ECAT — Engineering College Admission Test (UET)

ECAT Physics — Electronics, Modern, Atomic and Nuclear Physics (Set 2)

Take the ECAT Physics — Electronics, Modern, Atomic and Nuclear Physics (Set 2): 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. As the temperature of a pure (intrinsic) semiconductor such as silicon is raised, its resistance:
- A. decreases ✓
- B. stays constant
- C. increases
- D. becomes zero at once
Answer: A. decreases
Explanation: More electron–hole pairs are produced by thermal energy, so conductivity rises and resistance falls.
Q2. When a p–n junction diode is reverse biased:
- A. majority carriers cross the junction easily
- B. its depletion region narrows and a large current flows
- C. its potential barrier disappears
- D. its depletion region widens and only a tiny leakage current flows ✓
Answer: D. its depletion region widens and only a tiny leakage current flows
Explanation: Reverse bias pulls majority carriers away from the junction, widening the depletion layer and raising the barrier; only minority carriers give a small current.
Q3. How many diodes are used in a bridge full-wave rectifier?
- A. 3
- B. 2
- C. 1
- D. 4 ✓
Answer: D. 4
Explanation: A bridge rectifier uses four diodes arranged so that two conduct in each half-cycle.
Q4. A transistor has a current gain β = 100. If its base current is 20 μA, what is its emitter current?
- A. 2.02 mA ✓
- B. 1.98 mA
- C. 2 mA
- D. 0.2 mA
Answer: A. 2.02 mA
Explanation: I_C = βI_B = 2 mA; I_E = I_B + I_C = 0.02 + 2 = 2.02 mA.
Q5. A transistor has a common-emitter current gain β = 99. What is its common-base current gain α = I_C/I_E?
- A. 0.01
- B. 99
- C. 0.99 ✓
- D. 1.01
Answer: C. 0.99
Explanation: α = β/(1 + β) = 99/100 = 0.99.

