An incandescent light bulb in a room with an air temperature of 3 0 0 K radiates with a peak wavelength of 1 0 − 6 m . The bulb draws 0 . 5 Amperes from the 1 2 0 Volt wall electrical outlet. What is the surface area of the filament in the bulb in m 2 ?
Assumption
The filament may be modeled as a perfect "black body".
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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
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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.
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Yes I neglected it because 2898^4 >> 300^4, sorry I forgot to include it in the solution
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.
ya the same problem with me
It was the same for me.
I have posted this answer but brilliant showed i was wrong!! How to proceed?
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Strange. I input this answer and got it correct. If I remember correctly I typed in 1.50E-5.
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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
By wien's law , λ p e a k T = 2 . 9 × 1 0 − 3 , ⇒ T = 2 9 0 0 K , Also , T s u r r = 3 0 0 K
Heat dissipation = i V = 6 0 W
Power radiated through radiation is given by stephan's law, and it must be equal to heat dissipation , Therefore
6 0 = A σ ( T 4 − T s u r r 4 )
Plugging values, we get A = 1 . 5 × 1 0 − 5 m 2
We can assume that filament acts as a black body.
Using Wien's displacement law , the temperature of of the filament is:
T = λ m a x b
Then using Stefan-Boltzmann law , we get,
j ∗ A = I V
where j ∗ is the power emitted per unit area of the surface of filament and A is the area of the filament. Then solving for A , we get,
A = 1 . 5 × 1 0 − 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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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 (where β ≈ 2 . 8 9 8 × 1 0 − 3 m K is Wien's constant), which relates the peak wavelength λ m a x of an object's blackbody radiation spectrum to its temperature in Kelvin T . From this, we find that the incandescent light bulb's filament is at a temperature of T = 1 0 − 6 2 . 8 9 8 × 1 0 − 3 = 2 8 9 8 K
Next, we find the power input to the light bulb from the wall outlet. The power P from the wall is simply given by the input voltage V times the current I , giving us P = V I = 1 2 0 × 0 . 5 = 6 0 W
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 and temperature T . The law states: P = A σ T 4 where σ is the Stefan Boltzmann constant with a value of 5 . 6 7 × 1 0 − 8 W m − 2 K − 4
Substituting in all our previous values for P and T , we find that the area A of the filament is:
A = σ T 4 P = ( 5 . 6 7 × 1 0 − 8 ) ( 2 8 9 8 ) 4 6 0 = 1 . 5 0 × 1 0 − 5 m 2