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2 temperature questions stuck.

  1. Jul 11, 2004 #1
    The fahrenheit temp reading is 90 degrees on a hot summer day. What is the reading on the kelvin scale?

    I know the kelvin scale is the absolute scale...i'm not sure how to tackle this problem or in other words what equation to use.

    A 50.0 g silver spoon at 20.0 degrees is placed in a cup of coffee at 90.0 degrees C. How much heat does the spoon absorb from the coffee to reach a temperature of 89.0 degrees C?

    Would I use the equation Q= mc(change in)T ?I wasnt sure how to do this problem either.

    A 500.0 g pot of water at room temp (20.0 degrees C) is placed on a stove. How much heat is required to change this water to steam at 100.0 degrees C?

    Would I again use the above problems equation w/ the mass = 500.0 g?

    THX so much! Any ideas or tips on how to tackle these problems will help out tremendously..we just started on temperature and heat...and we're moving fast through this summer phyiscal science class.
     
  2. jcsd
  3. Jul 11, 2004 #2

    chroot

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    Look up the conversion from fahrenheit to celsius. From there, the kelvin scale is very simply related to celsius. Just add 273.15 to convert celsius to kelvin -- e.g. 100 degrees C is 373.15 kelvins.
    The spoon is going from 20 to 89 degrees, and weighs 50g. Use the formula you mentioned, where c is the specific heat of silver. Make sure your units are consistent.
    There are two steps to this one. The first step is just like the last problem -- you have some amount of substance going from one temperature to another. Use [itex]\Delta Q = m c \Delta T[/itex] for that. The second step deals with the conversion of the liquid water (which is at 100 degrees C) to gaseous steam (also at 100 degrees C). The phase change itself, from liquid to gas, requires additional energy input. While the phase change is in progress, the energy added to the system goes only into the phase change -- the temperature remains at 100 degrees C until all of the water is completely turned into steam. The amount of energy required to complete the phase change is called the heat of vaporization, and you should be able to look it up in your book.

    Add the energy required to get the water to 100 degrees C to the energy required to complete the phase change to arrive at the answer.

    - Warren
     
  4. Jul 14, 2004 #3
    The fahrenheit-celsius relationship is

    [tex]
    F = 1.8C + 32
    [/tex]

    so that
    [tex] C = \frac{5}{9}(F-32)[/tex].

    The absolute scale is related to the kelvin scale as [tex] K = 273.15 + C [/tex]. So the formula you end up using here is

    [tex] K = 273.15 + \frac{5}{9}(F-32) [/tex]

    where F = fahrenheit temperature
    C = centigrade temperature
    K = kelvin temperature.

    For the kind of calorimetric problems that you have posed here, I strongly urge you to follow this piece of advice that I learnt while solving them at school: Draw the temperatures on a rough scale as parallel horizontal lines (the higher temperature above the equilibrium temperature line and the lower one below it). Next, understand that your equations (Q = mcT) apply when thermodynamic equlibrium has been reached, that is all bodies that were involved are now at fixed temperature, depending on their initial states.

    Then use the equation, heat lost = heat gained (assuming no loss of energy to the surroundings). This really is a restatement of the Law of Conservation of Energy which states that energy can only be transformed from one form to another but cannot be created or destroyed. So the problem involving such a simple law should be systematic and simple and not as difficult as the language suggests. The line diagram method can eventually be discarded once you have a hang of whats happening in the problem, but for starters, its actually useful.

    Hope that helps...

    Cheers
    Vivek
     
  5. Jul 14, 2004 #4
    To add to what I said a while back...

    If something, say water, is at 20 degrees C and you are asked to find how much heat energy is required to convert all of it into steam at 110 degrees (for instance) then you cannot simply apply one equation and get your answer. To make the system of solving such problems foolproof, make a (mental) list of the various processes that need to occur for this conversion to take place:

    1. Heat energy is given to mass m of water initially at 20 degrees C to 100 degrees C. This is given by mass(m)*sp. ht. capacity of water * (temp change, which is 80 degrees C here).

    2. Heat energy is required to convert this mass of water at 100 degrees C to steam at 100 degrees C. This is where latent heat of vaporization of water comes into the picture. So here, the heat energy required to effect vaporization is given by mass(m) * specific latent heat of vaporization of water (L).

    3. Now that we have steam at 100 degrees, we would like to raise its temperature to 110 degrees C, for which we need some more heat energy, given by (mass of steam m)*(specific heat capacity of steam)*(temperature difference, which is 10 degrees C now).

    So the total heat energy required to effect the conversion of water at 20 degrees C to steam at 110 degrees C is equal to the sum of the energies computed in the three steps above.

    More difficult problems will have multiple bodies involved so you will have to consider temperature changes for all of them separately and add up the individual products to get the final answer. Once again, the diagrammatical approach suggested can prove to be useful.

    But I would like to know where you faced a difficulty in analyzing these situations before suggesting more tips and ideas to you...
     
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