ECAT — Engineering College Admission Test (UET)

ECAT Physics — Electronics, Modern, Atomic and Nuclear Physics (Set 3)

Take the ECAT Physics — Electronics, Modern, Atomic and Nuclear Physics (Set 3): 30 multiple-choice questions (40 minutes) with instant scoring and a full…

What this test covers

30 ECAT Physics

Sample questions with answers

The first 5 questions of this test, with the correct answer and explanation.

Q1. The energy of a photon of wavelength 500 nm is approximately (hc = 1240 eV nm)
- A. 1.24 eV
- B. 0.62 eV
- C. 4.96 eV
- D. 2.48 eV ✓
Answer: D. 2.48 eV
Explanation: E = hc/λ = 1240/500 = 2.48 eV.
Q2. The work function of a metal is 2.0 eV. Light of photon energy 3.5 eV falls on it. The maximum kinetic energy of the emitted electrons and the stopping potential are
- A. 1.5 eV and 1.5 V ✓
- B. 1.5 eV and 3.5 V
- C. 2.0 eV and 2.0 V
- D. 5.5 eV and 5.5 V
Answer: A. 1.5 eV and 1.5 V
Explanation: K_max = hf − φ = 3.5 − 2.0 = 1.5 eV; eV₀ = K_max gives V₀ = 1.5 V.
Q3. The threshold wavelength of a metal is 620 nm. The work function is approximately (hc = 1240 eV nm)
- A. 2 eV ✓
- B. 4 eV
- C. 1 eV
- D. 0.5 eV
Answer: A. 2 eV
Explanation: φ = hc/λ₀ = 1240/620 = 2 eV.
Q4. If the intensity of monochromatic light above the threshold frequency is doubled, the photoelectric current
- A. doubles, and the maximum kinetic energy also doubles
- B. stays the same, while the maximum kinetic energy doubles
- C. stays the same, and the maximum kinetic energy is unchanged
- D. doubles, while the maximum kinetic energy is unchanged ✓
Answer: D. doubles, while the maximum kinetic energy is unchanged
Explanation: Intensity sets the number of photons (and so photoelectrons) per second; K_max depends only on frequency.
Q5. An electron is accelerated through 100 V. Its de Broglie wavelength is approximately (h = 6.63 × 10⁻³⁴ J s)
- A. 12 nm
- B. 1.2 nm
- C. 0.012 nm
- D. 0.12 nm ✓
Answer: D. 0.12 nm
Explanation: λ = h/√(2meV) = 6.63×10⁻³⁴/√(2 × 9.1×10⁻³¹ × 1.6×10⁻¹⁷) = 0.12 nm.

