An electric flat-plate square heater of sides 10 cm provides 100 W power from each side. If the heater is assumed to be black, its temperature is approximately:

This question was previously asked in

BHEL ET Mechanical Held on May 2019

Option 2 : 648 K

BHEL Engineer Trainee Electrical 2019 Paper

1435

240 Questions
240 Marks
150 Mins

**Concept:**

**Emissive power (E)** of a radiating body is given by:

**E = σAT ^{4} (in W) **

where σ = Stefan boltzmann's constant = 5.67 × 10^{-8} W/m-K^{4}, A = Surface area in m^{2}, T = Temperature of body(in K).

**Black body:**

A black body is a body which **absorbs radiations from all the directions** at all wavelength and emits the radiations above the particular temperature.

**Calculation:**

**Given:**

a = 10 cm = 0.1 m, P = 100 W, T = ?

Now, we know that

A black body absorbs radiations from all directions, So from the energy balance of the body

Power absorb = Emissive power (E) of body

**100** = σAT4

100 = 5.67 × 10^{-8} × 0.1 × 0.1 × T^{4}

**∴ T = 648 K**