Tuesday 25 December 2012

Thermal Energy of Matter – Temperature, Mass and Heat Capacities

a)      Temperature and Heat:

o   Temperature refers to the degree of hotness of a substance. It is measured by thermometers (liquid-in-bulb thermometers, air thermometer, thermo-couple thermometer, etc.). When two bodies in contact have different temperatures, there will be net flow of heat (or thermal energy) from the body of higher temperature to that of lower temperature.

o   Heat therefore refers to thermal energy of the substance. Thermal energy Q that a substance has depends not only on its temperature T but also on its heat capacity C; or, its specific heat capacity c and mass m (please see (c) below).

o   The SI unit for temperature is K (Kelvin) and that for heat or thermal energy is J (joules). Temperature is often measured in oC (where, oC = K - 273.15 and 1oC = 1 K) and, at times, in oF (Farenheit)

b)      At absolute zero (0 Kelvin (K) temperature or -273.15o C), all matter theoretically has zero thermal energy (although it may have entry point energy). In the universe that we know so far, nothing has registered temperature of absolute zero or below. The lowest recorded temperature is about 2.73 K. However, in labs., scientists have achieved temperature near to but not below absolute zero.

c)      All matter with temperature above absolute zero has thermal energy which depends on

§  Its temperature T (K, Kelvin) and heat capacities C (Q K-1);

§  Its temp. T   (K, Kelvin), mass m (kg) and specific heat capacity c (Q kg-1 K-1).

Thus, the thermal energy Q of a solid substance may be calculated as follows:

·      Q = TC
·      Q = Tm c
where, C (heat capacity) = heat energy required to raise the substance by 1 K
 c (specific heat capacity) = heat energy needed to raise 1 kg of the matter by 1 K or 1 oC  (note: 1 K = 1 oC) .

·      From the above formulae, it is not difficult for you to figure out the definitions of heat capacity C and specific heat capacity c that we will be dealing with in greater depth in a later posting.

d)      Thermal equilibrium refers to the situation when two bodies, say A and B, in contact with one another are at equal temperature (though may not be at equal thermal energies) and the rate of flow of thermal energy (heat) from A to B is the same as from B to A – the net rate of heat transfer between them is zero

e)      Kinetic molecular (or particle) model of matter describes how temperature (and thus thermal energy) of a substance affects:

o   at the microscopic level

§  motion of its constituent particles: whether vibrating about fixed position; moving freely while still being attracted to one another; or moving freely and independently of one another;

§  distance between the particles: getting further apart (expansion) or getting closer (contraction)

§  force of attraction between particles: Strongest in solid state; present in liquid state; negligible in gaseous state

§  force and rate of collisions at which its constituent particles at gaseous state hit any surface in contact including the internal surface of its container resulting in pressure being exerted;


o   at the macroscopic level:

§  3 states of the matter – solid, liquid or gaseous states.

§  volume of the matter – due to expansion or contraction i.e. due to change in the average distance of particles at different temperatures; and

§  pressure and volume of gas due to different temperatures.

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