Overview: Heat Transfer | Physics for JAMB PDF Download

Conduction, Convection, and Radiation as Modes of Heat Transfer

• Conduction: Conduction is the transfer of heat through direct contact between particles of a substance. In solids, heat is transferred through the vibration and collision of atoms or molecules. When one end of a metal rod is heated, the particles near that end gain energy and vibrate more vigorously, transferring energy to neighboring particles and so on. This process continues, and eventually, heat is conducted throughout the entire rod.
• Convection: Convection is the transfer of heat through the movement of fluids (liquids or gases). It occurs due to the differences in density within a fluid. When a fluid is heated, it expands, becomes less dense, and rises. As it rises, cooler fluid from the surroundings replaces it. This creates a continuous circulation or convection current, transferring heat from one place to another.
• Radiation: Radiation is the transfer of heat through electromagnetic waves. Unlike conduction and convection, it does not require a medium to transfer heat. All objects with a temperature above absolute zero (-273.15°C) emit thermal radiation. The rate of radiation depends on the temperature and nature of the surface. Dark, rough surfaces are better emitters and absorbers of radiation compared to light, smooth surfaces.

Temperature Gradient, Thermal Conductivity, and Heat Flux

• Temperature Gradient: Temperature gradient refers to the change in temperature per unit length or distance. It represents the rate at which temperature changes in a particular direction. The temperature gradient is calculated by dividing the temperature difference by the distance over which the temperature changes.
• Thermal Conductivity: Thermal conductivity is a property of materials that determines their ability to conduct heat. It is denoted by the symbol "k" and is measured in units of watts per meter per Kelvin (W/(m·K)). Materials with high thermal conductivity, such as metals, are good conductors of heat. On the other hand, materials with low thermal conductivity, such as air or insulating materials, are poor conductors and are often used for thermal insulation.
• Heat Flux: Heat flux, also known as thermal flux or heat transfer rate, refers to the amount of heat transferred per unit time per unit area. It is denoted by the symbol "q" and has units of watts per square meter (W/m²). Mathematically, heat flux is calculated by dividing the amount of heat transferred (Q) by the time taken (t) and the surface area (A) through which the heat is transferred.

Effect of the Nature of the Surface on the Energy Radiated and Absorbed

The nature of the surface has a significant impact on the energy radiated and absorbed. According to the Stefan-Boltzmann law, the rate at which an object radiates thermal energy is directly proportional to the fourth power of its absolute temperature (in Kelvin). Mathematically, it can be expressed as:
Radiated Energy ∝ εσT⁴
Where:

• Radiated Energy represents the energy radiated by the surface.
• ε (epsilon) is the emissivity of the surface, which determines how well the surface emits radiation (ranging from 0 to 1).
• σ (sigma) is the Stefan-Boltzmann constant (approximately 5.67 × 10^-8 W/(m²·K^4)).
• T is the absolute temperature of the surface.

Thus, surfaces with higher emissivity and higher temperatures radiate more energy. Additionally, different surfaces also have varying abilities to absorb incoming radiation. Dark, rough surfaces tend to absorb more radiation compared to light, smooth surfaces.

Conductivities of Common Materials

Here are the approximate thermal conductivities (in W/(m·K)) of some common materials:

• Copper: 401
• Aluminum: 237
• Iron: 80
• Stainless Steel: 16
• Glass: 0.8
• Water: 0.6
• Air: 0.03

These values can vary depending on factors such as temperature, impurities, and material properties. It's important to note that these values are provided for reference and may not be accurate to several decimal places.

Thermos Flask

A thermos flask, also known as a vacuum flask or Dewar flask, is a container designed to maintain the temperature of its contents. It consists of several components working together:

• Inner Vessel: The inner vessel is made of a double-walled glass or metal container. The inner surface is usually coated with a reflective material to minimize radiation and energy transfer.
• Vacuum Layer: The space between the inner and outer walls of the flask is evacuated to create a vacuum. The absence of air or other gases prevents heat transfer by conduction and convection.
• Stopper/Cap: The flask has an airtight stopper or cap, usually made of plastic or rubber, to seal the opening tightly and prevent heat transfer.
• Outer Casing: The outer casing of the thermos flask is usually made of plastic or metal, providing insulation and protection for the inner components.

The combination of the vacuum layer and insulation materials helps to reduce heat transfer by conduction, convection, and radiation, making the thermos flask an efficient container for keeping the contents hot or cold for extended periods.

Land and Sea Breeze

Land and sea breezes are local winds that occur due to temperature differences between land and water surfaces during the day and night:

• Land Breeze: During the night, the land cools down faster than the adjacent water body. The cool air over the land becomes denser and starts flowing towards the warmer sea, creating a land breeze. This breeze is typically felt during the late evening and early morning.
• Sea Breeze: During the day, the land heats up more quickly than the nearby water. The warm air over the land rises, and cooler air from the sea flows in to replace it, resulting in a sea breeze. This breeze is felt during the daytime and is often accompanied by a cool, refreshing effect.

These local wind patterns are influenced by the differential heating of land and water, causing air to move from regions of higher pressure (cooler air) to regions of lower pressure (warmer air), thus creating the breezes.

Engines

Engines are devices that convert various forms of energy into mechanical work. Two common types of engines are:

• Internal Combustion Engines: Internal combustion engines burn fuel (typically gasoline or diesel) inside a combustion chamber. The combustion of the fuel-air mixture produces high-pressure gases, which expand and push a piston. The reciprocating motion of the piston is converted into rotary motion by a crankshaft. Examples of internal combustion engines include those found in cars, motorcycles, and small generators.
• Jet Engines: Jet engines are commonly used in aircraft. They work on the principle of jet propulsion. Air is compressed and mixed with fuel in a combustion chamber, where it ignites and expands rapidly. The expanding gases are expelled through a nozzle at the rear of the engine, creating a forward thrust according to Newton's third law of motion. Jet engines can be further classified into turbojet, turbofan, turboprop, and turboshaft engines, depending on their specific designs and applications.

Both internal combustion engines and jet engines rely on the combustion of fuel to generate high-pressure gases that produce mechanical work, either directly through a piston or indirectly through the expulsion of gases.