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Thermal Energy/ States of Matter- Help

  1. Aug 4, 2003 #1
    Thermal Energy/ States of Matter- Help!!

    Hey guys! I have been on the road traveling for 6 weeks straight with not much time for schoolwork. Now I am home for 4 days so I have to get to work. I have finished one of my physics tests but wanted some reassurance that the answers are correct- as I cannot afford a bad grade- must keep the grades up. I will post the problems and my solutions and if you could check over them that would be greatly appreciated! Thanks ahead of time!!!!!

    1) Which takes less time to melt, one Kg of ice, one Kg of lead, one Kg of copper, or one Kg of silver? Assume that ea h solid is at its melting point and the heat energy is being applied to each solid at the same rate.
    I beleieve that the answer to this one is silver because it has the lowest heat of vaporization at 1.04 x 10^4.

    2) Container A has 50 Kg of water in it. container B has 30 Kg of water in it. If both containers have equal quantities of heat energy then__________.

    I beleieve that container B and its contents would have a higher temperature than conatiner A due to the fact that there is less material within it. Is this right??

    3) If the density of mercury is 13.59 x 10^3 kg/m^3 at 20 * C, what will its density be at 65 * C?

    OK...this is where I am confused. I believe that you somehow have to incorporate the equation Vi(coefficient of volume expansion)(change in temperature)= m^3....so you would use 13.59 x 10^3 (180 x 10^-6)(65*-20*). Does this sound correct? I got the answer to be 675 N but that doesn't seem correct....

    4) In a certain hrdraulic liff, the small piston has a radius of 15 cm. The large piston has a radis of 26 cm. A force of 225 N is applied to the small piston. What is the mass of the crate that is being lited by the application of this 225 N force?

    Here I know that you have to use Pascal's principle and the equations P1= F1/A1...(pressure= force/surface area)and F1/A1=F2/A2....I don't understand this enough to solve it...

    5) The heat of vaporization of mercury is 2.72 x 10^5 J/Kg. How many grams of mercury at its boiling point can be vaporized by the addition of 4.53 kJ of heat energy?

    Here, I think you would use Q=m(Hv)
    4530 J= m(2,72 x 10^5)
    m= .01665 kg or 16.7 g
    Thank you so much for your time- I really appreciate it!!!
     
  2. jcsd
  3. Aug 4, 2003 #2

    HallsofIvy

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    "1) Which takes less time to melt, one Kg of ice, one Kg of lead, one Kg of copper, or one Kg of silver? Assume that ea h solid is at its melting point and the heat energy is being applied to each solid at the same rate.
    I beleieve that the answer to this one is silver because it has the lowest heat of vaporization at 1.04 x 10^4."

    Only problem I have with this is that "heat of vaporization" is irrelevant- the problems asked for time to MELT!

    "2) Container A has 50 Kg of water in it. container B has 30 Kg of water in it. If both containers have equal quantities of heat energy then__________.
    I beleieve that container B and its contents would have a higher temperature than conatiner A due to the fact that there is less material within it. Is this right??"

    Heat content is Mass*Temp*"heat coefficient" isn't it? If Heat content and heat coefficient are the same, increasing the mass while keeping heat content and coefficient the same must do what to the temperature?

    "3) If the density of mercury is 13.59 x 10^3 kg/m^3 at 20 * C, what will its density be at 65 * C?]"

    OK...this is where I am confused. I believe that you somehow have to incorporate the equation Vi(coefficient of volume expansion)(change in temperature)= m^3....so you would use 13.59 x 10^3 (180 x 10^-6)(65*-20*). Does this sound correct? I got the answer to be 675 N but that doesn't seem correct...."

    density is mass/volume. You know that 1 m^3 of mercury would have a mass of 13.59* 100^3 kg at 20 degrees C. Now find it's volume at 65 degrees C using the formula you have. Since mass does not change with temperature, divide 13.59* 10^3 by the new volume to get the density. You are correct that 675 Newtons can't possibly be correct since "Newtons" are a unit of force, not density!

    "4) In a certain hrdraulic liff, the small piston has a radius of 15 cm. The large piston has a radis of 26 cm. A force of 225 N is applied to the small piston. What is the mass of the crate that is being lited by the application of this 225 N force?

    Here I know that you have to use Pascal's principle and the equations P1= F1/A1...(pressure= force/surface area)and F1/A1=F2/A2....I don't understand this enough to solve it..."

    Yes, F1/A1= F2/A2. F1, the force applied to the small piston, is given as 225 N. Further, A1= pi(15^2) and A2= pi(26^2) so
    F2/(pi*26^2)= 225/(pi*15^2). Solve for F2.

    "5) The heat of vaporization of mercury is 2.72 x 10^5 J/Kg. How many grams of mercury at its boiling point can be vaporized by
    the addition of 4.53 kJ of heat energy?


    Here, I think you would use Q=m(Hv)
    4530 J= m(2,72 x 10^5)
    m= .01665 kg or 16.7 g"

    Yep.
     
  4. Aug 4, 2003 #3
    ok...using the given info....

    Hi! Thank you for your help!
    For #1, is silver the correct answer while taking into consideration the time to melt?

    For #2- Container B would have a higher temperature wouldn't it? How could Conatiner A have a higher temperature with more water inside of it?

    #3-After looking over your info I solved the problem and got 16.78 kg/m^3. Is this correct?

    #4- After working this out I arrived at 675 N. This sounds correct...

    Thank you so much for your help....
     
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