Heat transfer

Introduction to heat transfer

Heat is a form of energy that is transferred between objects due to a difference in temperature. In the study of physics and thermodynamics, understanding how heat is transferred is important for explaining many everyday phenomena, from how we cook food to how we cool our homes.

Throughout this explanation, we will take a deeper look at the mechanisms of heat transfer including conduction, convection, and radiation. These mechanisms govern the movement of thermal energy through different materials and spaces.

Conductivity

Conduction is the process by which heat energy is transmitted through collisions between neighbouring molecules or atoms. Conduction occurs mainly in solids where the particles are very closely spaced apart.

To visualize this, imagine a metal rod with one end placed in a flame. The heat from the flame causes the molecules at the hot end to vibrate faster. This increased speed results in energy being transferred to nearby molecules, causing heat to spread along the rod:

    Example of heat transfer in conduction:
    , Flame 🌶 => metal molecule: 
    (hot) --> (vibrates) --> (heat goes away)
    ,
    

Mathematically, heat transfer via conduction can be described by Fourier's law:

    q = -k * a * (dt/dx)
    

Where:

• Q is the heat transfer per unit time.
• k is the thermal conductivity of the material.
• A is the area of the cross section.
• dT/dx is the temperature gradient in the direction of heat flow.

Example: Consider a frying pan placed on a stove. The part of the pan that is directly above the heat source heats up first. The handle, being a part of the same solid object, heats up eventually through conduction.

Convection

Convection is the transfer of heat by the physical motion of a fluid (liquid or gas). This process is primarily responsible for heat transfer in fluids, where bulk motion moves energy from one place to another.

A common observation of convection can be seen when heating water in a pot. The water at the bottom, which is closest to the heat source, heats up, becomes less dense, and rises. Cooler, denser water then sinks down to take its place:

    Example of heat transfer in convection:
    ,
    Hot water ↑ 
    Cold water ↓ 
    (heat source below)
    ,
    

The manifestation of heat transfer by convection can be described by Newton's law of cooling:

    Q = h * A * (T_surface - T_fluid)
    

Where:

• Q is the heat transfer per unit time.
• h is the heat transfer coefficient.
• A is the surface area through which the heat is being transferred.
• T_surface is the temperature of the surface.
• T_fluid is the temperature of the fluid away from the surface.

Example: Think of how the fan inside an air-conditioning unit works. It circulates cool air throughout the room, constantly replacing warm air with cool air, and thus cooling the room through convection.

Radiation

Radiation is the transfer of heat via electromagnetic waves. Unlike conduction and convection, radiation requires no medium; heat can be transferred through the vacuum of space.

A classic example of radiation is the heat we feel from the Sun. Even though the Sun is millions of kilometres away from Earth, its thermal energy travels through the vacuum of space to reach us as infrared radiation.

    Example of heat transfer in radiation:
    ,
    Sun 🌞 ---> Earth 🌍
    (vacuum of space)
    ,
    

The amount of heat radiated can be calculated using the Stefan-Boltzmann law:

    q = ε * σ * a * t^4
    

Where:

• Q is the total energy radiated per unit time.
• ε is the emissivity of the material.
• σ is the Stefan-Boltzmann constant (5.67 × 10^-8 W/m^2K^4).
• A is the surface area of the object.
• T is the absolute temperature of the object in Kelvin.

Example: The heat you feel from a campfire when you're sitting nearby is mostly radiated by radiation. Even if the air between you and the fire isn't hot, you can still feel warm because of this type of heat transfer.

Assembly of heat transfer system

In real-life situations, heat transfer often occurs through a combination of conduction, convection, and radiation. For example, when water is boiled on the stove, conduction transfers heat from the burner to the pot, convection circulates the water as it is heated, and radiation may move heat from the surface of the pot to the surrounding environment.

Understanding these different modes of heat transfer helps design more efficient systems for heating and cooling, from household appliances to industrial machinery and spacecraft.

Conclusion

The study of how heat is transferred between objects helps us understand the principles behind many natural and man-made processes. By mastering the concepts of conduction, convection, and radiation, we can improve energy efficiency, understand weather patterns, and develop new technologies. Whether thinking about insulation in a home, cooling systems in a computer, or the body's own methods of temperature regulation, the basic principles of heat transfer are indispensable.

The continued exploration of these principles in thermodynamics not only improves our understanding of physics, but also enriches technology and innovation, leading to practical and environmental improvements in our daily lives.

Comments