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Imagine a space heater consisting of an iron ball (radius 10.0 cm) through which electrical current is passed in order to heat the ball. What temperature would it need to be in order to radiate a net power of 500 W into the surrounding air (which is at a temperature of 20 oC)? Assume the ball is a perfect blackbody.

Question

Imagine a space heater consisting of an iron ball (radius 10.0 cm) through which electrical current is passed in order to heat the ball. What temperature would it need to be in order to radiate a net power of 500 W into the surrounding air (which is at a temperature of 20 oC)? Assume the ball is a perfect blackbody.

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Solution

To solve this problem, we need to use the Stefan-Boltzmann Law, which states that the power radiated by a black body in watts is equal to the Stefan-Boltzmann constant times the surface area of the object times the temperature to the fourth power. The equation is:

P = σ * A * T^4

where: P is the power radiated, σ is the Stefan-Boltzmann constant (5.67 * 10^-8 W/m^2K^4), A is the surface area of the object, and T is the temperature in Kelvin.

We are given that P = 500 W and we need to find T.

First, we need to calculate the surface area of the iron ball. The surface area of a sphere is given by the equation A = 4πr^2. The radius r is given as 10 cm, but we need to convert this to meters, so r = 0.1 m.

A = 4π(0.1 m)^2 = 0.04π m^2

Now we can substitute P, σ, and A into the Stefan-Boltzmann Law and solve for T:

500 W = (5.67 * 10^-8 W/m^2K^4) * (0.04π m^2) * T^4

Solving for T gives:

T^4 = 500 W / [(5.67 * 10^-8 W/m^2K^4) * (0.04π m^2)]

T^4 = 2.21 * 10^9 K^4

Taking the fourth root of both sides gives:

T = (2.21 * 10^9 K^4)^(1/4) = 728 K

However, this is the temperature in Kelvin. To convert to Celsius, we subtract 273.15:

T = 728 K - 273.15 = 454.85 °C

So, the iron ball would need to be approximately 455 °C in order to radiate a net power of 500 W into the surrounding air.

This problem has been solved

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