NUMS — Medical & Dental Entry Test

NUMS Physics — Electronics, Modern and Nuclear Physics (Set 4)

Take the NUMS Physics — Electronics, Modern and Nuclear Physics (Set 4): 30 multiple-choice questions (40 minutes) with instant scoring and a full answer…

What this test covers

30 NUMS Physics

Sample questions with answers

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

Q1. Silicon at room temperature has an intrinsic carrier concentration nᵢ = 1.5 × 10¹⁶ m⁻³. It is doped with donors so that n = 1.0 × 10²² m⁻³. What is the hole concentration?
- A. 1.0 × 10²² m⁻³
- B. 1.5 × 10¹⁶ m⁻³
- C. 2.25 × 10¹⁰ m⁻³ ✓
- D. 2.25 × 10³² m⁻³
Answer: C. 2.25 × 10¹⁰ m⁻³
Explanation: Mass action: np = nᵢ², so p = (1.5 × 10¹⁶)²/10²² = 2.25e+10 m⁻³: holes are minority carriers in n-type silicon.
Q2. The band gap of silicon is about 1.1 eV. What is the longest wavelength of light that can create electron–hole pairs in it? (hc = 1240 eV nm)
- A. 550 nm
- B. 2254 nm
- C. 1127 nm ✓
- D. 700 nm
Answer: C. 1127 nm
Explanation: λ = hc/E_g = 1240/1.1 = 1127 nm, in the near infrared; silicon is transparent beyond this.
Q3. A silicon diode (forward drop 0.7 V) is connected in series with a 430 Ω resistor to a 5.0 V supply in forward bias. What is the current?
- A. 10 mA ✓
- B. 11.6 mA
- C. 0 mA
- D. 7.0 mA
Answer: A. 10 mA
Explanation: The resistor has 5.0 − 0.7 = 4.3 V across it: I = 4.3/430 = 10 mA.
Q4. A 6.0 V Zener diode regulates a 600 Ω load from a 12 V supply through a series resistor of 300 Ω. What current flows through the Zener diode?
- A. 20 mA
- B. 30 mA
- C. 10 mA ✓
- D. 0 mA
Answer: C. 10 mA
Explanation: Current through the series resistor = (12 − 6)/300 = 20 mA. The load takes 6/600 = 10 mA, leaving 10 mA through the Zener.
Q5. A full-wave rectifier with a peak output of 10 V is fed from the 50 Hz mains. What are the ripple frequency and the average (dc) output voltage?
- A. 100 Hz and 6.37 V ✓
- B. 50 Hz and 6.37 V
- C. 50 Hz and 3.18 V
- D. 100 Hz and 3.18 V
Answer: A. 100 Hz and 6.37 V
Explanation: Both half-cycles are used, so the ripple is 100 Hz. V_dc = 2V_m/π = 20/π = 6.37 V (a half-wave rectifier would give 3.18 V at 50 Hz).

