Understanding Heat Transfer: Cooking With Thermal Energy

Thermal energy is the energy contained within a system that is responsible for its temperature. Heat is the flow of thermal energy. When cooking, heat is transferred from the burner to the pot, and then from the pot to the food. This process involves three types of heat transfer: conduction, convection, and radiation. Conduction is the transfer of heat through direct contact, convection is the transfer of heat through the movement of molecules, and radiation is the transfer of heat through electromagnetic waves. In the context of cooking with a pan, conduction occurs when heat is transferred from the burner to the bottom of the pan, and convection occurs when the molecules of water in the pan move away, allowing colder molecules to settle and be heated.

Conduction

When a pot or pan is placed on a stove, conduction occurs as heat flows through the pan, warming it up. This is because the thermal energy of the hot stove, which is at a higher temperature, flows to the cooler pan at a lower temperature. The efficiency of conduction depends on the material of the pan. Metals, particularly those containing ferromagnetic materials such as iron, conduct heat better than non-metals. This is because electrons in metal atoms can break free and move more quickly compared to the atoms in non-metallic solids.

In non-metallic solids, heat energy is transferred through vibrational effects, where heat passes from one atom to another. This process is generally slower than in metals because the atoms are fixed.

To determine if a pan is compatible with an induction cooktop, a magnet test can be performed. If a magnet clings to the bottom of the pan, it will work on an induction cooktop. This is because induction cooktops create a magnetic field between the pot and the magnetic coils beneath the cooking surface, and the energy created in this electromagnetic field heats the contents of the pot.

Convection

In the context of cooking, convection ovens utilise a fan to circulate hot air and maintain a steady temperature. This forced convection results in faster and more even cooking compared to conventional ovens. The circulating hot air transfers heat more efficiently to the food, leading to reduced cooking times. However, the use of a convection fan may not be suitable for certain types of food, such as cookies and bread, as it dries out the oven more quickly.

Radiation

Thermal radiation is electromagnetic radiation emitted by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. The emission of energy arises from a combination of electronic, molecular, and lattice oscillations in a material. When matter absorbs thermal radiation, its temperature will tend to rise.

Thermal radiation reflects the conversion of thermal energy into electromagnetic energy. Thermal energy is the kinetic energy of random movements of atoms and molecules in matter. It is present in all matter of non-zero temperature. These atoms and molecules are composed of charged particles, i.e. protons and electrons. The kinetic interactions among matter particles result in charge acceleration and dipole oscillation. This results in the electrodynamic generation of coupled electric and magnetic fields, resulting in the emission of photons, radiating energy away from the body.

The characteristics of thermal radiation depend on various properties of the surface from which it is emanating, including its temperature and its spectral emissivity. The radiation is not monochromatic, i.e. it does not consist of only a single frequency, but comprises a continuous spectrum of photon energies, its characteristic spectrum.

The rate of heat transfer by radiation is largely determined by the colour of the object. Black is the most effective, and white is the least effective. The rate of heat transfer by emitted radiation is determined by the Stefan-Boltzmann law of radiation:

[latex]\displaystyle\frac{Q}{t}=\sigma{e}AT^{4}\\[/latex],

Where σ = 5.67 × 10−8 J/s · m2 · K4 is the Stefan-Boltzmann constant, A is the surface area of the object, and T is its absolute temperature in kelvin. The symbol e stands for the emissivity of the object, which is a measure of how well it radiates. An ideal jet-black (or black body) radiator has e = 1, whereas a perfect reflector has e = 0. Real objects fall between these two values.

Thermal equilibrium

When cooking, we are attempting to bring the food and the heat source into thermal equilibrium. Thermal equilibrium is achieved when two objects or systems reach the same temperature and cease to exchange energy through heat.

In the context of cooking, thermal equilibrium is reached when the temperature of the pan matches that of the burner or stove. However, true thermal equilibrium is rarely achieved in cooking because the burner is continuously adding energy and the burner will always be hotter than the water or food being heated.

Convection involves heat moving through a fluid or gas. When a pan of cold water is placed on a stove, thermal energy is transferred to the water through convection. As the water warms, it becomes less dense and rises, while the denser and cooler water sinks and then warms.

Conduction is the internal heat transfer within an object, whether it is a solid, liquid, or gas. In the context of cooking, conduction occurs as heat flows through the pan to its handle, warming it up. Conduction also occurs when heat flows from the burner to the pot, and then from the pot to its contents. Metals are better conductors of heat than non-metals due to the greater freedom of movement of electrons in their atoms.

Radiation is the transfer of thermal energy through electromagnetic waves, such as light waves. The sun is an example of thermal energy transfer through radiation. The sun's heat cannot reach Earth via convection or conduction because there is no way for molecules to collide, as no surfaces touch.

Understanding thermal equilibrium and the different modes of thermal energy transfer can help us become better cooks and improve our understanding of the world around us.

Heat transfer

There are three methods of heat transfer: conduction, convection, and radiation.

Conduction is the transfer of heat between objects in direct contact. For example, when cooking with a stove, the burners conduct heat to the bottom of a pan, which then conducts heat to its contents. Conduction is the slowest method of heat transfer but allows for even cooking.

Convection combines conduction heat transfer with circulation to force molecules in the air to move from warmer areas to cooler ones. For example, when a pot of water is placed on a stove, the pot heats up through conduction and then heats the water molecules inside. As these molecules heat up, convection causes them to move away from the interior of the pot as they are replaced by cooler molecules. This creates a continuous current that distributes heat evenly throughout the substance.

Radiation is the transfer of thermal energy through electromagnetic waves. It does not require a medium to travel through. An example of radiation cooking is using a microwave, where short, high-frequency waves penetrate food and agitate its water molecules to create friction and transfer heat.

Understanding heat transfer is crucial in cooking as it helps chefs choose the best techniques for their ingredients and create delicious meals. It also allows them to make better use of thermal energy, which is a part of our daily lives, from our morning coffee to the methods we use to power our appliances.

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