Question

A physical quantity of the dimensions of length that can be
formed out of c, G and e²/4πℰ₀ is [c is velocity of light, G is
universal constant of gravitation and e is charge] [2017]
(a)c²[G e²/4πℰ₀]¹/²
(b)1/c²[e²/G4πℰ₀]¹/²
(c)1/c G e²/4πℰ₀
(d)1/c²[G e²/4πℰ₀]¹/²

Answer 1

Answer:

As the temperature of the black body increases, the wavelength at which the spectral intensity (E

λ

) is maximum decreases.

Reason

The wavelength at which the spectral intensity will be maximum for a black body is proportional to the fourth power of its absolute temperature.

A .

Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

B .

Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

C .

Assertion is correct but Reason is incorrect

D .

Both Assertion and Reason are incorrect

December 20, 2019

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Viola Joon

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ANSWER

By Wein's Displacement Law

λT=constant of maximum intensity

So as temperature increases wavelength decreases

λ α

T

1

an not

T

4

1

Answer 2

Question:

A physical quantity of the dimensions of length that can be formed out of c, G and $\sf \dfrac{ {e}^{2} }{4\pi \varepsilon_{0}}$ is.

[c is velocity of light, G is the universal constant of gravitation and e is charge]

$\sf a) \: c^2\Bigg[G \dfrac{ {e}^{2} }{4\pi \varepsilon_{0}}\Bigg]^{1/2}$

$\sf b) \: \dfrac{1}{c^2}\Bigg[ \dfrac{ {e}^{2} }{G4\pi\varepsilon_{0}}\Bigg]^{1/2}$

$\sf c) \: \dfrac{1}{c}G \dfrac{ {e}^{2} }{4\pi\varepsilon_{0}}$

$\sf d) \: \dfrac{1}{c^2}\Bigg[ G\dfrac{ {e}^{2} }{4\pi\varepsilon_{0}}\Bigg]^{1/2}$

Solution:

Let's suppose the physical quantity formed by c, G and $\sf \dfrac{ {e}^{2} }{4\pi \varepsilon_{0}}$ is $[c]^a[G] ^b \bigg[ \dfrac{ {e}^{2} }{4\pi \varepsilon_{0}}\bigg]^c$

According to the question, it is a physical quantity for length.

So,

$L= [c]^a[G] ^b \bigg[ \dfrac{ {e}^{2} }{4\pi \varepsilon_{0}}\bigg]^c$  

Use dimensional analysis,

Dimensions of the following are:

• L= [L]
• $c=[LT^-1]$
• $G=[M^{-1}L^3T{-2}]$
• $\dfrac{ {e}^{2} }{4\pi \varepsilon_{0}} =[ML^3T^{-2}]$

Why $\dfrac{ {e}^{2} }{4\pi \varepsilon_{0}} =[ML^3T^{-2}]?$

Use dimensional analysis for each quantity in $\dfrac{ {e}^{2} }{4\pi \varepsilon_{0}}$

Here we have

$e^2=[q] ^2=[AT]^2$

$\epsilon_0=[M^{-1}L^{-3}T^4A^2]$

Rest are Constants

Combining these two according to the formula we get,

$\dfrac{ {e}^{2} }{4\pi \varepsilon_{0}}=\frac{[AT]^2}{[M^{-1}L^{-3}T^4A^2]}=\frac{1}{[M^{-1}L^{-3}T^2]} = [ML^3T^{-2}]$

Now, equating the dimensional formulas for

$L= [c]^a[G] ^b \bigg[ \dfrac{ {e}^{2} }{4\pi \varepsilon_{0}}\bigg]^c \\\\ L=[LT^-1]^a[M^{-1}L^3T{-2}]^b[ML^3T^{-2}] ^c \\\\L=[L^aT^{-a}] [M^{-b} L^{3b}T^{-2b}][M^cL^{3c}T^{-2c}] \\\\ L=[L^{a+3b+3c}][M^{-b+c}][T^{-a-2b-2c}]$

Now,equating the exponent power we have,

$a+3b+3c=1 - - - - [i]\\\\ -b+c=0 - - - - [ii] \\\\ - a-2b-2c=0 - - - - [iii] \\\\ From [ii], we~have~b=c,~using~this~value~in~[iii] ~we, ~get \\\\ - a-2c-2c=0 \\\\ - a-4c=0 a=-4c \\\\ Using~a=-4c~and~b=c~in~[i]~we~have~\\\\ - 4c+3c+3c=1 \\\\ 2c=1 \\\\ c=\frac{1}{2} =b \\\\ We~ know~that~a=-4c ~so, ~a=-4 \times \frac{1}{2} ~, a=-2$  

The physical quantity so obtained is

$L= [c]^-2[G] ^{\frac{1}{2}}\bigg[ \dfrac{ {e}^{2} }{4\pi \varepsilon_{0}}\bigg]^{\frac{1}{2}} \\\\ => \frac{1}{c^2} G^{\frac{1}{2}}\bigg[ \dfrac{ {e}^{2} }{4\pi \varepsilon_{0}}\bigg]^{\frac{1}{2}} \\\\ =\frac{1}{c^2} \bigg[G \frac{e^{2} }{4\pi \varepsilon_{0}} \bigg]^{\frac{1}{2}}$