The other 25 questions

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Q6. A transistor has current gain β = 100. If the base current is 20 μA, what are the collector and emitter currents?
- A. Iᴄ = 2.02 mA and Iₑ = 2.00 mA
- B. Iᴄ = 2.00 mA and Iₑ = 2.02 mA
- C. Iᴄ = 2.00 mA and Iₑ = 1.98 mA
- D. Iᴄ = 0.20 mA and Iₑ = 0.22 mA
Q7. A common-emitter amplifier has β = 50, input resistance 1.0 kΩ and collector load 4.0 kΩ. What are the voltage gain and the power gain?
- A. 200 and 10000
- B. 200 and 200
- C. 4 and 200
- D. 50 and 200
Q8. An ideal op-amp is connected as a non-inverting amplifier with R_f = 90 kΩ and R₁ = 10 kΩ. If the input is 0.80 V and the supply is ±12 V, what is the output voltage?
- A. 7.2 V
- B. 8.0 V
- C. 12 V
- D. −8.0 V
Q9. What single logic gate is equivalent to the Boolean expression Y = A·B + A·B̄ + Ā·B?
- A. NAND
- B. XOR
- C. AND
- D. OR
Q10. In a photoelectric experiment the stopping potential is 0.50 V for light of frequency 6.0 × 10¹⁴ Hz and 1.33 V for 8.0 × 10¹⁴ Hz. What value of Planck's constant does this give? (e = 1.602 × 10⁻¹⁹ C)
- A. 6.65 × 10⁻³⁴ J s
- B. 8.3 × 10⁻³⁵ J s
- C. 3.3 × 10⁻³⁴ J s
- D. 1.3 × 10⁻³³ J s
Q11. A thermal neutron at 300 K has kinetic energy (3/2)kT. What is its de Broglie wavelength? (mₙ = 1.675 × 10⁻²⁷ kg, k = 1.381 × 10⁻²³ J K⁻¹, h = 6.626 × 10⁻³⁴ J s)
- A. 0.0033 nm
- B. 0.0145 nm
- C. 0.145 nm
- D. 1.45 nm
Q12. X-rays of wavelength 0.1000 nm are Compton scattered through 60°. What is the wavelength of the scattered photon? (Compton wavelength h/mₑc = 2.43 × 10⁻³ nm)
- A. 0.10121 nm
- B. 0.09878 nm
- C. 0.10243 nm
- D. 0.10000 nm
Q13. A 3.0 W laser beam is completely absorbed by a small black target. What force does the beam exert on it? (c = 3.0 × 10⁸ m s⁻¹)
- A. 9.0 × 10⁸ N
- B. 2.0 × 10⁻⁸ N
- C. 1.0 × 10⁻⁸ N
- D. 3.0 × 10⁻⁹ N
Q14. The half-value layer of a lead sheet for a beam of gamma rays is 2.0 cm. What thickness reduces the intensity to one-eighth?
- A. 8.0 cm
- B. 16 cm
- C. 6.0 cm
- D. 4.0 cm
Q15. Muons have a proper mean lifetime of 2.2 μs. How far, on average, does a muon travel in the laboratory frame when moving at 0.80c? (c = 3.0 × 10⁸ m s⁻¹)
- A. 880 m
- B. 1100 m
- C. 528 m
- D. 396 m
Q16. An electron is accelerated until its kinetic energy equals its rest energy (0.511 MeV). What is its speed?
- A. 0.707c
- B. c
- C. 0.5c
- D. 0.866c
Q17. What energy is released if 1.0 mg of mass is completely converted to energy? (c = 3.0 × 10⁸ m s⁻¹)
- A. 9.0 × 10¹⁴ J
- B. 3.0 × 10⁵ J
- C. 9.0 × 10¹⁰ J
- D. 9.0 × 10⁸ J
Q18. An excited atomic state has a lifetime of 1.0 × 10⁻⁸ s. What is the minimum uncertainty in its energy, using ΔE·Δt ≥ ħ/2? (ħ = 1.055 × 10⁻³⁴ J s)
- A. 5.3 × 10⁻²⁷ J
- B. 1.1 × 10⁻³⁴ J
- C. 5.3 × 10⁻²⁵ J
- D. 1.1 × 10⁻²⁶ J
Q19. A star has twice the surface temperature and twice the radius of a second star. By what factor does its total radiated power exceed that of the second star?
- A. 16
- B. 8
- C. 64
- D. 32
Q20. Taking the nuclear radius as R = 1.2 × 10⁻¹⁵ A^⅓ m and the mass of a nucleon as 1.66 × 10⁻²⁷ kg, estimate the density of nuclear matter.
- A. 2.3 × 10¹⁴ kg m⁻³
- B. 4.6 × 10¹⁷ kg m⁻³
- C. 2.3 × 10³ kg m⁻³
- D. 2.3 × 10¹⁷ kg m⁻³
Q21. Using atomic masses ¹H = 1.007825 u, n = 1.008665 u and ⁴He = 4.002603 u (1 u = 931.5 MeV), what is the binding energy per nucleon of ⁴He?
- A. 14.2 MeV
- B. 7.07 MeV
- C. 28.3 MeV
- D. 3.54 MeV
Q22. For the fusion reaction ²H + ³H → ⁴He + n the atomic masses are 2.014102 u, 3.016049 u, 4.002603 u and 1.008665 u (1 u = 931.5 MeV). What energy is released?
- A. 8.8 MeV
- B. 17.6 MeV
- C. 3.5 MeV
- D. 28.3 MeV
Q23. Radium-226 has a half-life of 1600 years (1 year = 3.156 × 10⁷ s). Estimate the activity of 1.0 g of it. (Nₐ = 6.022 × 10²³ mol⁻¹)
- A. 3.7 × 10¹⁰ Bq
- B. 3.7 × 10⁷ Bq
- C. 1.4 × 10⁻¹¹ Bq
- D. 2.7 × 10²¹ Bq
Q24. What fraction of a radioactive sample has decayed after one mean life (τ = 1/λ)?
- A. 50%
- B. 63%
- C. 86%
- D. 37%
Q25. Thorium-232 (Z = 90) decays through a series to stable lead-208 (Z = 82). How many α and β⁻ particles are emitted in total?
- A. 8 α and 6 β⁻
- B. 6 α and 4 β⁻
- C. 6 α and 6 β⁻
- D. 4 α and 6 β⁻
Q26. A 1000 MW reactor derives all its power from fission of ²³⁵U, releasing about 200 MeV per fission (1 MeV = 1.602 × 10⁻¹³ J). Roughly how much ²³⁵U is consumed per day?
- A. 10.5 kg
- B. 105 kg
- C. 1.05 kg
- D. 0.11 kg
Q27. A 50 kg person absorbs 0.50 J of energy from alpha radiation (radiation weighting factor 20). What are the absorbed dose and the equivalent dose?
- A. 0.10 Gy and 2.0 Sv
- B. 0.50 Gy and 10 Sv
- C. 0.01 Sv and 0.2 Gy
- D. 0.01 Gy and 0.2 Sv
Q28. The dose rate from a small gamma source is 80 μSv h⁻¹ at 1.0 m. Assuming no absorption by air, what is the dose rate at 4.0 m?
- A. 20 μSv h⁻¹
- B. 10 μSv h⁻¹
- C. 1.3 μSv h⁻¹
- D. 5.0 μSv h⁻¹
Q29. What is the de Broglie wavelength of the electron in the n = 2 orbit of hydrogen? (a₀ = 0.529 Å)
- A. 13.3 Å
- B. 3.33 Å
- C. 1.06 Å
- D. 6.65 Å
Q30. Why is a photodiode normally operated in reverse bias?
- A. Only reverse bias creates the band gap in the junction
- B. Reverse bias makes the depletion layer disappear completely
- C. Light-generated carriers cause a large relative change in the tiny reverse current
- D. Light cannot be absorbed by a forward-biased junction at all