An incandescent light bulb in a room with an air temperature of 300 K 300~\mbox{K} radiates with a peak wavelength of 1 0 6 m 10^{-6}~\mbox{m} . The bulb draws 0.5 Amperes 0.5~\mbox{Amperes} from the 120 Volt 120~\mbox{Volt} wall electrical outlet. What is the surface area of the filament in the bulb in m 2 \mbox{m}^2 ?

Assumption

The filament may be modeled as a perfect "black body".


The answer is 0.00001496.

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4 solutions

Yan Qi Huan
Nov 18, 2013

This problem involves the usage of several basic laws from the field of radiation and thermodynamics.

First, we make use of Wien's Displacement Law, assuming that the filament can be taken to be blackbody λ m a x T = β \lambda_{max}T=\beta (where β 2.898 × 1 0 3 m K \beta\approx2.898\times10^{-3}m K is Wien's constant), which relates the peak wavelength λ m a x \lambda_{max} of an object's blackbody radiation spectrum to its temperature in Kelvin T T . From this, we find that the incandescent light bulb's filament is at a temperature of T = 2.898 × 1 0 3 1 0 6 = 2898 K T=\frac{2.898\times10^{-3}}{10^{-6}}=2898K

Next, we find the power input to the light bulb from the wall outlet. The power P P from the wall is simply given by the input voltage V V times the current I I , giving us P = V I = 120 × 0.5 = 60 W P=VI=120\times0.5=60W

Lastly, we assume that all the input power from the wall outlet has been converted into the blackbody radiation that the light bulb emits, and that the filament is a perfect blackbody. The latter assumption allows us to use the Stefan Boltzmann Law, which relates the power emitted by a blackbody to its area A A and temperature T T . The law states: P = A σ T 4 P=A\sigma T^4 where σ \sigma is the Stefan Boltzmann constant with a value of 5.67 × 1 0 8 W m 2 K 4 5.67\times10^{-8}Wm^{-2}K^{-4}

Substituting in all our previous values for P P and T T , we find that the area A A of the filament is:

A = P σ T 4 = 60 ( 5.67 × 1 0 8 ) ( 2898 ) 4 = 1.50 × 1 0 5 m 2 A=\frac{P}{\sigma T^4}=\frac{60}{(5.67\times10^{-8})(2898)^4}=1.50\times10^{-5}m^2

In the last line the temperature (T^4) should be (A^4)-(B^4)

where A=2897.76 and B=300 (The room Temperature)

Although both gives the same answer I think the latter is more correct

Santanu Banerjee - 7 years, 6 months ago

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Yes, I agree. I am not sure if Yan was aware of it but considering the second term (B^4) doesn't make a difference as it is far less than the fourth power of A.

Pranav Arora - 7 years, 6 months ago

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Yes I neglected it because 2898^4 >> 300^4, sorry I forgot to include it in the solution

Yan Qi Huan - 7 years, 6 months ago

They will probably email you letting you know that there was an error and that you have been awarded the points. That was the case with me.

Aaron Aquino - 7 years, 6 months ago

ya the same problem with me

naitik bhise - 7 years, 6 months ago

It was the same for me.

bobby jim - 7 years, 6 months ago

I have posted this answer but brilliant showed i was wrong!! How to proceed?

Lucas Guimarães - 7 years, 6 months ago

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Strange. I input this answer and got it correct. If I remember correctly I typed in 1.50E-5.

Yan Qi Huan - 7 years, 6 months ago

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I received a e-mail from Brilliant's Staff and it was their mistake! I see the correct mark now : D

Lucas Guimarães - 7 years, 6 months ago
Jatin Yadav
Nov 19, 2013

By wien's law , λ p e a k T = 2.9 × 1 0 3 , T = 2900 K \lambda_{peak}T = 2.9 \times 10^{-3}, \Rightarrow T = 2900K , Also , T s u r r = 300 K T_{surr} = 300K

Heat dissipation = i V = 60 W iV = 60W

Power radiated through radiation is given by stephan's law, and it must be equal to heat dissipation , Therefore

60 = A σ ( T 4 T s u r r 4 ) 60 = A \sigma (T^4 - T_{surr}^{4})

Plugging values, we get A = 1.5 × 1 0 5 m 2 A = \boxed{1.5 \times 10^{-5} m^2}

Snehal Shekatkar
Nov 20, 2013

We can assume that filament acts as a black body.

Using Wien's displacement law , the temperature of of the filament is:

T = b λ m a x T=\frac{b}{\lambda_{max}}

Then using Stefan-Boltzmann law , we get,

j A = I V j^{\ast}A=IV

where j j^{\ast} is the power emitted per unit area of the surface of filament and A is the area of the filament. Then solving for A A , we get,

A = 1.5 × 1 0 5 m 2 \boxed{A=1.5 \times 10^{-5} m^{2}}

Let the temperature of the surroundings be \­( T_{o}\­) and the temperature of the object at maximum wavelength \­( λ\­) be \­( T\­).

The rate of power radiated is described by

\­( P) = \(A e σ ( T^{4} - T_{o}^{4}\­) (1)

assuming that the bulb is a blackbody (e = 1) and σ is the Stefan-Boltzmann constant.

The rate of power radiated is described by

\­( P\) = \(A e σ ( T^{4} - T_{o}^{4}\­) (1)

and the temperature of the object at maximum wavelength \­( λ\­) can be determined using Wien's displacement law

\­( T\­) = (\frac{2.898 mm. K}{λ}\) (2)

and the power delivered by the bulb is

\­( P\­) = \­(IV\­) (3)

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