IMK114: Introduction to Food Physics Chapter 4: Thermal Properties PDF

IMK114: Introduction to Food Physics IMK 114 (Introduction to Food Physics) Chapter 4: Thermal Properties 1 Chapter Outline...

IMK114: Introduction to Food Physics IMK 114 (Introduction to Food Physics) Chapter 4: Thermal Properties 1 Chapter Outline Temperature Heat Capacity Enthalpy Heat transfer in food Thermal Conductivity Thermal diffusivity Caloric value of food Refer to the Thermal analysis handouts on e-learn 2 2 IMK114: Introduction to Food Physics The thermodynamic laws 1st Law: The first law of thermodynamics states that energy can neither be created nor destroyed, only altered in form. For any system, energy transfer is associated with mass crossing the control boundary, external work, or heat transfer across the boundary. These produce a change of stored energy within the control volume. 3 The thermodynamic laws 2nd Law: The Second Law of Thermodynamics states that: “in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state.” 4 IMK114: Introduction to Food Physics The thermodynamic laws 2nd Law: The measure of this disorder is referred to as entropy. In the process of energy transfer, some energy will dissipate as heat. Simply, entropy is the amount of energy which is unavailable to do work. 5 Heat The internal energy U of a thermodynamic system exists in the forms of both heat and work. Therefore, two transformations are possible for internal energy. Transfer of heat Q and/or Eq. 4.11 transfer of work W’. So, we can express internal energy in the following way: 6 6 IMK114: Introduction to Food Physics Heat In a thermodynamic system, we treat work only in the form of displacement. Work, W (force–displacement, or pressure– volume). Eq. 4.12 We assume that other forms of work like electric, magnetic, elastic and frictional are not involved. 7 Heat With the definition of work displacement: Eq. 4.13 we have Eq. 4.14 or Eq. 4.15 8 IMK114: Introduction to Food Physics Enthalpy The term enthalpy H now is used for the sum of internal energy and the product pV Eq. 4.16 For a change in enthalpy, we have: Eq. 4.17 If we consider only cases where the pressure is constant (dp =0), then we have: Eq. 4.18 9 Enthalpy Enthalpy is the measurement of energy in a thermodynamic system. The quantity of enthalpy equals to the total content of heat of a system, equivalent to the system’s internal energy plus the product of volume and pressure 10 IMK114: Introduction to Food Physics Heat and Enthalpy The term enthalpy H now is used for the sum of internal energy and the product pV Eq. 4.19 This means that the amount of heat dQ which occurs during an isobaric (constant pressure) process is the same as the change in enthalpy of the system. So, when we investigate material properties under constant pressure (e.g. 1 atm) ,we talk about enthalpy instead of energy of a system. 11 Heat and enthalpy Definition of Heat: Heat is the transfer of energy across a system's boundary due to a temperature difference between the system and its surroundings. Energy Conservation: Governed by the First Law of Thermodynamics, energy is conserved and can be transformed between forms. Theoretical Efficiency: In theory, energy transformation could be 100% efficient. Practical Efficiency: Due to the Second and Third Laws of Thermodynamics, real-world efficiencies are always below 100%. Implication: Heat cannot be entirely converted into other forms of energy with perfect efficiency. This unique characteristic highlights the special nature of heat as a form of energy. 12 12 IMK114: Introduction to Food Physics Heat and enthalpy This limitation is a result of the Second Law of Thermodynamics, which introduces the concept of irreversibility and entropy. Here’s what makes heat unique: 1. Irreversibility:When heat flows from a hot object to a cold one, some of the energy is "lost" in terms of its ability to do useful work. This loss is not because energy disappears (it’s conserved), but because the energy becomes less organized or less useful. 2. Entropy:Heat transfer increases the disorder (entropy) of a system, and this increase makes it impossible to convert all heat into other energy forms like mechanical work. 13 13 Heat and enthalpy 3. Dependence on Temperature Difference: Heat transfer only occurs due to a temperature difference. It cannot occur spontaneously in the absence of such a gradient, which limits its usability as a form of energy. In summary, the "special nature" of heat is its inherent inefficiency in being transformed entirely into other forms of energy, a principle that distinguishes it from other energy forms like kinetic or potential energy. This unique behavior is crucial in understanding thermodynamic systems and energy processes. 14 14 IMK114: Introduction to Food Physics Heat Transfer in Food Conduction Convection Radiation 15 15 Conduction Convection Direct heat transfer Requires medium to transfer Follow Fourier’s law heat (i.e. gas, air, liquid) Follow Newton’s law of T cooling q = −kA x q = hAT No movement Natural and forced convection 16 16 IMK114: Introduction to Food Physics 17 17 18 18 IMK114: Introduction to Food Physics Radiation Combination Transfer/emit energy/ heat as waves Follow Stefan-Boltzman law q = ATA 4 19 19 Radiation heat transfer thermal radiation is in the infrared band of frequencies, some of the thermal radiation takes the form of microwaves, visible light, and ultraviolet 20 20 IMK114: Introduction to Food Physics Q= heat transfer (J) q= heat transfer rate (J/s) 21 21