Concept Notes 1: Temperature, Thermometry & Conduction
Heat is a form of energy that flows spontaneously from an object at a higher temperature to an object at a lower temperature. Understanding temperature measurement and thermal conduction forms the bedrock of thermal physics.
1. Heat vs. Temperature
- Heat: The total internal kinetic energy transferred between systems due to a temperature difference. Measured in Joules (J) or calories (cal).
- Temperature: A reliable, objective measure of the degree of hotness or coldness of an object. Measured using the Celsius scale ($^\circ\text{C}$), Fahrenheit scale ($^\circ\text{F}$), or the SI base unit Kelvin ($\text{K}$).
$$\text{Temperature in Kelvin} = \text{Temperature in } ^\circ\text{C} + 273.15$$
2. Thermometers: Clinical vs. Laboratory
Our sense of touch is subjective and easily deceived. Standardized instruments called Thermometers provide accurate, reproducible measurements.
A. The Clinical Thermometer:
- Purpose: Specifically calibrated to measure the internal temperature of the human body.
- Temperature Scale Range: $35^\circ\text{C}$ to $42^\circ\text{C}$ (or $94^\circ\text{F}$ to $108^\circ\text{F}$). Normal human body temperature is approximately $37^\circ\text{C}$ ($98.6^\circ\text{F}$).
- The Kink (Constriction): Features a sharp bend or constriction in the capillary tube just above the mercury bulb.
- Function: Prevents the mercury thread from falling back into the bulb when the thermometer is removed from the patient's armpit or mouth, allowing accurate reading.
- To reset the thermometer for another reading, it must be gently jerked downward.
- Modern Digital Thermometers: Eliminate the use of toxic mercury by utilizing temperature-sensitive electronic thermistors, providing a safe digital readout.
B. The Laboratory Thermometer:
- Purpose: Used in scientific experiments to measure temperatures of chemical reactions, boiling liquids, and ambient environments.
- Temperature Scale Range: Typically $-10^\circ\text{C}$ to $110^\circ\text{C}$.
- Absence of Kink: Does not have a constriction. Therefore, its reading must be recorded while the bulb is still in direct contact with the substance being measured!
3. Conduction: Heat Transfer in Solids
- Definition: The process by which heat is transferred from the hotter end to the colder end of an object through the progressive vibrational collisions of adjacent atoms or molecules, without any bulk movement of the material itself.
- Mechanism: When one end of a solid metal bar is heated, particles at that end gain kinetic energy and vibrate intensely. These vibrating particles collide with their neighbors, transferring vibrational energy down the lattice.
- Thermal Conductors: Substances that allow heat to pass through them readily and rapidly (e.g., Silver, Copper, Aluminium, Iron, Brass).
- Thermal Insulators (Poor Conductors): Substances that resist the passage of heat (e.g., Wood, Plastic, Glass, Rubber, Cork, Ebonite, Air, Water).
Practical Engineering Applications:
- Cooking pans are fabricated from copper or aluminium for rapid, uniform heat distribution, but are fitted with handles made of bakelite or wood to protect hands from thermal burns.
- Trapped Air as an Insulator: Still air has an extremely low thermal conductivity ($k \approx 0.026\text{ W/m}\cdot\text{K}$).
- Woolen clothing keeps us warm because curly wool fibers trap a high volume of air.
- Using two thin blankets traps an insulating air layer between them, providing superior thermal protection compared to a single thick blanket of identical total weight!