http://www.ck12.org Chapter 19. Thermodynamics and Heat Engines
Once heat (a transfer of thermal energy) is absorbed by a substance, it becomes indistinguishable from the thermal
energy already present: what methods achieved that level of thermal energy is no longer relevant. In other words,
’to heat’ is a well defined concept as a verb: its use automatically implies some kind of transfer. When heat using as
a noun, one needs to be realize that it must refer to this transfer also, not something that can exist independently.
Specific Heat Capacity and Specific Latent Heat
The ideas in the paragraphs above can be understood better through the concept ofspecific heat capacity(or specific
heat for short), which relates an increase in temperature of some mass of a substance to the amount of heat required
to achieve it. In other words, for any substance, itrelates thermal energy transfers to changes in temperature.It
has units of Joules per kilogram Kelvin. Here is how we can define and apply specific heat (Qrefers to heat supplied,
mto the mass of the substance andcto its specific heat capacity):
Q=cm∆T [2]Heat capacity is largely determined by the number of degrees of freedom of the molecules in a substance (why?).
However, it also depends on other parameters, such as pressure. Therefore, the formula above implicitly assumes
that these external parameters are held constant (otherwise we wouldn’t know if we’re measuring a change in specific
heat is real or due to a change in pressure).
When a substance undergoes a phase change, its temperature does not change as it absorbs heat. We referred to this
as an increase or decrease in latent energy earlier. In this case, the relevant question is how much heat energy does
it require to change a unit mass of the substance from one phase to another? This ratio is known aslatent heat, and
is related to heat by the following equation (Lrefers to the latent heat):
Q=Lm [3]During a phase change, the number of degrees of freedom changes, and so does the specific heat capacity. Heat
capacity can also depend on temperature within a given phase, but many substances, under constant pressure, exhibit
a constant specific heat over a wide range of temperatures. For instance, look at the graph of temperature vs heat
input for amole(6. 0221415 × 1023 molecules) of water at http://en.wikipedia.org/wiki/File:Energy_thruphase
changes.png. Note that the x-axis of the graph is called ’relative heat energy’ because it takes a mole of water at 0
degrees Celsius as the reference point.
The sloped segments on the graph represent increases in temperature. These are governed by equation [1]. The
flat segments represent phase transitions, governed by equation [2]. Notice that the sloped segments have constant,
though different, slopes. According to equation [1], the heat capacity at any particular phase would be the slope of
the segment that corresponds to that phase on the graph. The fact that the slopes are constant means that, within a
particular phase, the heat capacity does not change significantly as a function of temperature.
MEDIA
Click image to the left for more content.