Thermal Physics: Temperature, Heat Transfer, and Phase Changes – Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Thermal Physics: Temperature, Heat Transfer, and Phase Changes

Non-Thermal Equilibrium Relationships

Thermal equilibrium is a state in which two objects in contact with each other no longer transfer heat, meaning they are at the same temperature. However, systems can have other types of equilibrium or relationships that are not thermal in nature.

Non-thermal equilibrium: A situation where two systems share a property (such as pressure or chemical potential) but are not in thermal equilibrium with a third system.
Example: If A is in mechanical equilibrium (same pressure) with B, and B is in mechanical equilibrium with C, but A and C are not in thermal equilibrium (different temperatures), then the transitive property does not apply for thermal equilibrium.

Temperature Scales and Conversion Errors

Temperature can be measured in different scales, most commonly Celsius (°C) and Fahrenheit (°F). Converting between these scales requires careful attention to both the scale factor and the offset.

Celsius to Fahrenheit conversion: The correct formula is:
Common mistakes:
- Omitting the negative sign (e.g., using +20°C instead of -20°C).
- Forgetting the offset (32) and using only the scale factor (e.g., ).
Example: If the actual temperature is -40°C, omitting the negative sign gives +40°C. Using only the scale factor: °F. Coincidentally, -40°C is also -40°F, so the error can sometimes yield the correct Fahrenheit value by accident.

Bimetallic Thermometers

Bimetallic thermometers use two metals with different coefficients of thermal expansion bonded together. As temperature changes, the metals expand at different rates, causing the strip to bend or coil, which moves a pointer to indicate temperature.

Structure: A spiral or coiled strip made of two metals.
Function: The difference in expansion causes the strip to bend as temperature changes.
Why spiral shape? The spiral amplifies the movement, making small temperature changes easier to read.
Why two metals? Using two metals with different expansion rates creates the bending effect. A single metal would not bend, only expand or contract linearly.
Temperature response: The strip is tightest (smallest spiral) at the lowest temperature and loosest (largest spiral) at the highest temperature.

Heat Transfer: Conduction and Convection

Heat transfer is the process by which thermal energy moves from a region of higher temperature to one of lower temperature. The main mechanisms are conduction, convection, and radiation.

Conduction

Definition: Transfer of heat through a material without the movement of the material itself.
Equation: Where: = heat transfer rate (W), = thermal conductivity (W/m·K), = area (m²), = temperature difference (K or °C), = thickness of material (m).
Example: Calculating heat loss through a window with two glass panes and an air gap.

Convection

Definition: Transfer of heat by the movement of fluids (liquids or gases).
Application: Air gaps in windows reduce heat transfer by limiting conduction and promoting less efficient convection.

Phase Changes and Pressure-Temperature Diagrams

Phase diagrams show the state of a substance (solid, liquid, gas) at different temperatures and pressures. The lines indicate equilibrium between phases.

Melting of ice under pressure: Applying pressure can lower the melting point of ice, causing it to melt at temperatures below 0°C.
Pressure calculation: Pressure is force per unit area:
Example: A skater's blade exerts high pressure on ice, causing it to melt and reduce friction.

Sample Table: Properties of Glass and Air (for Heat Transfer Calculations)

Purpose: To compare the thermal conductivities of glass and air, which are needed for calculating heat transfer through windows.

Material	Thermal Conductivity (W/m·K)
Glass	~0.8
Air	~0.024

Additional info: Values are typical for room temperature; actual values may vary slightly depending on composition and conditions.

Key Equations

Celsius to Fahrenheit:
Heat conduction:
Pressure: