BackPhysics 3B: Chapter 12 – Thermodynamics, Heat, and Thermal Expansion
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Thermodynamics and the First Law
Introduction to Thermodynamics
Thermodynamics is the branch of physics that deals with the relationships between heat, work, temperature, and energy. The First Law of Thermodynamics is a fundamental principle describing the conservation of energy in thermodynamic processes.
Internal Energy (U): The total energy contained within a system, including kinetic and potential energies of its molecules.
Work (W): Energy transferred to or from a system by mechanical means.
Heat (Q): Energy transferred due to temperature difference.
First Law of Thermodynamics Equation:
Positive Work: Work done on the system increases internal energy.
Negative Work: Work done by the system decreases internal energy.
Thermal Energy Transfer: Heat added to the system increases internal energy.
Example: Stirring water with a paddle does work on the water, increasing its temperature.
States of Matter and Phase Changes
Matter exists in different states: solid, liquid, and gas. The arrangement and movement of molecules differ in each state.
Solid: Molecules are closely packed and vibrate in place. Example: Ice.
Liquid: Molecules are close but can slide past each other. Example: Water.
Gas: Molecules are far apart and move freely. Example: Steam.
Phase Changes: Transitions between states (e.g., melting, freezing, boiling, condensing) involve energy transfer.
Temperature Scales and Conversions
Temperature is a measure of the average kinetic energy of particles in a substance. Common scales include Celsius (°C) and Kelvin (K).
Conversion:
Example: Room temperature (20°C) is K.
Important Temperatures:
Water boils: 100°C (373 K)
Water freezes: 0°C (273 K)
Absolute zero: -273.15°C (0 K)
Thermal Expansion
Linear and Volumetric Expansion
When materials are heated, they generally expand. The amount of expansion depends on the material and the temperature change.
Linear Expansion: Change in length of a material due to temperature change.
Volumetric Expansion: Change in volume of a material due to temperature change.
Linear Expansion Formula:
= change in length
= original length
= coefficient of linear expansion (material-dependent)
= change in temperature
Volumetric Expansion Formula:
= change in volume
= original volume
= coefficient of volumetric expansion
Coefficients of Expansion
Different materials expand at different rates. The coefficients and are experimentally determined for each material.
Material | Linear Expansion (°C-1) | Volumetric Expansion (°C-1) |
|---|---|---|
Steel | 1.7 × 10-5 | 5.0 × 10-5 |
Aluminum | 2.3 × 10-5 | 6.9 × 10-5 Additional info: typical value |
Glass | 8.5 × 10-6 Additional info: inferred | 2.5 × 10-5 Additional info: inferred |
Example Problem: Thermal Expansion
A cryogenic unit keeps tissue samples frozen at liquid nitrogen temperature (-196°C). Suppose a 55-cm-tall stainless steel rod is lowered into the liquid. By how much does it contract from its room-temperature (20°C) length?
Given: cm, °C, for steel = °C-1
Solution:
Calculation:
Result: The rod contracts by approximately cm.
Additional info: The contraction is negative, indicating a decrease in length due to cooling.
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