ECAT — Engineering College Admission Test (UET)

ECAT Physics — Electronics, Modern, Atomic and Nuclear Physics (Set 1)

Take the ECAT Physics — Electronics, Modern, Atomic and Nuclear Physics (Set 1): 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. When a p–n junction is forward biased, its depletion region:
- A. becomes wider
- B. becomes narrower ✓
- C. disappears permanently
- D. is unaffected
Answer: B. becomes narrower
Explanation: The forward voltage opposes the barrier potential, pushing majority carriers towards the junction and narrowing the depletion layer.
Q2. A full-wave rectifier is fed with a 50 Hz AC supply. What is the fundamental frequency of the ripple in its output?
- A. 50 Hz
- B. 200 Hz
- C. 100 Hz ✓
- D. 25 Hz
Answer: C. 100 Hz
Explanation: A full-wave rectifier gives one output pulse for each half-cycle, so the ripple frequency is 2 × 50 = 100 Hz (a half-wave rectifier gives 50 Hz).
Q3. In a transistor, a base current of 50 μA produces a collector current of 5 mA. What is the current gain β?
- A. 10
- B. 100 ✓
- C. 1000
- D. 0.01
Answer: B. 100
Explanation: β = I_C/I_B = 5 × 10⁻³/(50 × 10⁻⁶) = 100.
Q4. An inverting op-amp amplifier has an input resistance of 10 kΩ and a feedback resistance of 100 kΩ. What is its voltage gain?
- A. +10
- B. +11
- C. −11
- D. −10 ✓
Answer: D. −10
Explanation: G = −R_f/R_i = −100/10 = −10 (the minus sign shows a 180° phase inversion).
Q5. Which logic gate gives an output of 1 only when its two inputs are different?
- A. NOR
- B. XOR ✓
- C. AND
- D. XNOR
Answer: B. XOR
Explanation: Exclusive-OR outputs 1 for inputs (0,1) and (1,0) and 0 when the inputs are equal.

The other 25 questions

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Q6. Adding a small amount of a pentavalent impurity such as phosphorus to pure silicon produces:
- A. a p-type semiconductor
- B. an insulator
- C. an n-type semiconductor
- D. an intrinsic semiconductor
Q7. What is the energy of a photon of wavelength 620 nm? (hc = 1240 eV nm)
- A. 1 eV
- B. 0.5 eV
- C. 3.1 eV
- D. 2 eV
Q8. Light of photon energy 4 eV falls on a metal with work function 2.5 eV. What is the stopping potential for the emitted photoelectrons?
- A. 2.5 V
- B. 6.5 V
- C. 4 V
- D. 1.5 V
Q9. A metal has a work function of 2 eV. What is its threshold wavelength? (hc = 1240 eV nm)
- A. 2480 nm
- B. 620 nm
- C. 310 nm
- D. 124 nm
Q10. In the photoelectric effect, increasing the intensity of the incident light (frequency unchanged and above threshold):
- A. increases the maximum kinetic energy of the photoelectrons
- B. changes the threshold frequency
- C. increases the number of photoelectrons but not their maximum kinetic energy
- D. decreases the stopping potential
Q11. An electron is accelerated from rest through a potential difference of 150 V. What is its de Broglie wavelength approximately?
- A. 0.1 nm
- B. 10 nm
- C. 1 nm
- D. 0.01 nm
Q12. In the Compton effect, the change in wavelength of the scattered X-ray photon is maximum when the scattering angle is:
- A. 0°
- B. 180°
- C. 45°
- D. 90°
Q13. A clock on a spaceship moving at 0.6c relative to the Earth records an interval of 4 s. What interval does an observer on Earth measure?
- A. 3.2 s
- B. 5 s
- C. 6.67 s
- D. 4 s
Q14. If 1 g of matter were completely converted into energy, how much energy would be released? (c = 3 × 10⁸ m/s)
- A. 9 × 10¹³ J
- B. 3 × 10⁵ J
- C. 9 × 10¹⁰ J
- D. 9 × 10¹⁶ J
Q15. A rod 10 m long (in its rest frame) moves along its length at 0.8c relative to an observer. What length does the observer measure?
- A. 8 m
- B. 2 m
- C. 6 m
- D. 16.7 m
Q16. What is the minimum photon energy required for the production of an electron–positron pair?
- A. 1.02 MeV
- B. 0.51 MeV
- C. 2.04 MeV
- D. 13.6 eV
Q17. If the absolute temperature of a black body is doubled, the wavelength at which its emission is maximum:
- A. is doubled
- B. is halved
- C. is unchanged
- D. becomes one quarter
Q18. Using Eₙ = −13.6/n² eV for hydrogen, what is the energy of the photon emitted when the electron falls from n = 3 to n = 2?
- A. 3.4 eV
- B. 12.09 eV
- C. 1.51 eV
- D. 1.89 eV
Q19. How much energy is needed to ionize a hydrogen atom whose electron is in the n = 2 state? (E₁ = −13.6 eV)
- A. 10.2 eV
- B. 13.6 eV
- C. 3.4 eV
- D. 6.8 eV
Q20. The radius of the first Bohr orbit of hydrogen is 0.053 nm. What is the radius of the third orbit?
- A. 0.212 nm
- B. 0.0059 nm
- C. 0.159 nm
- D. 0.477 nm
Q21. Which series of hydrogen spectral lines falls in the visible region?
- A. Paschen series
- B. Balmer series
- C. Brackett series
- D. Lyman series
Q22. Hydrogen atoms excited to the n = 4 level return to the ground state. What is the maximum number of different spectral lines that can be emitted?
- A. 4
- B. 10
- C. 6
- D. 3
Q23. Laser action requires:
- A. only spontaneous emission
- B. a continuous spectrum of energy levels
- C. population inversion involving a metastable state
- D. equal populations of the ground and excited states
Q24. An X-ray tube operates at 31 kV. What is the shortest wavelength in the X-rays it produces? (hc = 1240 eV nm)
- A. 0.04 nm
- B. 0.025 nm
- C. 0.4 nm
- D. 0.004 nm
Q25. How many neutrons are there in the nucleus of ²³⁵₉₂U?
- A. 327
- B. 92
- C. 235
- D. 143
Q26. A radioactive sample of 80 g has a half-life of 5 days. How much of it remains undecayed after 15 days?
- A. 10 g
- B. 5 g
- C. 40 g
- D. 20 g
Q27. The nucleus ²³⁸₉₂U emits one α-particle and then two β⁻-particles. What is the resulting nucleus?
- A. ²³⁴₈₈Ra
- B. ²³⁰₉₂U
- C. ²³⁴₉₂U
- D. ²³⁴₉₀Th
Q28. The mass defect of a nucleus is 0.02 u. What is its binding energy? (1 u = 931 MeV)
- A. 46.55 MeV
- B. 931 MeV
- C. 1.862 MeV
- D. 18.62 MeV
Q29. A radioactive isotope has a half-life of 693 s. What is its decay constant?
- A. 0.693 s⁻¹
- B. 1 × 10⁻³ s⁻¹
- C. 4.8 × 10⁻⁴ s⁻¹
- D. 1 × 10³ s⁻¹
Q30. Nuclear fusion requires extremely high temperatures because:
- A. the nuclei must have enough kinetic energy to overcome their electrostatic repulsion
- B. the strong nuclear force is repulsive at short range
- C. neutrons must be slowed down first
- D. heat is needed to split the nuclei