for a train Half a Kilometre long to cross a bridge almost kilometre long racing at 100km/h
Ademiye
method of polarization
Ajayi
What is atomic number?
Makperr Reply
The number of protons in the nucleus of an atom
Deborah
type of thermodynamics
Yinka Reply
oxygen gas contained in a ccylinder of volume has a temp of 300k and pressure 2.5×10Nm
Taheer Reply
why the satellite does not drop to the earth explain
Emmanuel Reply
what is a matter
Yinka
what is matter
Yinka
what is matter
Yinka
what is a matter
Yinka
I want the nuclear physics conversation
Mohamed
because space is a vacuum and anything outside the earth 🌎 can not come back without an act of force applied to it to leave the vacuum and fall down to the earth with a maximum force length of 30kcm per second
Clara
at t=0second,aparticles moving in x-y plain with aconstant acceleration has avelocity of initial velocity =(3i-2j)m/s and is at the origion.at t=3second the particle's velocity is final velocity=(9i+7j)then how to find the acceleration?
Yoni Reply
how about the formula like v^2=u^2+2as
Bayuo
a=v-u/t
Doreen
what is physics
Yinka

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics' conversation and receive update notifications?

Ask