VITEEE 2019 :: Physics :: General questions

• Physics - General questions

The ratio of the energies of the hydrogen atom in its first to second excited states is

Answer:

Explanation:

1^st excited state corresponds to n = 2

2^nd excited state corresponds to n = 3

As E $\propto\frac{1}{n^{2}}$

$\frac{E_{1}}{E_{2}}$ = $\frac{n_2^2}{n_1^2}$

= $\frac{3^{2}}{2^{2}} = \frac{9}{4}$

If the kinetic energy of a free electron doubles, it's de-Broglie wavelength changes by the factor

Answer:

Explanation:

de-Broglie wavelength,

$\lambda = \frac{h}{p} = \frac{h}{\sqrt{2.m.(K.E)}}$

$\lambda \propto \frac{1}{\sqrt{K.E}}$

If K.E is doubled, then de-Broglie  wavelength becomes $\frac{\lambda}{\sqrt{2}}$

Two thin lenses are in contact and the focal length of the combination is 80 cm. If the focal length of one lens is 20 cm, then the power of the other lens will be

Answer:

Explanation:

P₂ =P - P₁

= $\frac{100}{80} -  \frac{100}{20}$ = -3.75 D

The electric resistance of a certain wire of iron is R. If its length and radius are both doubled, then

Answer:

Explanation:

R_{1 =} $\frac{\rho l_{1}}{A_{1}}$ , now l₂ = 2l₁  r₂ = 2r₁

A₁ = πr₁²

A₂ = π(r₂)² = π(2r₁)² = 4πr₁ ²= 4A₁

R₂ = $\frac{\rho \left(2l_{1}\right)}{4A_{1}} = \frac{\rho l_{1}}{2A_{1}}$

= $\frac{R_{1}}{2}$

.'. Resistance is halved, but specific resistance remains the same.

• Physics - General questions