Chapter 6 Thermal Energy and Thermodynamics

Chapter 6: Thermal Energy and Thermodynamics

Introduction

• Overview of key concepts:

  • Temperature

  • Absolute Zero

  • Heat

  • Quantity of Heat

  • Laws of Thermodynamics

  • Entropy

  • Specific Heat Capacity

  • Thermal Expansion

  • Expansion of Water

Temperature

• Definition: A measure corresponding to the warmth or coldness of an object, measured by a thermometer.

  • It is a per-particle property with no upper limit and a definite lower limit.

• Kinetic Theory: Temperature correlates with the average translational kinetic energy per particle.

  • Gas: Speed of gas particles bouncing.

  • Liquid: Speed of particles sliding past one another.

  • Solid: Vibration of particles in place.

Thermometers

• Function by the expansion or contraction of liquid (mercury colored alcohol).

• Reading achieved upon reaching thermal equilibrium with the object.

• Infrared Thermometers: Operate by sensing IR radiation.

Temperature Scales

• Celsius: 0°C (freezing) to 100°C (boiling).

• Fahrenheit: 32°F (freezing) to 212°F (boiling).

• Kelvin: 273 K (freezing) to 373 K (boiling), absolute zero at -273°C.

Absolute Zero

• Definition: Lowest possible temperature at -273°C, where atoms lose all available kinetic energy.

• Effects: Volume of gases changes with temperature adjustments.

Thermal Energy

• Described through a sparkler’s temperature of 2000°C, illustrating high temperature but limited energy due to a small number of molecules.

Heat

• Definition: Flow of thermal energy due to temperature difference; naturally flows from higher to lower temperature substances.

• Practical Example: 1L vs. 3L of water absorbing the same heat results in a threefold temperature rise in the smaller volume.

Quantity of Heat

• Measurement: Expressed in joules or calories.

  • Calorie: Heat needed to raise 1g of water by 1°C.

  • Relation: 4.19 joules = 1 calorie.

• Kilocalorie: Used in food labeling, equivalent to raising 1kg of water by 1°C.

The Laws of Thermodynamics

1. First Law: Heat added equals the increase in internal energy plus external work done; total energy is conserved.

2. Second Law: Heat flows naturally from hot to cold; cannot flow reverse without external work.

3. Third Law: Absolute zero cannot be reached.

Entropy

• Definition: A measure of disorder in a system; energy transformations tend to increase disorder.

• Natural systems evolve toward states of higher entropy over time.

Specific Heat Capacity

• Definition: Amount of heat required to raise the temperature of 1 unit mass by 1 degree; indicates resistance to temperature change.

• Example: Water’s high specific heat capacity affects climate, taking longer to heat and cool compared to land.

Thermal Expansion

• Cause: Rise in temperature leads to increased molecular motion, causing expansion.

• Notable Examples: Railroad tracks buckling in heat; expansion joints in bridges.

• Special Consideration: Telephone lines need to be strung with sag to account for thermal expansion.

Expansion of Water

• Unique property: Water expands upon freezing, making ice less dense. Water between 0°C and 4°C contracts before starting to expand.

• Implications: In winter, colder water sinks until the temperature of the entire pond reaches 4°C. Ice forms on top, insulating the lower water layers.