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dusya [7]
3 years ago
11

Which types of heat transfer involve heat flow from hot objects to colder objects? A. convection, conduction, and radiation B. c

onvection and radiation, but not conduction C. convection, but not conduction or radiation D. convection and conduction, but not radiation
Physics
2 answers:
bulgar [2K]3 years ago
6 0

its all 3 convection, radiation, and conduction  

Soloha48 [4]3 years ago
4 0
B is definitely the answer. Please let it be
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Two parallel disks of diameter D 5 0.6 m separated by L 5 0.4 m are located directly on top of each other. Both disks are black
oksian1 [2.3K]

To solve this problem it is necessary to apply the concepts related to the Stefan-Boltzmann law which establishes that a black body emits thermal radiation with a total hemispheric emissive power (W / m²) proportional to the fourth power of its temperature.

Heat flow is obtained as follows:

Q = FA\sigma\Delta T^4

Where,

F =View Factor

A = Cross sectional Area

\sigma = Stefan-Boltzmann constant

T= Temperature

Our values are given as

D = 0.6m

L = 0.4m\\T_1 = 450K\\T_2 = 450K\\T_3 = 300K

The view factor between two coaxial parallel disks would be

\frac{L}{r_1} = \frac{0.4}{0.3}= 1.33

\frac{r_2}{L} = \frac{0.3}{0.4} = 0.75

Then the view factor between base to top surface of the cylinder becomes F_{12} = 0.26. From the summation rule

F_{13} = 1-0.26

F_{13} = 0.74

Then the net rate of radiation heat transfer from the disks to the environment is calculated as

\dot{Q_3} = \dot{Q_{13}}+\dot{Q_{23}}

\dot{Q_3} = 2\dot{Q_{13}}

\dot{Q_3} = 2F_{13}A_1 \sigma (T_1^4-T_3^4)

\dot{Q_3} = 2(0.74)(\pi*0.3^2)(5.67*10^{-8})(450^4-300^4)

\dot{Q_3} = 780.76W

Therefore the rate heat radiation is 780.76W

5 0
2 years ago
A child throws a ball vertically upward to a friend on a balcony 28 m above him. The friend misses the ball on its upward flight
photoshop1234 [79]

Answer:

t=1.9 sec

Explanation:

From the question we are told that:

Height h=28m

Time t=3s

Generally the Newton's equation for Initial velocity upward is mathematically given by

 s=ut+\frtac{1}{2}at^2

 28=3u-\frac{1}{2}*9.8*3^2

 u=24.03m/s

Generally the velocity at  elevation and depression occurs  as ball arrives and passes through S=28

 v=\sqrt{24.03-2*9.8*28}

 v=5.35m/s and -5.35m/s

Generally the Newton's equation for time to reach initial velocity  is mathematically given by

 v=u+at

 5.35=24.03-9.8t

 t=\frac{28.03-5.35}{9.8}

 t=1.9 sec

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