Joule Heating

6 Key excerpts on "Joule Heating"

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  eBook - ePub

  Thermal Energy

  Sources, Recovery, and Applications

  • Yatish T. Shah(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  7 Electrical Heating—Part 1 Joule Heating 7.1      INTRODUCTION

  Electrical heating is any process in which electrical energy is converted into heat. Common applications include space heating, cooking, water heating, numerous industrial processes, waste treatment, and so on. Different types of electrical heating technologies that are currently practiced in homes and industries are as follows:

  1.  Joule Heating which include (a) indirect resistance heating, (b) direct resistance or conduction heating, and (c) encased resistance heating 2.  Infrared and ultraviolet radiation heating 3.  Induction heating 4.  Dielectric (microwave) heating 5.  Electric arc heating 6.  Plasma heating 7.  Power beam heating which include (a) electron beam heating and (b) laser heating

  These seven different types of electrical heating technologies are covered in the current and next six chapters. The current chapter discusses Joule Heating and its applications.

  7.2      Joule Heating

  Joule Heating, also known as ohmic heating and resistance heating, is the process by which the passage of an electric current through a conductor releases heat. The amount of heat released is proportional to the square of the current such that

  Q   ∝   I 2   ×   R

  (7.1)

  where:

  Q is heat produced

  I is the current

  R is the resistance

  This relationship is known as Joule’s first law or Joule–Lenz law [1 ]. Joule Heating affects the whole electric conductor, unlike the Peltier effect that transfers heat from one electrical junction to another.

  Resistance heating was first studied by Joule in 1841 and independently by Lenz in 1842. Joule immersed a length of wire in a fixed mass of water and measured the temperature rise due to a known current flowing through the wire for a 30 min period. By varying the current and the length of the wire, he deduced that the heat produced was proportional to the square of the current multiplied by the electrical resistance of the immersed wire. The SI unit of energy was subsequently named the joule and given the symbol J. The commonly known unit of power, the watt, is equivalent to one joule per second [4

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  eBook - ePub

  An Introduction to Electrical Science

  • Adrian Waygood(Author)
  • 2018(Publication Date)
  • Routledge

    (Publisher)

  from the water. That is:

  increase in internal energy = work − □ heat

  Note, we cannot measure the absolute amount of internal energy in the water, only its change – in this case, the amount of increase . We will learn more about this relationship later in this chapter.

  POOL

  Joule Heating

  ‘Joule Heating ’, which is also known as ‘resistance heating ’ or ‘ohmic heating ’, describes the process which results in an increase in a conductor’s temperature due to an electric current.Sometimes, as in the example of the electric kettle, this is desirable. But, in the case of an electric wire or conductor, this represents an energy loss through heat transfer into the surroundings which, of course, is undesirable but, to a large extent, unavoidable.

  Joule Heating is the result of collisions between a conductor’s free electrons and its fixed atoms/ions. The sudden loss of the electrons’ kinetic energy causes a corresponding increase in the conductor’s internal energy and, because the two are linked, a corresponding increase in its temperature .

  The kinetic energy of any object is proportional to the square of its velocity which, in the case of free electrons, corresponds to the square of their drift velocity . As we learnt in a previous chapter, the drift velocity increases significantly in wires having a smaller cross-section — which explains why, for example, the temperature of a fuse wire, whose cross-sectional area is far lower than the circuit conductor which it protects, increases so much faster than that conductor and reaches its melting point sooner.

  Power

  Suppose two vehicles, say, an SUV and a compact car, are each supplied with, say, exactly one litre of fuel. Both vehicles would do work as they convert the potential (chemical) energy supplied by that litre of fuel into kinetic energy, as they move along a road. However, the SUV would use up its supply of energy (i.e. its fuel) much faster than the compact car and, therefore, not travel as far before running out of fuel. This is because the SUV has a much more powerful engine than the car, and expends its source of energy at a much higher rate

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  Handbook of Food Processing

  Food Preservation

  • Theodoros Varzakas, Constantina Tzia, Theodoros Varzakas, Constantina Tzia(Authors)
  • 2015(Publication Date)
  • CRC Press

    (Publisher)

  Other synonyms used in the literature to describe this principle of heating are direct resistance heating, Joule effect heating, electroconductive heating, and electro-resistive heating. Ohmic heating is comparable to microwave heating without the intermediary step of converting electricity into microwaves through the magnetron before heating the product (Ruan et al., 2001). However, as a heating technology, Ohmic heating has a very high coefficient of per-formance, close to one, meaning that every 1 W of electrical power is converted to almost 1 W of heat. Ohmic heating technology offers an alternate way to rapidly heat the food considered as the resistance. In this method, when an alternating current is passed through a food that has appropriate EC, heat generation takes place through the foodstuff, and the electrical energy is directly converted into heat, causing a temperature rise. Classical heat transfer mechanisms such as convection or conduction are minimal. In food application, the system is similar to an electrical circuit as shown in Figure 10.1, which is comprised of a resistance and a source of current with appropriate voltage gradient. The food product which is placed between the two electrodes acts as the resistance when an alternating cur-rent passes through it. However, the most important factor is the EC of the product which is a temperature-dependent parameter. An analogue of electrical circuit V Electrode Electrode Food materials R FIGURE 10.1 Principle of Ohmic heating process for food materials. 393 Ohmic Heating The rate of heating is directly proportional to the square of the electric field strength and the EC or the resistance of material based on Joule’s first law: P = V 2 / R ; or P = I 2 R considering Ohm’s law: V = IR ; where P is the energy per unit time in W or J/s; V is the potential difference in V; R is the electrical resistance measured in Ω ; and I is the electrical current in A.

