7 An electric heater is made of thin conducting wires forming a mesh. The diameter of the wire is 0.5 mm and the total length of all the wires is 2 m. The temperature of the wire is 1,000 K. The heat from the wire is dissipated to the air at 300 K flowing through the wire mesh at a velocity of 5 m/s. Ignore radiation of the heater and assume it has a uniform temperature. T Const = (a) Calculate the convection heat transfer coefficient(h) between the air and the heater. (b) If the total electrical resistance of the heater is 10 2, what is the electrical current? (c) Calculate the Biot number of the heater. V=IR (d) The electrical current is suddenly cut off. How long does it take for the heater temperature to drop to 500 K? Properties of the air: pa = 0.7 kg/m³, Cp,a= 1030 J/kg K, va = 38.8×106 m²/s, ka=0.041 W/m•K, aa =56.7×106 m²/s, and Pra = 0.684. Properties of the heater: ph = 8900 kg/m³, Cp,h= 380 J/kg K, and kn = 400 W/m.K.
7 An electric heater is made of thin conducting wires forming a mesh. The diameter of the wire is 0.5 mm and the total length of all the wires is 2 m. The temperature of the wire is 1,000 K. The heat from the wire is dissipated to the air at 300 K flowing through the wire mesh at a velocity of 5 m/s. Ignore radiation of the heater and assume it has a uniform temperature. T Const = (a) Calculate the convection heat transfer coefficient(h) between the air and the heater. (b) If the total electrical resistance of the heater is 10 2, what is the electrical current? (c) Calculate the Biot number of the heater. V=IR (d) The electrical current is suddenly cut off. How long does it take for the heater temperature to drop to 500 K? Properties of the air: pa = 0.7 kg/m³, Cp,a= 1030 J/kg K, va = 38.8×106 m²/s, ka=0.041 W/m•K, aa =56.7×106 m²/s, and Pra = 0.684. Properties of the heater: ph = 8900 kg/m³, Cp,h= 380 J/kg K, and kn = 400 W/m.K.
Elements Of Electromagnetics
7th Edition
ISBN:9780190698614
Author:Sadiku, Matthew N. O.
Publisher:Sadiku, Matthew N. O.
ChapterMA: Math Assessment
Section: Chapter Questions
Problem 1.1MA
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Transcribed Image Text:7
An electric heater is made of thin conducting wires forming a mesh. The diameter of the wire is
0.5 mm and the total length of all the wires is 2 m. The temperature of the wire is 1,000 K. The
heat from the wire is dissipated to the air at 300 K flowing through the wire mesh at a velocity of
5 m/s. Ignore radiation of the heater and assume it has a uniform temperature. T Const
=
(a) Calculate the convection heat transfer coefficient(h) between the air and the heater.
(b) If the total electrical resistance of the heater is 10 2, what is the electrical current?
(c) Calculate the Biot number of the heater. V=IR
(d) The electrical current is suddenly cut off. How long does it take for the heater
temperature to drop to 500 K?
Properties of the air: pa = 0.7 kg/m³, Cp,a= 1030 J/kg K, va = 38.8×106 m²/s, ka=0.041 W/m•K,
aa =56.7×106 m²/s, and Pra = 0.684.
Properties of the heater: ph = 8900 kg/m³, Cp,h= 380 J/kg K, and kn = 400 W/m.K.
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