CHAPTER 5
Next to time, temperature may be the most commonly used physical quantity in our daily lives. We might loosely define temperature as a measure of hotness or coldness. The idea of temperature is distinct from that of heat or internal energy.
Thermometers are devices that measure the temperature of a substance. To function, they all depend on some physical property that changes with temperature. A common type exploits the fact that a liquid, usually mercury or red-colored alcohol, will expand or contract when heated or cooled, rising or falling in a glass tube as the temperature varies.
There are three different temperatures scales in common use for calibrating thermometers: FAHRENHEIT (represented by oF), CELSIUS (oC) and KELVIN (K). The normal freezing and boiling temperatures of water –called the phase transition temperatures – may be used to compare the three scales. There is no upper limit on temperature. However, there is a limit on cold temperature. The coldest temperature called Absolute Zero (- 273 0C).
What determines the temperature of matter? At higher temperatures, the atoms and molecules in matter move faster and have higher kinetic energies. Because of collisions between particles, during which energy is exchanged, the particles do not have exactly the same kinetic energy at each instant. But the average kinetic energy of all the particles is constant as the temperatures stays constant.
The formula that relates temperatures in Fahrenheit to the corresponding temperature in Celsius is
(C/5) = [(F- 32)/9]
Temperature in kelvin = temperature in celsius + 273.
Thermal expansion is an important phenomenon that is exploited by the common types of thermometers and by a variety of other useful devices. In almost all cases, substances that are not constrained expand when their temperatures increase (exceptions include water below 4oC and some compounds of tungsten)
We can use logic and basic mathematics to predict the amount of expansion that occurs in solids. If a rod of length L undergoes thermal expansion when the change in temperature is dT, then the change in length dL is
dL = L(dT)(A), where A is known as the co-efficient of linear expansion.
Example 5.1 (page 79, 7th edition) is an useful application of this equation.
The change in length in this example is considerable and must be allowed for in the bridge design. Expansion joints, which act somewhere like loosely interlocking fingers, are placed in bridges, elevated roadways and other structures to allow thermal expansion to safely occur. The equation for thermal expansion also occurs when the temperature decreases. When this occurs, the change in temperature is negative, so the change in length is also negative, the solid becomes shorter.
The BIMETALLIC STRIP is widely used application of thermal expansion. It consists of two strips of different metals bonded to one another. The two metals have different coefficients of linear expansion, so they expand by different amounts when heated. The result is that the strip bends-one way when heated and the other way when cooled.
The behavior of liquids is quite similar to that of solids. Because liquids do not hold a certain shape, it is best to consider the change in volume caused by thermal expansion.
Above 4oC, water expands when heated like ordinary liquids. But between 0oC
and 4oC, water actually contracts when heated and expands when cooled. Water is also unusual in that its density when in solid phase (ice) is less that its density when in the liquid phase. Because of this, ice floats in water , whereas most solids (candle wax) sink in their own liquid.
The volume expansion of gases is larger than that of solids and liquids. Also, the amount of expansion does not vary with different gases (except at very low temperatures or very high pressures).
The IDEAL GAS LAW expresses the interdependence of the pressure, volume and temperature of gas. In a gas with a density that is low enough that interactions between its constituent particles can be ignored, the pressure p, volume V and the temperature T of the gas are related by the following equation:
pV = (constant) T.
The constant depends on the quantity of gas present but not its specific type.
The constant depends on the mass of the gas present and not on what kind of gas it is.
Transferring heat to a substance or doing work on it increases its internal energy and thus changes the temperature of the substance. In developing the theory we assume, that there is no change in phase.
The amount of heat required to change the temperature of a substance of mass M, where the change in temperature is denoted by dT and the amount of heat required is denoted by dQ,
dQ = (M) (dT) C, where C is known as the specific heat capacity of that substance. The SI Unit of specific heat capacity is joule per kilogram – degree Celsius.
Table 5.3 (Page 193) shows specific heat capacity values for a few substances.
Let us try to understand Example 5.2 (page 194)
dT = (100- 20 )0 C = 80 0 C
Specific heat capacity of water (Table 5.3, page 193) = 4,180 J/kg 0 C.
Therefore,
Heat that must be transferred
= (mass of water) (specific heat capacity of water) (change in temperature)
= 73, 600 J
Sections covered from this chapter : Sec 5.1, 5.2 and 5.5 ( till page 194)
Homework 4
Due date : Monday, June 1,2020 ( 6 pm)
- On a nice winter day at the South pole, the temperature rises
to – 60 0 F. What is the approximate temperature in degrees Celsius?
- How much heat is needed to raise the temperature of 5 kg of silver
from 20 0 C to 960 0 C?
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