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  Science In The Making

  Scientific Development As Chronicled Historic Papers In The Philosophical Magazine, with commentaries and illustrations

  • E. A. Davis(Author)
  • 1995(Publication Date)
  • CRC Press

    (Publisher)

  Joule expands this statement by referring to: the great electro-chemical discovery of Faraday, by which we know that each atomic element is associated with the same absolute quantity of electricity. Let us suppose that these atmospheres of electricity, endowed to a certain extent with the ordinary properties of matter, revolve with great velocity around their respective atoms and that the velocity of rotation determines what we call temperature. In referring to the consequences of his experiments for the efficiency of steam engines, Joule makes a prophetic statement which is essentially that of the principle of conservation of energy: ‘Believing that the power to destroy belongs to the Creator alone, I entirely coincide with Roget and Faraday in the opinion that any theory which, when carried out, demands the annihilation of force, is necessarily erroneous.’ 1845 27 On the Existence of an Equivalent Relation between Heat and the ordinary Forms of Mechanical Power. By James P.Joule, Esq. Here Joule refers to his famous paddle-wheel experiment in which falling weights rotated a paddle in water, thereby raising its temperature. The mechanical equivalent of heat determined by these experiments was 890 lb. (referred, as in his earlier papers, to the weight which when raised by 1 foot raises the temperature of 1 lb. of water by 1°F). The average of this value and those determined by other experiments is 817 lb. or, as Joule so picturesquely puts it: James prescott joule 277 Any of your readers who are so fortunate as to reside amid the romantic scenery of Wales or Scotland, could, I doubt not, confirm my experiments by trying the temperature of the water at the top and at the bottom of a cascade. If my views be correct, a fall of 817 feet will of course generate one degree of heat; and the temperature of the river Niagara will be raised about one-fifth of a degree by its fall of 160 feet. 1847 31 On the Theoretical Velocity of Sound. By J.P.Joule.

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  Thermal Design

  Heat Sinks, Thermoelectrics, Heat Pipes, Compact Heat Exchangers, and Solar Cells

  • HoSung Lee(Author)
  • 2022(Publication Date)
  • Wiley

    (Publisher)

  There is another form of heat, called Joule Heating (I 2 R), which is irreversible and is always generated as current flows in a wire. The Thomson heat is reversible between heat and electricity. 5.1 Introduction 339 5.1.5 Thomson (or Kelvin) Relationships The interrelationships between the three thermoelectric effects are important in order to understand the basic phenomena. In 1854, Thomson [2] studied the relationships thermodynamically and provided two relationships as shown in Eqs. (5.4) and (5.5) by applying the first and second laws of thermodynamics with the assumption that the reversible and irreversible processes in thermoelectricity are separable. The necessity for the assumption remained an objection to the theory until the advent of the new thermodynamics. The Thomson effect is relatively small compared to the Peltier effect, but it plays an important role in deducing the Thomson relationships. These relationships were later completely confirmed by experiments. 𝜋 AB = 𝛼 AB T (5.4) 𝜏 AB = T d𝛼 AB dT (5.5) Equation (5.4) leads to the very useful Peltier cooling as ̇ Q Peltier = 𝛼 AB TI (5.6) where T is the temperature at a junction between two different materials and the dot above the heat Q indicates the amount of heat transported per unit time. 5.1.6 The Figure of Merit The performance of thermoelectric devices is measured by the figure of merit (Z), with units 1/K: Z = 𝛼 2 𝜌k = 𝛼 2 𝜎 k (5.7) where 𝛼 = Seebeck coefficient, μV/K; 𝜌 = electrical resistivity, Ωcm σ = 1/ρ = electrical conductivity, (Ωcm) −1 k = thermal conductivity, W/mK The dimensionless figure of merit is defined by ZT , where T is the absolute temperature. There is no fundamental limit on ZT , but for decades it was limited to values around ZT ≈1 in existing devices. The greater the value of ZT is, the greater the energy conversion efficiency of the material. The quantity of 𝛼 2 𝜎 is defined as the power factor.

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  Energy Conversion for Space Power

  • Nathan Snyder(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  (Ref. 18) Also it is shown that the only effect of Joule Heating, both of the sample and the lead wires, is in first approximation to raise the average temperature of the specimen. (Refs. 18, 19) If one carries out measurements in transverse magnetic fields, then the number of phenomena which are influenced by isothermal or adiabatic conditions is vastly increased. * Although many relations connecting these phenomena are given in the litera-ture, their application is complicated by the fact that inter-pretation in terms of fundamental quantities is generally de-pendent on a knowledge of the conduction processes in the conductor. Contributions From the Electron Theory of Solids Our treatment thus far has been of general validity and has not been restricted to particular models representative of the microscopic processes involved in the transport of charge or of heat energy. Thus the measurable quantities, the phenomenological relations, the Onsager equations, the continuity conditions — all of these are relatively independent of the particular characteristics in the solid, which influ-ence the electrical or thermal transport. To go further, we need to examine the dynamic processes in the solid. As you know, we can obtain expressions for each of the kinetic coef-ficients in terms of microscopic parameters. In doing this, however, our results become more specialized. We need *See for example, references 1, 4, 5, 15, 20, and 21. 10 ENERGY CONVERSION FOR SPACE POWER particular information on the density of states of the charge c a r r i e r s , their distribution in energy, and the relationship between their velocity and energy; their interaction with the thermal vibrations of the atoms in the crystal or with im-purity atoms or other defects. Finally, it is necessary to know something about the lattice dynamics and the modes of propagation of heat energy, which provide additional con-tributions to the thermal conductivity.

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