Thermal Energy and Heat

Chapter 16: Thermal Energy and Heat Overview

• Essential Questions and Concepts:     * Understanding how thermal energy (microscopic motions of atoms/molecules) relates to temperature.     * Examining differences in concepts by measuring temperature changes and determining specific heat capacity of water.     * Differentiating between conduction, convection, and radiation as types of energy transfer.     * Describing benefits and risks of renewable energy forms: solar, wind, geothermal, fusion, and biofuels.     * Maintaining an acceptable environment through efficient use of renewable and non-renewable energy sources.     * Explaining the special properties of water: cohesive behavior, ability to moderate temperature, expansion upon freezing, and versatility as a solvent.

Section 16.1: Thermal Energy and Matter

• Historical Context of Heat:     * In the 1700s, heat was thought to be a fluid called caloric that flowed between objects.     * Benjamin Thompson (Count Rumford): In 1798, Thompson challenged the caloric theory while supervised the drilling of brass cannons in Bavaria.     * Rumford observed that the drilling process produced enough heat to boil water as long as the drilling continued; when drilling stopped, boiling stopped. He concluded heat was not matter but related to the motion of the drill.

• Heat and Work:     * Heat: The transfer of thermal energy from one object to another because of a temperature difference.     * Heat flows spontaneously from hot objects to cold objects.     * Work done by machines (like a drill) involves friction, which converts some work into thermal energy.

• Temperature:     * Temperature: A measure of how hot or cold an object is compared to a reference point.     * Reference points include the freezing ($0^{\circ} C$) and boiling ($100^{\circ} C$) points of water on the Celsius scale.     * The Kelvin scale uses absolute zero ($0\,K$) as a reference point.     * Temperature is related to the average kinetic energy of the particles in an object due to their random motions. As temperature increases, average kinetic energy increases.

• Thermal Energy:     * Thermal Energy: The total potential and kinetic energy related to the motion of all particles in an object.     * Factors affecting thermal energy: mass, temperature, and phase (solid, liquid, or gas).     * Mass Relationship: A teapot of tea at the same temperature as a cup of tea has more thermal energy because it contains more particles.     * Temperature Relationship: A cup of hot tea has more thermal energy than a cup of cold tea of the same mass because the particles move faster (higher average kinetic energy).

• Thermal Expansion and Contraction:     * Thermal Expansion: An increase in the volume of a material due to a temperature increase.     * Mechanism: Particles move farther apart as temperature increases.     * Phases: Gases expand more than liquids, and liquids usually expand more than solids due to differences in forces of attraction.     * Thermal Contraction: A decrease in volume as temperature decreases and particles move more slowly/collide less often.     * Applications: Glass thermometers (alcohol expansion) and oven thermometers (bimetallic strips of brass and steel that expand at different rates).

• Specific Heat:     * Specific Heat ($c$): The amount of heat needed to raise the temperature of one gram of a material by one degree Celsius.     * Formula: $Q = m \cdot c \cdot \Delta T$         * $Q$ is heat absorbed in joules ($J$).         * $m$ is mass in grams ($g$).         * $c$ is specific heat in $J/g \cdot ^{\circ} C$.         * $\Delta T$ is the change in temperature in degrees Celsius ($^{\circ} C$).     * Example: Specific heat of water is $4.18\,J/g \cdot ^{\circ} C$. Plastic (polypropylene) is $1.84$-$2.09\,J/g \cdot ^{\circ} C$. Air is $1.01\,J/g \cdot ^{\circ} C$. Iron is $0.449\,J/g \cdot ^{\circ} C$. Silver is $0.235\,J/g \cdot ^{\circ} C$.     * Calculation Example: For an iron skillet with $m = 500.0\,g$, $c = 0.449\,J/g \cdot ^{\circ} C$, and $\Delta T = 95.0^{\circ} C$:         * $Q = 500.0\,g \times 0.449\,J/g \cdot ^{\circ} C \times 95.0^{\circ} C = 21,375\,J \approx 21.4\,kJ$.

• Calorimetry:     * Calorimeter: An instrument used to measure changes in thermal energy.     * Principle: Heat flows from a hotter object to a colder object until they reach the same temperature.     * Law of Conservation of Energy: Thermal energy released by a test sample = thermal energy absorbed by its surroundings (usually water).     * The device is sealed to prevent energy escape and typically includes a stirrer to distribute energy evenly.

