4 Marks Questions

Question 14 Marks

A hot body is placed in a closed room maintained at a lower temperature. Is the number of photons in the room increasing?

Answer

As the hot body is placed in a closed room maintained at a lower temperature, there will be transfer of heat in the room through convection and radiation. Heat radiation also consists of photons; therefore, photons will be emitted by the hot body. Hence, the number of photons in the room will increase.

View full question & answer→

Question 24 Marks

A silver ball of radius 4.8cm is suspended by a thread in a vacuum chamber. Ultraviolet light of wavelength 200 run is incident on the ball for some time during which a total light energy of 1.0 × 10^-7J falls on the surface. Assuming that on the average one photon out of every ten thousand is able to eject a photoelectron, find the electric potential et the surface of the bell assuming zero potential at infinity. What is the potential at the centre of the bell?

Answer

$\lambda=200\text{nm}=2\times10^{-7}\text{m}$

E of one photon $=\frac{\text{hc}}{\lambda}$

$=\frac{6.63\times10^{-34}\times3\times10\times^{8}}{2\times10^{-7}}=9.945\times10^{-19}$

No. of photons

$=\frac{1\times10^{-7}}{9.945\times10^{-19}}=1\times10^{11}\text{no.s}$

Hence, No.of photo electrons

$=\frac{1\times10^{11}}{10^{4}}=1\times10^{7}$

Net amount of positive charge ‘q’ developed due to the outgoing electrons

= 1 × 10⁷ × 1.6 × 10^-19 = 1.6 × 10^-12 C.

Now potential developed at the centre as well as at the surface due to these charger

$=\frac{\text{kq}}{\text{r}}=\frac{9\times10\times1.6\times10^{-12}}{4.8\times10^{-2}}=3\times10^{-1}\text{v}=0.3\text{v.}$

View full question & answer→

Physics 4 Marks Questions

Test yourself on this topic