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Homework Help: [COLOR=Navy]Thermal Energy Look Over![/COLOR]:

  1. Apr 21, 2005 #1
    Thermal Energy Look Over!!

    bugeye: o:)

    Hi I was wondering if someone could just quickly look over my answers, as I taught myself this lesson on thermal energy and want to make sure I have done the work right. Below I have placed the questions with the answers directly below them. Thanks in advance... :!!)

    1. The transfer of thermal energy is known as: ?

    2. The measure of the average speed of the molecules in a substance is known as: ?

    3. The lowest temperature possible has a value of: ?
    0 K or 0 °R (−273.15 °C or −459.67 °F)

    4. When 1 kg of water at 1000C is mixed with 10 kg of water at 00C, the equilibrium temperature will be:
    exactly 100°C

    5. Two objects, A and B, have the same mass and the same initial temperature. However, object A has a smaller specific heat capacity than object B. If the same amount of heat of heat is added to each object, then:

    object A will have the higher final temperature

    6. Two people, Tony and Tania, are employed in a store where they lift heavy objects. Tony can do twice as much work as Tania in twice as much as time. How do their power ratings compare?
    they are equal

    7. A metal absorbs 1200 J of heat energy from its surroundings and then does 700 J of work back on its surroundings. Calculate the change of energy for the metal. ?
    Energy absorbed = change of internal energy + work done
    Change of energy = Energy absorbed - work done = 1200 – 700 = 500 J
    Change of energy = 500 J

    8. The temperature of O2 is measured to be 273.15 ° C. What is this temperature measured in Kelvin?
    Answer: 273.15oC = 273.15 + 273.16 = 546.31 K
    273.15oC = 546.3 K

    9. Amy pours 1 kg of water at 7.0 ° C into a bowl. Calculate the amount of heat energy needed to bring this water to a boil. The specific heat for water is 4186 J/kg. ° C.
    Answer: Heat energy = mc(Tf-Ti) = 1*4186*(100-7) = 4186*93 = 389298 J
    Heat energy needed = 389298 J

    10. A block made of an unknown substance has a mass of 1.5 kg and a temperature of 15.0 ° C. When 2.6 x 104 J of heat is added to the block, its temperature rises to 90 ° C. Calculate the specific heat capacity to the unknown substance.
    Answer: Specific heat capacity, c = Q/[m(Tf - Ti)]
    = (2.6*104)/[1.5*(90 - 15)] = 231.11 J/KgoC
    Specific heat capacity of the unknown substance = 231.11 J/KgoC

    11. Calculate the amount of heat energy required to melt a 10 kg block of ice at 0° C into liquid water at 0 ° C. The latent heat of fusion for water is 3.34 x 105 J/kg.
    Answer: Heat energy = mLf = 10*(3.34*105) = 3.34*106 J
    Heat energy required = 3.34*106 J

    12. Calculate the amount of heat energy required to boil 1.0 kg of water at 100 ° C into steam at 100 ° C. The latent heat of vaporization for water is 2.26 x 106 J/kg.
    Answer: Heat energy = mLv = 1*(2.26*106) = 2.26*106 J
    Heat energy required = 2.26*106 J

    13. An elevator motor lifts the elevator full of passengers to a height of 12 m in 30 s. If the mass of the elevator and passengers is 1000 kg, what is the power developed by the motor?
    Answer: Force = mg = 1000*9.8 = 9800 N
    Work done = Force*distance = 9800*12 = 117600 J
    Power = Work done / time = 117600/30 = 3920 W
    Power developed by the motor = 3920 W = 3920 J/s

    14. The efficiency of an incandescent light bulb is only 4.0%. Calculate the amount of electric energy that must be input to the bulb in order to get 100 J of light energy from the bulb. Answer: input electric energy = output light energy/efficiency = 100/0.04 = 2500 J
    Input electric energy = 2500
    Last edited: Apr 21, 2005
  2. jcsd
  3. Apr 21, 2005 #2


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    Recheck answer #4.
  4. Apr 21, 2005 #3
    does that mean the rest are right? for #4 the question should be....

    4. When 1 kg of water at 100 degrees Celcius is mixed with 10 kg of water at 0 degrees Celcuis, the equilibrium temperature will be:

    is it still wrong if the question should be like the one I reqwrote...

  5. Apr 22, 2005 #4


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    All others are correct.

    For #4 : Assume the final, equilibrium temperature is T. How much heat energy is gained by the cold water, and how much is lost by the hot water to get to this temperature ? How are these two quantities related ?
  6. Apr 23, 2005 #5
    Hi so wud the answer for 4 be.....

    either 50degrees celcius or 0 degrees celcius??

    am I right there?
  7. Apr 23, 2005 #6
    anyone please?!?
  8. Apr 23, 2005 #7


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    No, that's not correct. How did you calculate the final temperature ? Show what you did, and someone will tell you where you're wrong.
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