The other 25 questions

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Q6. A photon has momentum 3.3 × 10⁻²⁷ kg m/s. Its wavelength is approximately (h = 6.63 × 10⁻³⁴ J s)
- A. 200 nm
- B. 100 nm
- C. 2000 nm
- D. 20 nm
Q7. A hydrogen atom in the n = 4 level jumps to the ground state, emitting a single photon. Its energy is approximately
- A. 12.75 eV
- B. 13.6 eV
- C. 3.4 eV
- D. 0.85 eV
Q8. In the hydrogen spectrum, the photon emitted in the transition n = 3 → n = 2 has wavelength approximately
- A. 1875 nm
- B. 486 nm
- C. 656 nm
- D. 122 nm
Q9. The ionization energy of hydrogen in its first excited state (n = 2) is
- A. 3.4 eV
- B. 1.7 eV
- C. 6.8 eV
- D. 13.6 eV
Q10. Which series of the hydrogen spectrum lies in the visible region?
- A. Balmer series
- B. Brackett series
- C. Paschen series
- D. Lyman series
Q11. A radioactive sample has a half-life of 10 days. The fraction remaining after 30 days is
- A. 1/4
- B. 1/8
- C. 1/6
- D. 1/3
Q12. A radioactive element has a half-life of 20 s. Its decay constant is approximately
- A. 0.14 s⁻¹
- B. 0.35 s⁻¹
- C. 0.035 s⁻¹
- D. 0.0035 s⁻¹
Q13. A sample initially has 4 × 10¹⁰ undecayed nuclei and an activity of 2 × 10⁷ decays per second. The decay constant is
- A. 2 × 10³ s⁻¹
- B. 8 × 10¹⁷ s⁻¹
- C. 0.5 s⁻¹
- D. 5 × 10⁻⁴ s⁻¹
Q14. An alpha particle is emitted from a nucleus ²³⁸₉₂U. The daughter nucleus has
- A. mass number 234 and atomic number 92
- B. mass number 238 and atomic number 90
- C. mass number 236 and atomic number 91
- D. mass number 234 and atomic number 90
Q15. In β⁻ decay, the daughter nucleus compared with the parent has
- A. atomic number one lower
- B. the same atomic number and mass number one lower
- C. mass number lower by 4
- D. the same mass number and atomic number one higher
Q16. A nuclear reaction has a mass defect of 0.03 u. The energy released is approximately (1 u = 931.5 MeV/c²)
- A. 2.79 MeV
- B. 27.9 MeV
- C. 279 MeV
- D. 0.03 MeV
Q17. The energy equivalent of 1 g of mass is
- A. 3 × 10⁵ J
- B. 9 × 10¹⁰ J
- C. 9 × 10¹⁶ J
- D. 9 × 10¹³ J
Q18. A nucleus ¹²₆C has total binding energy 92 MeV. Its binding energy per nucleon is approximately
- A. 15.3 MeV
- B. 3.8 MeV
- C. 7.7 MeV
- D. 92 MeV
Q19. A nucleus has 26 protons and 30 neutrons. Its mass number and the number of electrons in the neutral atom are
- A. 56 and 30
- B. 30 and 26
- C. 56 and 26
- D. 26 and 56
Q20. A rod of rest length 1 m moves along its length at 0.6c relative to an observer. The length measured by the observer is
- A. 0.36 m
- B. 0.6 m
- C. 1.25 m
- D. 0.8 m
Q21. A clock moving at 0.8c relative to an observer ticks off 6 s on its own time. The time elapsed for the observer is
- A. 7.5 s
- B. 6 s
- C. 3.6 s
- D. 10 s
Q22. The relativistic mass of a body moving at 0.6c, compared with its rest mass m₀, is
- A. 1.67 m₀
- B. 0.8 m₀
- C. 1.6 m₀
- D. 1.25 m₀
Q23. In an NPN transistor connected in common-emitter mode, the base current is 40 μA and the current gain β is 50. The collector current is
- A. 0.8 mA
- B. 2 mA
- C. 2.04 mA
- D. 50 μA
Q24. An inverting op-amp amplifier has R_f = 100 kΩ and R_in = 10 kΩ. Its voltage gain is
- A. +11
- B. +10
- C. −10
- D. −11
Q25. The output of a two-input XOR gate is 1 when
- A. the inputs are equal
- B. the two inputs differ
- C. both inputs are 0
- D. both inputs are 1
Q26. In a reverse-biased p–n junction, the current that flows is
- A. a large current from majority carriers
- B. a current that grows exponentially with the voltage
- C. a very small leakage current from minority carriers
- D. exactly zero at every applied voltage
Q27. A full-wave rectifier fed by a 50 Hz supply produces a ripple of frequency
- A. 50 Hz
- B. 25 Hz
- C. 100 Hz
- D. 200 Hz
Q28. Silicon is doped with a small amount of phosphorus (five valence electrons). The result is an
- A. p-type semiconductor with holes as majority carriers
- B. insulator
- C. n-type semiconductor with holes as majority carriers
- D. n-type semiconductor with electrons as majority carriers
Q29. The peak of a black body’s emission spectrum is at 500 nm. If its temperature is doubled, the new peak wavelength is
- A. 250 nm
- B. 707 nm
- C. 500 nm
- D. 1000 nm
Q30. An X-ray tube operates at 20 kV. The minimum wavelength of the X-rays produced is approximately (hc = 1240 eV nm)
- A. 0.0062 nm
- B. 0.062 nm
- C. 1.24 nm
- D. 0.62 nm