## Joule heating – wikipedia gas efficient cars 2015

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A voltage difference between two points of a conductor creates an electric field that accelerates charge carriers in the direction of the electric field, giving them kinetic energy. When the charged particles collide with ions in the conductor, the particles are scattered; their direction of motion becomes random rather than aligned with the electric field, which constitutes thermal motion. Thus, energy from the electrical field is converted into thermal energy.  Power loss and noise [ edit ]

Joule heating is referred to as ohmic heating or resistive heating because of its relationship to Ohm’s Law. It forms the basis for the large number of practical applications involving electric heating. However, in applications where heating is an unwanted by-product of current use (e.g., load losses in electrical transformers) the diversion of energy is often referred to as resistive loss. The use of high voltages in electric power transmission systems is specifically designed to reduce such losses in cabling by operating with commensurately lower currents. The ring circuits, or ring mains, used in UK homes are another example, where power is delivered to outlets at lower currents, thus reducing Joule heating in the wires. Joule heating does not occur in superconducting materials, as these materials have zero electrical resistance in the superconducting state.

where t is time and P is the instantaneous power being converted from electrical energy to heat. Far more often, the average power is of more interest than the instantaneous power: P a v g = U rms I rms = I rms 2 R = U rms 2 / R {\displaystyle P_{avg}=U_{\text{rms}}I_{\text{rms}}=I_{\text{rms}}^{2}R=U_{\text{rms}}^{2}/R}

These formulas are valid for an ideal resistor, with zero reactance. If the reactance is nonzero, the formulas are modified: P a v g = U rms I rms cos ⁡ ϕ = I rms 2 Re ⁡ ( Z ) = U rms 2 Re ⁡ ( Y ∗ ) {\displaystyle P_{avg}=U_{\text{rms}}I_{\text{rms}}\cos \phi =I_{\text{rms}}^{2}\operatorname {Re} (Z)=U_{\text{rms}}^{2}\operatorname {Re} (Y^{*})}

In plasma physics, the Joule heating often needs to be calculated at a particular location in space. The differential form of the Joule heating equation gives the power per unit volume. d P / d V = J ⋅ E {\displaystyle \mathrm {d} P/\mathrm {d} V=\mathbf {J} \cdot \mathbf {E} }

Here, J {\displaystyle \mathbf {J} } is the current density, and E {\displaystyle \mathbf {E} } is the electric field. For a neutral plasma not in magnetic field and with a conductivity σ {\displaystyle \sigma } , J = σ E {\displaystyle \mathbf {J} =\sigma \mathbf {E} } and therefore d P / d V = J ⋅ E = J ⋅ J / σ = J 2 ρ {\displaystyle \mathrm {d} P/\mathrm {d} V=\mathbf {J} \cdot \mathbf {E} =\mathbf {J} \cdot \mathbf {J} /\sigma =J^{2}\rho }

• Some food processing equipment may make use of Joule heating: running current through food material (which behave as an electrical resistor) causes heat release inside the food.  The alternating electrical current coupled with the resistance of the food causes the generation of heat.  A higher resistance increases the heat generated. Ohmic heating allows for fast and uniform heating of food products, which keeps the high quality in foods. Products with particulates heat up faster in Ohmic heating (as compared to conventional heat processing) due to higher resistance. 

Joule heating (Ohmic Heating) is a flash pasteurization (also called "high-temperature short-time" (HTST)) aseptic process that runs an alternating current of 50–60 Hz through food.  Heat is generated through the electrical resistance of the food.  As the product heats up, electrical conductivity increases linearly.  A higher electrical current frequency is best as it reduces oxidation and metallic contamination.  This heating method is best for foods that contain particulates suspended in a weak salt containing medium due to their high resistance properties.  Ohmic heating allows for a maintained quality of foods due to the uniform heating that decreases deterioration and over-processing of food.  Benefits [ edit ]

Ohmic heating has similar benefits than other rapid heating methods. This method can destroy microorganisms achieving sterility through electroporation of cell membranes, membrane rupture from the voltage drop across cell membranes, and cell lysis.  Ideal Food Products [ edit ]

In particulate foods, the particles heat up faster than the liquid matrix due to the higher resistance to electricity.  This prevents overheating of the liquid matrix while the particles receive sufficient heat processing.  Below are some examples of different electrical conductivity values of certain foods to display composition and salt concentration affect electrical conductivity. Electrical conductivity of selected foods  Food

Ohmic heating is highly influenced by the electrical conductivity of the product.  Dependent on the position of the food relative to the electrodes,there can be areas of overprocessing and underprocessing.  Fats, oils, alcohols, bone, and crystalline structures cannot be heated directly by ohmic heating due to their low electrical conductivity values.  Similarly, it is difficult to obtain uniform heating in non-homogenous food particulates making it difficult to assure sterility.  Heating efficiency [ edit ]