# What is Temperature – Physics

In physics and everyday life, a temperature is an objective comparative measurement of hot or cold based on our sense of touch.  A body that usually feels hot has a higher temperature than a similar body that feels cold. But this definition is not a simple matter. For example, a metal rod feels colder than a plastic rod at room temperature simply because metals are generally better at conducting heat away from the skin, as are plastics.

Simply hotness may be represented abstractly, and therefore it is necessary to have an objective way of measuring temperature. It is one of the basic thermodynamic properties.

## Thermal Equilibrium

A particularly important concept is thermodynamic equilibrium. In general, when two objects are brought into thermal contact, heat will flow between them until they come into equilibrium with each other.  When a temperature difference does exist, heat flows spontaneously from the warmer system to the colder system. Heat transfer occurs by conduction or by thermal radiation. When the flow of heat stops, they are said to be at the same temperature. They are then said to be in thermal equilibrium.

For example, you leave a thermometer in a cup of coffee. As the two objects interact, the thermometer becomes hotter, and the coffee cools off a little until they come into thermal equilibrium. Two objects are defined to be in thermal equilibrium if, when placed in thermal contact, no net energy flows from one to the other, and their temperatures don’t change. We may postulate:

When the two objects are in thermal equilibrium, their temperatures are equal.

This is a subject of a law that is called the “zeroth law of thermodynamics”.

Zeroth Law of Thermodynamics

We can discover an important property of thermal equilibrium by considering three systems. A, B, and C are initially not in thermal equilibrium. We separate systems A and B with an adiabatic wall (ideal insulating material), but we let system C interact with systems A and B. We wait until thermal equilibrium is reached; then, A and B are in thermal equilibrium with C. But are they in thermal equilibrium with each other?

According to many experiments, there will be no net energy flow between A and B. This is experimental evidence of the following statement:

If two systems are both in thermal equilibrium with a third, then they are in thermal equilibrium with each other.

This statement is known as the zeroth law of thermodynamics. It has this unusual name because it was not until after the great first and second laws of thermodynamics were worked out that scientists realized that this obvious postulate needed to be stated first.

This law provides a definition and method of defining temperatures, perhaps the most important intensive property of a system when dealing with thermal energy conversion problems. Temperature is a system property that determines whether the system will be in thermal equilibrium with other systems. When two systems are in thermal equilibrium, their temperatures are, by definition, equal, and no net thermal energy will be exchanged between them. Thus the importance of the zeroth law is that it allows a useful definition of temperature.

Temperature is a very important characteristics of matter. Many properties of matter change with temperature. The length of a metal rod, steam pressure in a boiler, the ability of a wire to conduct an electric current, and the color of a very hot glowing object. All these depend on temperature.

For example, most materials expand when their temperature is increased. This property is very important in all of science and engineering, even in nuclear engineering. The thermodynamic efficiency of power plants changes with inlet steam temperature or even with the outside temperature. At higher temperatures, solids such as steel glow orange or even white, depending on temperature. The white light from an ordinary incandescent lightbulb comes from an extremely hot tungsten wire. It can be seen temperature is one of the fundamental characteristics that describe matter and influences matter behavior.

## Kinetic Temperature

The kinetic theory of gases provides a microscopic explanation of temperature. It is based on the fact that during an elastic collision between a molecule with high kinetic energy and one with low kinetic energy, part of the energy will transfer to the molecule of lower kinetic energy. Temperature is therefore related to the kinetic energies of the molecules of a material. Since this relationship is fairly complex, it will be discussed later.[/su_spoiler][/su_accordion]

## Temperature Scales

When using a thermometer, we need to mark a scale on the tube wall with numbers. We have to define a temperature scale. A temperature scale is a way to measure temperature relative to a starting point (0 or zero) and a unit of measurement.

These numbers are arbitrary, and historically many different schemes have been used. For example, this was done by defining some physical occurrences at given temperatures—such as the freezing and boiling points of water — and defining them as 0 and 100, respectively.

Some several scales and units exist for measuring temperature. The most common are:

• Celsius (denoted °C),
• Fahrenheit (denoted °F),
• Kelvin (denoted K, especially in science).
Fahrenheit Scale

The Celsius scale and the Fahrenheit scale are based on the number of increments between the freezing point and boiling point of water at standard atmospheric pressure. The Celsius scale has 100 units between these points, and the Fahrenheit scale has 180 units, where each unit represents 1°C or 1 °F, respectively. The zero points on the scales are arbitrary.

Fahrenheit scale is based on two points:

• The lower defining point, 0 °F, was established as the temperature of a solution of brine made from equal parts of ice and salt.
• The upper defining point, 96 °F, was established as the average human body temperature (96 °F, about 2.6 °F less than the modern value due to a later redefinition of the scale)

The difference in height between the two points would then be marked off in 180 divisions, with each division representing 1 °F. Today, the scale is usually defined by two fixed points: the temperature at which water freezes into ice is 32 °F, and the boiling point of water is 212 °F.

Temperature Conversion – Fahrenheit – Celsius

To convert from a Fahrenheit temperature to a Celsius temperature, we have to subtract 32 degrees from the Fahrenheit reading to get to the zero point on the Celsius scale and then adjust for the different size degrees. The ratio of the size of the degrees is 5/9 so that the relationship between the scales is represented by the following equations:

°F = 32.0 + (9/5)°C

°C = (°F – 32.0)(5/9)

Celsius Scale

About 20 years after Fahrenheit proposed its temperature scale for thermometer, Swedish professor Anders Celsius defined a better scale for measuring temperature. He proposed using the boiling point of water as 100° C and the freezing point of water as 0° C. Water was chosen as the reference material because it was always available in most laboratories worldwide.

