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Thermal Properties of Matter · Question 12

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1

A wire of length 10 cm and diameter 0.5 mm is used in a bulb. The temperature of the wire is 1727 and power radiated by the wire is 94.2 W. Its emissivity is x8 where x = . (Given = 6.0 10-8 W m-2 K-4, = 3.14 and assume that the emissivity of wire material is same at all wavelengths.)

Answer

5

Solution

Stefan–Boltzmann law: P = e A T4

T = 1727 + 273 = 2000 K

Radiating (curved) surface area of the wire:

A = d L = 3.14 0.5 10-3 0.1 = 1.57 10-4 m2

T4 = 6.0 10-8 (2000)4 = 6.0 10-8 1.6 1013 = 9.6 105 W m-2

⇒ 94.2 = e 1.57 10-4 9.6 105

⇒ 94.2 = e 150.72

⇒ e = 0.625 = 58

∴ x = 5

Answer: 5

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2

Two cylindrical rods A and B made of different materials, are joined in a straight line. The ratios of lengths, radii and thermal conductivities of these rods are: LALB = 12, rArB = 2 and KAKB = 12. The free ends of rods A and B are maintained at 400 K, 200 K, respectively. The temperature of rods interface is K, when equilibrium is established.

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3

Consider a rectangular sheet of solid material of length l = 9 cm and width d = 4 cm. The coefficient of linear expansion is = 3.1 10-5 K-1 at room temperature and one atmospheric pressure. The mass of sheet m = 0.1 kg and the specific heat capacity Cv = 900 J kg-1 K-1. If the amount of heat supplied to the material is 8.1 102 J then change in area of the rectangular sheet is :

  1. 2.0 10-6 m2
  2. 6.0 10-7 m2
  3. 3.0 10-7 m2
  4. 4.0 10-7 m2
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4

Pressure of an ideal gas, contained in a closed vessel, is increased by 0.4\% when heated by 1 . Its initial temperature must be:

  1. 2500 K
  2. 25
  3. 250
  4. 250 K
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5

Match List - I with List - II. Choose the correct answer from the options given below :

  1. (A)-(IV), (B)-(III), (C)-(II), (D)-(I)
  2. (A)-(III), (B)-(I), (C)-(II), (D)-(IV)
  3. (A)-(III), (B)-(IV), (C)-(I), (D)-(II)
  4. (A)-(II), (B)-(III), (C)-(I), (D)-(IV)
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