Section 16.2: Heat and Thermodynamics

• Conduction:     * Conduction: The transfer of thermal energy with no overall transfer of matter.     * Occurs through collisions between particles within a material or between touching materials.     * Conduction in gases is slower than in liquids and solids because gas particles are farther apart and collide less often.     * Metals are fast conductors due to free electrons that collide with atoms and other electrons.     * Thermal Conductor: A material that conducts thermal energy well (e.g., copper, aluminum, tile floors).     * Thermal Insulator: A material that conducts thermal energy poorly (e.g., wood, air, wool, plastic foam, argon gas in windows).

• Convection:     * Convection: The transfer of thermal energy when particles of a fluid (gas or liquid) move from one place to another.     * Convection Current: Occurs when a fluid circulates in a loop as it alternately heats up (expanding/becoming less dense/rising) and cools down (contracting/becoming denser/sinking).     * Natural Cycles: Ocean currents, weather systems, and movements of hot rock in Earth's interior.

• Radiation:     * Radiation: The transfer of energy by waves moving through space. It does not require matter.     * All objects radiate energy; the rate increases as temperature increases.

• Laws of Thermodynamics:     * Thermodynamics: The study of conversions between thermal energy and other forms of energy.     * First Law of Thermodynamics: Energy is conserved. Energy added to a system can increase thermal energy or do work on the system ($Q = \Delta U + W$).     * Second Law of Thermodynamics: Thermal energy flows spontaneously only from hotter to colder objects. It can flow from cold to hot only if work is done on the system (e.g., a refrigerator).         * Waste Heat: Thermal energy not converted into work by a heat engine; it is lost to the environment.         * Efficiency of a heat engine is always less than $100\%$.         * Disorder (entropy) in the universe is always increasing.     * Third Law of Thermodynamics: Absolute zero ($0\,K$) cannot be reached.

Section 16.3: Using Heat

• Heat Engines:     * External Combustion Engine: Burns fuel outside the engine (e.g., steam engine).         * Thomas Newcomen (1712) developed the first practical version to pump water from mines.         * James Watt (1765) improved efficiency by operating at higher temperatures.     * Internal Combustion Engine: Burns fuel inside the engine (e.g., gasoline car engine).         * Four-Stroke Cycle:             1. Intake Stroke: Air-fuel mixture enters the cylinder.             2. Compression Stroke: Piston compresses the gas; spark plug ignites it.             3. Power Stroke: Hot gas expands and drives the piston down (useful work).             4. Exhaust Stroke: Exhaust gases leave the cylinder.         * Only about one-third of fuel energy is converted to work.

• Heating Systems:     * Central Heating System: Heats many rooms from one central location (basement).     * Hot-Water Heating: Water heated by a boiler, circulated by a pump to radiators; heat transferred by conduction, radiation, and convection.     * Steam Heating: Similar to hot-water but uses steam; often used in older buildings.     * Electric Baseboard Heating: Converts electrical energy to thermal energy using a conductor (coil).     * Forced-Air Heating: Uses fans to circulate warm air through ducts; includes filters to clean air.

• Cooling Systems and Heat Pumps:     * Heat Pump: A device that reverses the normal flow of thermal energy by doing work on a refrigerant.     * Refrigerant: A fluid that vaporizes (absorbs heat) and condenses (gives off heat) inside tubing.     * Refrigerators: Transfer thermal energy from the cold compartment to the warm room. Leaving the door open heats the room because of waste heat from the motor.     * Air Conditioners: Use a compressor, condenser coil (outside), and evaporator coil (inside) to move heat from indoors to outdoors.

Solar Energy in Home Design

• Passive Strategies: Large south-facing windows to trap radiant energy; north-facing walls with high insulation and few windows; deciduous trees for summer shade and winter light.

• Active Strategies:     * Solar Collectors: On the roof to heat water.     * Solar Panels (Photovoltaic cells): Linked cells made of silicon that convert sunlight into electric current.     * Rechargeable Batteries: Store electrical energy for use when there is no sunlight.

• Insulation: Timber-framed walls filled with insulating material seal against drafts and reduce heat flow.

Questions & Discussion

• Q: Why is the metal door hotter than the plastic bumper of a car in the sun?

• A: Metal has a lower specific heat than plastic, so it rises in temperature more when absorbing the same energy.

• Q: Why doesn't hot air burn your arm when reaching into an oven?

• A: Air is a poor thermal conductor (good insulator).

• Q: Can you cool a kitchen by leaving the refrigerator door open?

• A: No, the refrigerator releases more heat from its motor and coils than it removes from the compartment, thus heating the room.

• Q: Why does a bicycle pump heat up when pumping a tire?

• A: According to the first law of thermodynamics, the work done on the system (compressing air) is partially converted into thermal energy.

• Q: What is the record lowest temperature achieved?

• A: 3 billionths of a kelvin above absolute zero ($3 \times 10^{-9}\,K$).