The other 25 questions

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Q6. A non-inverting op-amp amplifier uses R₁ = 10 kΩ (to ground) and a feedback resistor R_f = 90 kΩ. What is its voltage gain?
- A. −9
- B. 9
- C. 10
- D. 0.11
Q7. Which property is characteristic of an ideal operational amplifier?
- A. Very high input resistance and very high open-loop gain
- B. Gain that depends strongly on frequency only
- C. Very high output resistance
- D. Very low input resistance and unity gain
Q8. A two-input NAND gate gives an output of 0:
- A. only when both inputs are 0
- B. for every input combination
- C. when the inputs are different
- D. only when both inputs are 1
Q9. What is the output of a NOR gate when both of its inputs are 0?
- A. undefined
- B. 1
- C. equal to the supply current
- D. 0
Q10. An X-ray photon has an energy of 6.2 keV. What is its momentum? (1 eV = 1.6 × 10⁻¹⁹ J, c = 3 × 10⁸ m/s)
- A. 2.98 × 10¹⁴ kg m/s
- B. 3.31 × 10⁻²⁴ kg m/s
- C. 3.31 × 10⁻²⁷ kg m/s
- D. 9.92 × 10⁻¹⁶ kg m/s
Q11. Ultraviolet light of wavelength 310 nm falls on a metal of work function 2.5 eV. What is the maximum kinetic energy of the emitted photoelectrons? (hc = 1240 eV nm)
- A. 2.5 eV
- B. 1.5 eV
- C. 4 eV
- D. 6.5 eV
Q12. A graph of stopping potential V₀ against frequency f of the incident light is a straight line for any metal. Its slope equals:
- A. the work function, different for each metal
- B. e/h
- C. h/e, the same for all metals
- D. the threshold frequency
Q13. A 1 mW laser emits light of wavelength 663 nm. About how many photons does it emit per second? (h = 6.63 × 10⁻³⁴ J s, c = 3 × 10⁸ m/s)
- A. 3 × 10⁻¹⁹
- B. 3.33 × 10¹⁵
- C. 3.33 × 10¹⁸
- D. 1.5 × 10¹²
Q14. A 0.1 kg cricket ball moves at 6.63 m/s. What is its de Broglie wavelength, and why is it never noticed? (h = 6.63 × 10⁻³⁴ J s)
- A. 10⁻³³ m — large enough to be observed easily
- B. 10⁻⁷ m — similar to visible light
- C. 6.63 × 10⁻³⁴ m — equal to Planck's constant
- D. 10⁻³³ m — far too small to show any wave effects
Q15. By how much does the wavelength of an X-ray photon increase when it is Compton-scattered through 90° by a free electron? (h = 6.63 × 10⁻³⁴ J s, mₑ = 9.1 × 10⁻³¹ kg, c = 3 × 10⁸ m/s)
- A. 4.86 pm
- B. zero
- C. 2.43 pm
- D. 1.21 pm
Q16. An electron is known to be somewhere within a region 1 × 10⁻¹⁰ m wide. Using Δx Δp ≈ h, what is the order of the uncertainty in its momentum? (h = 6.63 × 10⁻³⁴ J s)
- A. 6.63 × 10⁻⁴⁴ kg m/s
- B. 6.63 × 10⁻²⁴ kg m/s
- C. 6.63 × 10⁻¹⁴ kg m/s
- D. 6.63 × 10⁻³⁴ kg m/s
Q17. At what speed does the relativistic mass of a body become 1.25 times its rest mass?
- A. 0.75c
- B. 0.25c
- C. 0.6c
- D. 0.8c
Q18. At what speed must a metre stick move along its length so that a stationary observer measures it as 50 cm long?
- A. 0.75c
- B. 0.5c
- C. 0.866c
- D. 0.25c
Q19. One of the two postulates of Einstein's special theory of relativity is that:
- A. the speed of light in free space is the same for all inertial observers
- B. the laws of physics differ between inertial frames
- C. time is absolute for all observers
- D. mass and energy are separately conserved
Q20. The Sun's spectrum peaks at about 500 nm. Taking Wien's constant as 2.9 × 10⁻³ m K, estimate the temperature of the Sun's surface.
- A. 1450 K
- B. 5800 K
- C. 580 K
- D. 14,500 K
Q21. Using the Rydberg constant R_H = 1.097 × 10⁷ m⁻¹, what is the wavelength of the line emitted when a hydrogen electron falls from n = 3 to n = 2?
- A. 122 nm
- B. 656 nm
- C. 486 nm
- D. 365 nm
Q22. In the Bohr model of hydrogen, what is the ratio E₂/E₄ of the energies of the second and fourth levels?
- A. 2
- B. 1/2
- C. 4
- D. 1/4
Q23. The Lyman series of hydrogen spectral lines lies in the:
- A. microwave region
- B. visible region
- C. ultraviolet region
- D. infrared region
Q24. How much energy is needed to excite a hydrogen atom from its ground state to the n = 3 level? (E₁ = −13.6 eV)
- A. 1.51 eV
- B. 13.6 eV
- C. 12.09 eV
- D. 10.2 eV
Q25. The characteristic (line) X-ray spectrum of a target metal is produced when:
- A. electrons from outer shells fill vacancies in inner shells of the target atoms
- B. fast electrons are decelerated by the nuclei of the target
- C. nuclei of the target undergo radioactive decay
- D. the target is heated to a very high temperature
Q26. Which of these is NOT a property of laser light?
- A. It is very intense
- B. It contains a wide range of wavelengths
- C. It is highly coherent
- D. It is highly directional
Q27. Isotopes of an element have nuclei with:
- A. different numbers of electrons only
- B. the same number of neutrons but different numbers of protons
- C. the same number of protons but different numbers of neutrons
- D. the same mass number but different atomic numbers
Q28. Of a 64 g sample of a radioactive isotope, only 2 g remains undecayed after 30 days. What is its half-life?
- A. 3.75 days
- B. 15 days
- C. 6 days
- D. 5 days
Q29. ²³²₉₀Th decays through a series of α and β⁻ emissions to the stable isotope ²⁰⁸₈₂Pb. How many α and β⁻ particles are emitted in all?
- A. 6 α and 8 β⁻
- B. 4 α and 6 β⁻
- C. 8 α and 4 β⁻
- D. 6 α and 4 β⁻
Q30. Approximately how much energy is released in the fission of one uranium-235 nucleus?
- A. 20 keV
- B. 2000 MeV
- C. 200 MeV
- D. 2 MeV