answer before p612 lt1

8/12/2019 Answer Before P612 LT1

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Question 1

Calculate the energy released when

(a) 10 g water at 100 C and

(b) 10 g of steam at 100 C

are each spilt on the hand.Take the specific heat capacity of water to be 4200 J kg

1K

1and the specific latent heat of

vaporisation of water to be 2.2 MJ kg1

. Assume that the temperature of the skin is 33 C.

Solution

(a) E = mcD0 = 0.01 4200 (100 33) = 2814 J = 2.8 kJ

(b) The latent heat given out in changing from steam at 100 C to water at the same temperature is

E = ml = 0.01 2.2 106 = 22 000 J

The heat given out when this condensed water drops in temperature from 100 C to 33 C is

E = mcq = 0.01 4200 (100 33) = 2814 J

So, the total heat given out is = 25 kJ

Question 2

When a falling hailstone is at a height of 2.00 km its mass is 2.50 g. What is its potential energy?

Assuming that all of this potential energy is converted to latent heat during the fall, calculate the mass

of the hailstone on reaching the ground. Take the specific latent heat of fusion of ice to be 3.36 105

J kg1

and the acceleration due to gravity to be 9.81 m s2

Solution

Potential energy = mgh = 2.5 10

3 9.81 2 103= 49.05 J

The falling hailstone loses potential energy, and this is used to partly melt the hailstone.

ml= 49.05

m 3.36 105 = 49.05

m = 1.4598 104 kg (mass of hailstone that melted)

Total mass of hailstone = 2.50 g

=> Remaining mass that reaches the ground = 2.50 0.1458 g

= 2.354 g

Question 3

0.30 kg of ice at 0 C is added to 1.0 kg of water at 45 C. What is the final temperature, assuming no

heat exchange with the surroundings? Take the specific heat capacity of water to be 4200 J kg-1

K-

1and the specific latent heat of fusion of ice to be 3.4 10 5 J kg

-1.


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Solution

Let q be the final temperature.

Heat lost by water = heat gained in melting the ice + heat gained in warming the ice water

mwcwDqw = micelice + micecwDqmelted ice

mwcw(45 q) = micelice + micecwD0 melted ice

1 4200 (45 q) = (0.3 3.4 105) + (0.3 4200 q)

4200 (45 q) = 1.02 105+ 1260 q1.89 1051.02 105= 1260 q + 4200 q

q = 16 oC

- See more at:

http://www.seai.ie/Schools/Post_Primary/Subjects/Physics/Unit_5_-_Heat_Quantity/Sample_Problems_-

_Specific_Latent_Heat/#sthash.w3NfDug6.dpuf

10. A refrigerator uses 400 J of work to remove 200 J of heat from its contents. How much heat

must it reject to its surroundings?

10. The refrigerator must reject an amount of heat to the room equal to the sum of the work done

by the refrigerator and the heat removed from its contents or 400 J + 200 J = 600 J.

7. Work of 2000 J is done on an ideal gas which experiences an internal energy increase of 800

J. Calculate the heat transferred during this process and indicate whether heat was added to or

removed from the gas.

7. This problem requires a direct application of the first law of thermodynamics.

U = Q - W

or

Q = U + W

The problem stated that work was done on the system, so by our sign convention this is

expressed with a negative sign.


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Q = (800 J) + ( - 2000 J) = - 1200 J

The negative sign indicates that heat is transferred from the system. This is the case, because the

work done on the system of 2000 J was sufficient to increase the internal energy by 800 J and to

have excess energy that is manifested as heat transferred from the system.

An ice cube having a mass of 50 gramsand an initial temperature of -10

degrees Celsius is placed in 400 gramsof 40 degreesCelsius water. What

is the final temperature of the mixture if the effects of the container can be

neglected?

Answer:http://www.solvephysics.com/thermodynamics_problem5.shtml

Even if a man shows no visible perspiration he still evaporates about 500

gramsof water per day from his lungs. How many calories of heat are

removed by this evaporation? What is the rate of heat loss in watts due to this

process?

Answer:http://www.solvephysics.com/thermodynamics_problem6.shtml

Estimate how far away a cicada can be heard if the lowest possible audible

intensity of a sound it produces is and the power of the

cicada's sound source is .

Answer:http://www.solvephysics.com/waves_problem4.shtml
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If you know this: then answer:

Light of wavelength 497.0 nmappers to have a wavelength of 500.2 nmwhen

it reaches eart from a distance star. find the velocity of the star if the speed of

light is 300000000 m/s.

Answer:http://www.solvephysics.com/waves_problem5.shtml

Questions 8 and 9 refer to a police car with its siren on, traveling at a velocity toward a

pe rson st andi ng on a st re et co rner . As th e ca r appr oaches , th e person hears th e sound at a

frequency of . Take the speed of sound to bev

.8. What is the frequency produced by the siren?

(A)

(B)

(C)

(D)

(E)

9. What is the wavelength of the sound produced by the siren?

(A)

(B)

(C)

(D)

(E)

10.

An ambulance driving with velocity where is the speed of sound, emits a siren with a
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frequency of . What is the frequency heard by a stationar y observer toward whom the

ambulance is driving?

(A)

(B)

(C)

(D)

(E)

Answers:http://www.sparknotes.com/testprep/books/sat2/physics/chapter17section9.rhtml

A baseball coach shouts loudly at an umpire standing 5.0 meters away. If the sound power produced bythe coach is 3.1 x 103 W, what is the decibel level of the sound when it re aches the umpire? (Let I0 =

1.0 x 10 -12 W/m2)

Answer:http://lchs.lpsd.ca/teachers/b_proctor/00ACCB50-

011F7A57.45/Intensity%20problems%20answers.pdf
http://www.sparknotes.com/testprep/books/sat2/physics/chapter17section9.rhtmlhttp://www.sparknotes.com/testprep/books/sat2/physics/chapter17section9.rhtmlhttp://www.sparknotes.com/testprep/books/sat2/physics/chapter17section9.rhtmlhttp://lchs.lpsd.ca/teachers/b_proctor/00ACCB50-011F7A57.45/Intensity%20problems%20answers.pdfhttp://lchs.lpsd.ca/teachers/b_proctor/00ACCB50-011F7A57.45/Intensity%20problems%20answers.pdfhttp://lchs.lpsd.ca/teachers/b_proctor/00ACCB50-011F7A57.45/Intensity%20problems%20answers.pdfhttp://lchs.lpsd.ca/teachers/b_proctor/00ACCB50-011F7A57.45/Intensity%20problems%20answers.pdfhttp://lchs.lpsd.ca/teachers/b_proctor/00ACCB50-011F7A57.45/Intensity%20problems%20answers.pdfhttp://lchs.lpsd.ca/teachers/b_proctor/00ACCB50-011F7A57.45/Intensity%20problems%20answers.pdfhttp://www.sparknotes.com/testprep/books/sat2/physics/chapter17section9.rhtml

answer before p612 lt1

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