Celsius temperature scale is also called the centigrade temperature scale because of the 100-degree interval between the defined points. The Celsius temperature for a state colder than freezing water is a negative number. The Celsius scale is used, both in everyday life and in science and industry, almost everywhere globally.

Absolute zero, the lowest temperature possible, is defined as being precisely 0 K and −273.15 °C. The temperature of the triple point of water is defined as precisely 273.16 K and 0.01 °C. This definition fixes the magnitude of both the degree Celsius and the kelvin as precisely 1 part in 273.16 of the difference between absolute zero and the triple point of water.

It must be added, by international agreement the unit “degree Celsius” and the Celsius scale are currently defined by two different points: absolute zero and the triple point of water (instead of boiling and freezing points). This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature.

Temperature Conversion – Fahrenheit – Celsius

To convert from a Fahrenheit temperature to a Celsius temperature, we have to subtract 32 degrees from the Fahrenheit reading to get to the zero point on the Celsius scale and then adjust for the different size degrees. The ratio of the size of the degrees is 5/9 so that the relationship between the scales is represented by the following equations:

°F = 32.0 + (9/5)°C
°C = (°F – 32.0)(5/9)

Kelvin Scale

The Kelvin temperature scale is the base unit of thermodynamic temperature measurement in the International System (SI). The Kelvin scale was determined based on the Celsius scale but with a starting point at absolute zero. Temperatures in the Kelvin scale are 273 degrees less than in the Celsius scale. The kelvin is defined as the fraction  1⁄273.16 of the thermodynamic temperature of the triple point of water. By international agreement, the triple point of water has been assigned a value of 273.16 K (0.01 °C; 32.02 °F) and partial vapor pressure of 611.66 pascals (6.1166 mbar; 0.0060366 atm). In other words, it is defined such that the triple point of water is exactly 273.16 K.

Note that the unit on the absolute scale is Kelvins, not degrees Kelvin. It was named in honor of Lord Kelvin, who had a great deal with temperature measurement and thermodynamics development.

The absolute temperature scale that corresponds to the Celsius scale is called the Kelvin (K) scale, and the absolute scale that corresponds to the Fahrenheit scale is called the Rankine (R) scale. The zero points on both absolute scales represent the same physical state. The relationships between the absolute and relative temperature scales are shown in the following equations.

Kelvin – Celsius

K = °C + 273.15

°C = K – 273.15

Rankine – Fahrenheit

R = °F + 460

°F = R – 460

## Absolute Zero

Such a scale has as its zero point. The coldest theoretical temperature is called absolute zero, at which the thermal motion of atoms and molecules reaches its minimum.  This is a state at which the enthalpy and entropy of a cooled ideal gas reach its minimum value, taken as 0. Classically, this would be a state of motionlessness, but quantum uncertainty dictates that the particles still possess finite zero-point energy. Absolute zero is denoted as 0 K on the Kelvin scale, −273.15 °C on the Celsius scale, and −459.67 °F on the Fahrenheit scale.

### Absolute Zero and Third Law of Thermodynamics

Third law of thermodynamics states:

The entropy of a system approaches a constant value as the temperature approaches absolute zero.
Based on empirical evidence, this law states that the entropy of a pure crystalline substance is zero at the absolute zero of temperature, 0 K and that it is impossible using any process, no matter how idealized, to reduce the temperature of a system to absolute zero in a finite number of steps. This allows us to define a zero point for the thermal energy of a body.

## The temperature required nuclear fusion to occur

Deuterium-Tritium Fusion. The fusion reaction of deuterium and tritium is particularly interesting because of its potential of providing energy for the future.

3T (d, n) 4He

The reaction yields ~17 MeV of energy per reaction. Still, it requires an enormous temperature of approximately 40 million Kelvins to overcome the coulomb barrier by the attractive nuclear force, stronger at close distances. The deuterium fuel is abundant, but tritium must be either bred from lithium or gotten in the operation of the deuterium cycle.

## Highest manufactured temperature

On 13 August 2012, scientists at CERN’s Large Hadron Collider (LHC), Geneva, Switzerland, announced that they created a quark-gluon plasma with a record-smashing temperature of 5.5 trillion K.

The team had been using the ALICE experiment, which is focused on studying the QCP and other conditions in the primordial Universe to smash together lead ions at 99% of the speed of light to create a quark-gluon plasma. It is believed that up to a few milliseconds after the Big Bang, the Universe was in a quark-gluon plasma state, which is an exotic state of matter. It is thought quark-gluon plasma consists of asymptotically free quarks and gluons.

## Lowest manufactured temperature

According to Guinness World Records, the lowest manufactured temperature achieved is 450 picokelvin above absolute zero (only half a billionth of a degree above absolute zero). It was achieved by a team of scientists at the Massachusetts Institute of Technology, Cambridge, Massachusetts, USA.

References:
Reactor Physics and Thermal Hydraulics:
1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
5. Todreas Neil E., Kazimi Mujid S. Nuclear Systems Volume I: Thermal Hydraulic Fundamentals, Second Edition. CRC Press; 2 edition, 2012, ISBN: 978-0415802871
6. Zohuri B., McDaniel P. Thermodynamics in Nuclear Power Plant Systems. Springer; 2015, ISBN: 978-3-319-13419-2
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8. Kleinstreuer C. Modern Fluid Dynamics. Springer, 2010, ISBN 978-1-4020-8670-0.
9. U.S. Department of Energy, THERMODYNAMICS, HEAT TRANSFER, AND FLUID FLOW. DOE Fundamentals Handbook, Volume 1, 2, and 3. June 1992.

## See above:

Thermodynamic Properties

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