Temperature, Heat, and Measurement
Introduction
Thermodynamics is a branch of physics that deals with heat, work, temperature, and energy, and the laws that govern their transformations. It describes how thermal energy is transferred and converted into other forms of energy, and how these processes relate to the macroscopic properties of matter.
At its core, thermodynamics is concerned with energy transformations, particularly those involving heat and work. While the mechanical energy concepts (kinetic and potential energy) describe energy at the macroscopic level, thermodynamics often deals with energy at the microscopic level – the internal energy associated with the random motion of atoms and molecules.
The concepts of temperature and heat are central to thermodynamics. We experience temperature every day – it tells us how hot or cold something is. Heat is the energy that flows from a hotter object to a colder object. Understanding these concepts and how to measure temperature is the starting point for studying thermal physics.
Thermodynamics has wide-ranging applications, from the design of engines and power plants to understanding chemical reactions, biological systems, and even the evolution of the universe.
Temperature And Heat
While often used interchangeably in everyday language, temperature and heat are distinct physical concepts. Understanding their difference is crucial in thermal physics.
Temperature (T)
Temperature is a measure of the average kinetic energy of the atoms or molecules in a substance. It is a property that determines the direction of heat flow between two objects in thermal contact.
- Temperature tells us the degree of hotness or coldness of an object.
- Temperature is a scalar quantity.
- Temperature is related to the microscopic motion of particles: Higher temperature means, on average, the particles are moving faster (in translation, rotation, or vibration).
- Two objects in thermal contact that are at the same temperature are in thermal equilibrium, and there is no net flow of heat between them.
Temperature scales are used to quantify temperature. Common scales include Celsius (°C), Fahrenheit (°F), and Kelvin (K). The Kelvin scale is the SI unit of temperature and is an absolute scale, meaning 0 K is absolute zero, the theoretical temperature at which particles have minimum possible motion.
Heat (Q)
Heat is the transfer of thermal energy between objects or systems due to a temperature difference. It is energy in transit.
- Heat is a form of energy transfer, not energy stored within a system. The term used for the energy stored within a system due to the random motion and interactions of its particles is internal energy.
- Heat flows spontaneously from a region of higher temperature to a region of lower temperature.
- Heat transfer stops when the objects in contact reach thermal equilibrium (same temperature).
- Heat is a scalar quantity, but its transfer has direction (from hot to cold).
The SI unit of heat is the same as that of energy, the joule (J). Another common unit is the calorie (cal). $1 \text{ calorie}$ is the amount of heat required to raise the temperature of 1 gram of water by 1°C (from 14.5°C to 15.5°C). $1 \text{ calorie} \approx 4.184$ Joules. The 'calorie' used in nutrition is actually a kilocalorie (kcal), $1 \text{ kcal} = 1000 \text{ calories}$.
Key Differences Summarised
| Feature | Temperature | Heat |
|---|---|---|
| What it is | Measure of the average kinetic energy of particles; indicates degree of hotness/coldness. | Energy transferred due to temperature difference. |
| Property vs. Transfer | Property of a system. | Energy in transit (transfer). |
| Units | Celsius (°C), Fahrenheit (°F), Kelvin (K). | Joule (J), calorie (cal). |
| Nature | Intensive property (does not depend on the amount of substance). | Extensive property (depends on the amount of substance and temperature difference). |
| Measurement | Measured using a thermometer. | Calculated or measured by methods like calorimetry (measuring temperature changes caused by heat transfer). |
Think of it like a tank of water: Temperature is like the water level (an intensive property indicating the "potential" for flow), while heat is like the flow of water between tanks at different levels (energy transfer). A large tank and a small tank can be at the same temperature (same level), but the large tank contains more internal energy (more water).
Measurement Of Temperature
Temperature is measured using a device called a thermometer. Thermometers work by utilising some physical property of a substance that changes reliably with temperature. This changing property is used to indicate the temperature on a calibrated scale.
Thermometric Properties
Common physical properties used for thermometry include:
- Length or volume of a liquid (e.g., mercury or alcohol in a glass thermometer - thermal expansion).
- Pressure of a gas at constant volume.
- Volume of a gas at constant pressure.
- Electrical resistance of a metal wire (resistance changes with temperature - Resistance Thermometer, e.g., Platinum Resistance Thermometer - PRT).
- Voltage generated at the junction of two different metals (Seebeck effect - Thermocouple).
- Radiation emitted by a hot object (used for very high temperatures - Pyrometer).
Temperature Scales
To establish a temperature scale, we need reference points (fixed points) and a way to divide the interval between them. Historically, the melting point of ice and the boiling point of water at standard atmospheric pressure have been used as fixed points.
- Celsius Scale (°C): Widely used in most parts of the world (including India) for everyday temperature measurements. The melting point of ice is defined as 0°C, and the boiling point of water as 100°C at standard atmospheric pressure. The interval between these points is divided into 100 equal degrees.
- Fahrenheit Scale (°F): Primarily used in the United States. The melting point of ice is defined as 32°F, and the boiling point of water as 212°F. The interval is divided into 180 degrees.
- Kelvin Scale (K): The SI unit of temperature and an absolute thermodynamic scale. It is based on the concept of absolute zero. The triple point of water (the unique temperature and pressure at which water, ice, and water vapour coexist in equilibrium) is a key reference point, defined as 273.16 K. The size of one Kelvin is defined such that it is exactly 1/273.16 of the thermodynamic temperature of the triple point of water. The difference between the boiling and freezing points of water is 100 K, same as 100°C. Absolute zero is 0 K.
Converting Between Temperature Scales
The relationships between the three scales are:
- Celsius to Fahrenheit: $ T_F = \frac{9}{5} T_C + 32 $
- Fahrenheit to Celsius: $ T_C = \frac{5}{9} (T_F - 32) $
- Celsius to Kelvin: $ T_K = T_C + 273.15 $ (often rounded to 273 for calculations)
- Kelvin to Celsius: $ T_C = T_K - 273.15 $
Note that temperature difference in Celsius is equal to the temperature difference in Kelvin ($\Delta T_C = \Delta T_K$), but different from Fahrenheit difference.
Example 1. The normal human body temperature is approximately 37°C. Convert this temperature to the Fahrenheit and Kelvin scales.
Answer:
Given temperature in Celsius, $T_C = 37^\circ\text{C}$.
Convert to Fahrenheit ($T_F$):
Use the formula $ T_F = \frac{9}{5} T_C + 32 $.
$ T_F = \frac{9}{5} \times 37 + 32 $
$ T_F = (1.8 \times 37) + 32 $
$ T_F = 66.6 + 32 $
$ T_F = 98.6 ^\circ\text{F}$.
Normal human body temperature is 98.6°F.
Convert to Kelvin ($T_K$):
Use the formula $ T_K = T_C + 273.15 $. (Using 273 for simplicity if allowed).
$ T_K = 37 + 273.15 = 310.15 $ K.
Using 273: $ T_K = 37 + 273 = 310 $ K.
Normal human body temperature is approximately 310.15 K (or 310 K).