Modern Physics Test

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Modern Physics Test - 2

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Weekly Quiz Competition

Question 1
4 / -1

A radioactive material decays by simultaneous emission of two particles with respective half-lives 1620 and 810 years.
The time, in years, after which one-fourth of the material remains is

A
1080

B
2430

C
3240

D
4860

Solution

when N_o is initial number of atoms
Question 2
4 / -1

The probability of electrons to be found in the conduction band of an intrinsic semiconductor at a finite temperature

A
increases exponentially with increasing band gap

B
decreases exponentially with increasing band gap

C
decreases with increasing temperature

D
is independent of the temperature and the band gap

Solution

KEY CONCEPT : For a semi conductor n = n₀e^–Eg/kT where n₀ = no. of free electrons at absolute zero, n = no. of free electrons at T kelvin, E_g = Energy gap, k = Boltzmann constant.
As E_g increases, n decreases exponentially.
Question 3
4 / -1

A full-wave rectifier circuit along with the out-put is shown in Figure. The contribution (s) from the diode 1 is (are)

A
C

B
A,C

C
B, D

D
A,B,C, D.

Solution

As shown in the fig. (i) during one half cycle the polarity of P and S are opposite such that diode (1) is reversed biased and hence non conducting.
During the other half cycle, diode (1) gets forward biased and is conducting. Thus diode (1) conducts in one half cycle and does not conduct in the other so the correct option is (b) (a and c.)
Question 4
4 / -1

As per Bohr model, the minimum energy (in eV) required to remove an electron from the ground state of doubly ionized Li atom (Z = 3) is

A
1.51

B
13.6

C
40.8

D
122.4

Solution

KEY CONCEPT :
Therefore, ground state energy of doubly ionized lithium atom (Z = 3, n = 1) will be
∴ Ionization energy of an electron in ground state of doubly ionized lithium atom will be 122.4eV.
Question 5
4 / -1

The circuit shown in the figure contains two diodes each with a forward resistance of 50 ohms and with infinite backward resistance. If the battery voltage is 6V, the current through the 100 ohm resistance (in amperes) is

A
zero

B
0.02

C
0.03

D
0.036.

Solution

In the circuit, diode D₁ is forward biased, while D₂ is reverse biased. Therefore, current i (through D₁ and 100Ω resistance) will be
Here, 50Ω is the resistance of D₁ in forward biasing.
Question 6
4 / -1

Which of the following statements is not true?

A
The resistance of intrinsic semiconductors decrease with increase of temperature

B
Doping pure Si with trivalent impurities give p-type semiconductors

C
The majority carriers in n-type semiconductors are holes

D
A p-n junction can act as a semiconductor diode

Solution

In n-type semiconductors, electrons are the majority charge carriers.
Question 7
4 / -1

The maximum kinetic energy of photoelectrons emitted from a surface when photons of energy 6 eV fall on it is 4 eV. The stopping potential, in volt, is

A
2

B
4

C
6

D
10

Solution

Stopping potential is the negative potential applied to stop the electrons having maximum kinetic energy.
Therefore, stopping potential will be 4 volt.
Question 8
4 / -1

In hydrogen spectrum the wavelength of H_α line is 656 nm, whereas in the spectrum of a distant galaxy, H_α line wavelength is 706 nm. Estimated speed of the galaxy with respect to earth is,

A
2 × 10⁸ m/s

B
2 × 10⁷ m/s

C
2 × 10⁶ m/s

D
2 × 10⁵ m/s

Solution

KEY CONCEPT : According to Doppler’s effect of light, the wavelength shift is given by
Question 9
4 / -1

A particle of mass M at rest decays into two particles of masses m₁ and m₂, having non-zero velocities. The ratio of the de Broglie wavelengths of the particles, λ₁/λ₂, is

A
m_1/m₂

B
m₂/m₁

C
1.0

D

Solution

Applying conservation of linear momentum, Initial momentum = Final momentum
0 = m₁v₁ – m₂v₂ ⇒ m₁v₁ = m₂v₂
Question 10
4 / -1

Which of the following is a correct statement?

A
Beta rays are same as cathode rays

B
Gamma rays are high energy neutrons

C
Alpha particles are singly ionised helium atoms

D
Protons and neutrons have exactly the same mass

Solution

Beta rays are same as cathode rays as both are stream of electrons.