Heat Transfer

Definition

Heat transfer is the movement of thermal energy from a region of higher temperature to a region of lower temperature. Key principle: Heat flows until thermal equilibrium is reached (equal temperatures).

Three Mechanisms of Heat Transfer

Heat can be transferred through three distinct mechanisms:

1. Conduction
2. Convection
3. Radiation

1. Conduction

Definition

Conduction is heat transfer through direct contact via molecular vibrations and electron movement. Process:

• Particles vibrate more at hot end
• Vibrations transfer to neighboring particles
• Energy spreads from hot to cold end
• No particle movement, only energy transfer

Materials and Conduction

Thermal Conductivity

Thermal conductivity (k) measures how well a material conducts heat.

Where:

• Q = heat transferred (Joules)
• k = thermal conductivity (W/m·°C)
• A = cross-sectional area (m²)
• ΔT = temperature difference (°C)
• d = thickness (m)
• t = time (seconds)

Good vs. Poor Conductors

Good Conductors (High k):

• Metals: copper, aluminum, iron
• k values: 50-400 W/m·°C
• Used for: cookware, heat sinks, radiators

Poor Conductors (Low k) - Insulators:

• Air: 0.026 W/m·°C
• Wood: 0.1 W/m·°C
• Fiberglass: 0.04 W/m·°C
• Used for: insulation, handles on hot pans

Examples of Conduction

Daily life:

• Metal spoon in hot soup gets hot
• Heat through metal pot to cook food
• Touch cold object and feel heat loss
• Thermal conductivity of different metals

Industrial:

• Heat exchangers in engines
• Thermal management in electronics
• Insulation in buildings

2. Convection

Definition

Convection is heat transfer through movement of fluids (liquids or gases). Process:

• Heated fluid becomes less dense (expands)
• Less dense fluid rises (buoyancy)
• Cool fluid sinks
• Creates circular current: convection current

Forced vs. Natural Convection

Natural Convection

• Driven by density differences alone
• Examples: warm air rising from heater, ocean currents
• Slower process

Forced Convection

• Driven by external pump or fan
• Examples: hair dryer, car radiator fan, air conditioning
• More efficient (faster heat transfer)

Convection Currents

A convection current is a circular flow pattern:

1. Hot fluid rises (less dense)
2. Rises to top/cool surface
3. Cools and becomes denser
4. Sinks back down
5. Process repeats

Examples of Convection

Atmospheric:

• Wind formation (solar heating of air)
• Thunderstorms (warm air rising)
• Ocean currents

Household:

• Boiling water (bubbles rising)
• Heating a room (warm air circulates)
• Refrigerator (cold air circulation)

Industrial:

• Cooling systems in power plants
• Heating systems in buildings
• Furnace operation

3. Radiation

Definition

Radiation is heat transfer through electromagnetic waves without requiring a medium. Process:

• Hot objects emit electromagnetic radiation
• Radiation travels through space
• Objects absorb radiation and warm up
• No direct contact needed

Infrared Radiation

Thermal radiation is primarily infrared radiation:

• Wavelength: 700 nm - 1 mm
• Invisible to human eye
• All objects above absolute zero emit some radiation

Stefan-Boltzmann Law

Power radiated by an object:

Where:

• P = radiant power (Watts)
• σ = Stefan-Boltzmann constant (5.67 × 10⁻⁸ W/m²·K⁴)
• A = surface area (m²)
• T = absolute temperature (Kelvin)

Key insight: Power depends on T⁴ - small temperature changes create large power changes!

Absorptivity and Emissivity

Darker surfaces:

• Absorb more radiation (high absorptivity)
• Emit more radiation (high emissivity)
• Examples: black paint, dark clothing

Lighter surfaces:

• Absorb less radiation (low absorptivity)
• Emit less radiation (low emissivity)
• Examples: white paint, light-colored clothing

Examples of Radiation

Solar:

• Sun's heat reaches Earth via radiation
• No medium between Sun and Earth
• Ultraviolet and infrared radiation

Household:

• Heat from light bulb (filament radiation)
• Heat from fire (flames radiate)
• Thermal imaging cameras detect radiation

Industrial:

• Temperature measurement with infrared sensors
• Microwave ovens (microwave radiation heats water)
• Sterilization with ultraviolet radiation

Comparing Heat Transfer Mechanisms

Thermal Equilibrium

Reaching Equilibrium

When objects at different temperatures are in contact:

1. Heat flows from hot to cold object
2. Temperature of hot object decreases
3. Temperature of cold object increases
4. Process continues until thermal equilibrium

At equilibrium: no net heat flow.

Mixing Example

Hot water (80°C) mixed with cold water (20°C):

• Both combine to intermediate temperature
• Thermal energy is conserved
• Heat lost by hot = heat gained by cold

Insulation and Heat Loss Prevention

Purpose of Insulation

Reduces heat transfer:

• Conduction: Low thermal conductivity material
• Convection: Traps air pockets (air is poor conductor)
• Radiation: Reflective surfaces bounce radiation

Common Insulation Materials

Houses:

• Fiberglass (0.04 W/m·°C)
• Foam (0.03 W/m·°C)
• Cork (0.05 W/m·°C)
• Wool (0.04 W/m·°C)

Spacecraft:

• Multi-layer insulation (MLI) - vacuum reduces conduction/convection
• Thermal shields - reflect radiation

Clothing:

• Layered materials trap air
• Prevents convection loss
• Reflective inner layer reduces radiation

Real-World Applications

Home Heating/Cooling

• Insulation: Reduces conduction through walls
• Double-pane windows: Air gap reduces conduction
• Weatherstripping: Prevents convection loss
• Reflective roof: Reduces radiation absorption

Cooking

• Metal pans: Good thermal conductivity transfers heat
• Pot handles: Insulating material prevents burn
• Lids: Trap convection heat
• Heat diffusers: Spread heat evenly

Industrial Heat Exchangers

• Transfer heat between fluids
• Use conduction through metal walls
• Increase surface area for efficiency
• Critical for power plants, refrigeration

Clothing and Comfort

• Winter: Thick insulation prevents heat loss
• Summer: Light colors reflect radiation
• Desert: Layering traps air, preventing rapid heating/cooling
• Water sports: Wetsuits trap water layer, insulate body

Thermal Management in Electronics

• Heat sinks: High thermal conductivity material
• Fans: Force convection cooling
• Thermal paste: Improves conduction between components
• Radiators: Large surface area for radiation/convection

Energy Efficiency

Reducing Heat Loss

In homes:

• Insulation R-value indicates thermal resistance
• Higher R-value = better insulation
• Proper sealing prevents convection
• Thermal mass (concrete, water) stores heat

In industry:

• Heat exchangers recover wasted heat
• Improved insulation saves energy
• Thermostat control optimizes temperature

Greenhouse Effect

How it works:

1. Solar radiation enters atmosphere
2. Earth absorbs radiation and heats up
3. Earth emits infrared radiation
4. Greenhouse gases trap infrared (prevent escape)
5. Heat accumulates - temperature rises

Key Takeaways

1. Conduction: Heat transfer through direct contact (molecular vibrations)
2. Thermal conductivity (k): Property of material; metals conduct well
3. Convection: Heat transfer via fluid movement (liquids/gases)
4. Convection currents: Circular flow from hot rising to cold sinking
5. Radiation: Heat transfer via electromagnetic waves (no medium needed)
6. Stefan-Boltzmann law: P = σAT⁴ (power depends on T⁴)
7. Thermal equilibrium: Reached when temperatures equal
8. Insulation: Reduces heat transfer in homes and devices