NUMS — Medical & Dental Entry Test
Take the NUMS Physics — Electronics, Modern and Nuclear Physics (Set 5): 30 multiple-choice questions (40 minutes) with instant scoring and a full answer…
What this test covers
The first 5 questions of this test, with the correct answer and explanation.
Q1. Pure silicon has an intrinsic carrier concentration nᵢ = 1.5 × 10¹⁶ m⁻³ at room temperature. It is doped with donors so that the electron concentration is 1.0 × 10²² m⁻³. Using the mass-action law np = nᵢ², what is the hole concentration?
Answer: A. 2.3 × 10¹⁰ m⁻³
Explanation: p = nᵢ²/n = (1.5 × 10¹⁶)²/10²² = 2.25 × 10³²/10²² = 2.3 × 10¹⁰ m⁻³. Doping with donors greatly raises n and suppresses p.
Q2. A silicon diode (forward drop 0.70 V) is connected in series with a 430 Ω resistor across a 5.0 V battery in the forward direction. What current flows?
Answer: A. 10 mA
Explanation: The resistor has 5.0 − 0.70 = 4.3 V across it. I = 4.3/430 = 0.010 A = 10 mA.
Q3. A 12 V supply feeds a 6.8 V Zener diode through a 220 Ω series resistor, and a 1.0 kΩ load is connected across the Zener diode. What current flows through the Zener diode?
Answer: B. 16.8 mA
Explanation: Current through the series resistor = (12 − 6.8)/220 = 23.6 mA. The load takes 6.8 V/1.0 kΩ = 6.8 mA, so the Zener takes 23.6 − 6.8 = 16.8 mA.
Q4. A full-wave bridge rectifier uses silicon diodes (0.70 V drop each, two conducting at a time). The secondary of the transformer gives a peak voltage of 15.7 V. What is the approximate average (d.c.) output voltage? (V_dc = 2V_peak/π for a full-wave rectified wave)
Answer: B. 9.1 V
Explanation: Peak output = 15.7 − 2 × 0.70 = 14.3 V. V_dc = 2 × 14.3/π = 9.1 V.
Q5. In a common-emitter circuit V_CC = 12 V and the collector resistor is 2.2 kΩ. The collector current is 3.0 mA. What is the collector–emitter voltage? (Neglect any emitter resistor.)
Answer: C. 5.4 V
Explanation: V_CE = V_CC − I_C R_C = 12 − 3.0 × 10⁻³ × 2200 = 12 − 6.6 = 5.4 V.
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Q6. A transistor has a common-base current gain α = 0.98 and an emitter current of 5.0 mA. What is its base current?
Q7. A summing amplifier uses an ideal op-amp with feedback resistor R_f = 10 kΩ and two input resistors R₁ = 5.0 kΩ and R₂ = 10 kΩ. The inputs are V₁ = 1.0 V and V₂ = 2.0 V. What is the output voltage?
Q8. How many 2-input NAND gates are needed (minimum) to build one 2-input AND gate?
Q9. For a half-adder, the sum output S and carry output C for inputs A = 1 and B = 1 are respectively:
Q10. A graph of stopping potential against frequency for a photoelectric metal is a straight line cutting the frequency axis at 5.0 × 10¹⁴ Hz and passing through a stopping potential of 2.07 V at 1.0 × 10¹⁵ Hz. What value of Planck's constant does this give? (e = 1.60 × 10⁻¹⁹ C)
Q11. Violet light of wavelength 400 nm falls on a sodium surface of work function 2.3 eV. Using hc = 1240 eV nm, what is the maximum kinetic energy of the emitted photoelectrons?
Q12. In Bohr's model the electron speed in the nth orbit of hydrogen is vₙ = (2.19 × 10⁶/n) m s⁻¹. What is the speed in the n = 3 orbit?
Q13. A ground-state hydrogen atom absorbs a photon of energy 12.75 eV. (Eₙ = −13.6/n² eV.) To which level is the electron raised?
Q14. In a Franck–Hertz-type experiment, mercury atoms are first excited by electrons of 4.9 eV, and the atoms then emit radiation as they return to the ground state. What is the wavelength emitted? (hc = 1240 eV nm)
Q15. The Kα X-ray wavelength of copper (Z = 29) is 0.154 nm. Using Moseley's law (frequency ∝ (Z − 1)²), what is the Kα wavelength of molybdenum (Z = 42)?
Q16. An X-ray beam passes through a lead sheet whose half-value thickness is 2.0 mm. What thickness reduces the transmitted intensity to ⅛ of the incident value?
Q17. Cosmic-ray muons travel at 0.998c over a 15 km path through the atmosphere, as measured in Earth's frame. How long is that path in the muon's own rest frame? (γ = 15.8)
Q18. A helium–neon laser emits 5.0 mW of light at 632.8 nm. About how many photons does it emit per second? (h = 6.63 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹)
Q19. A thermal neutron has mean kinetic energy (3/2)kT at 300 K. What is its approximate de Broglie wavelength, using λ = h/√(3mkT)? (m = 1.675 × 10⁻²⁷ kg, k = 1.38 × 10⁻²³ J K⁻¹, h = 6.63 × 10⁻³⁴ J s)
Q20. Using atomic masses ²³⁸U = 238.050788 u, ²³⁴Th = 234.043601 u and ⁴He = 4.002603 u (1 u = 931.5 MeV/c²), what is the energy released in the alpha decay ²³⁸U → ²³⁴Th + ⁴He?
Q21. A technetium-99m dose (half-life 6.0 h) has an activity of 600 MBq at 8:00 a.m. What is its activity at 5:00 p.m. on the same day?
Q22. How many decays per second occur in 1.0 µg of radium-226 (half-life 1600 years; M = 226 g mol⁻¹; N_A = 6.02 × 10²³ mol⁻¹; 1 year = 3.16 × 10⁷ s)?
Q23. A reactor releases about 200 MeV per fission of ²³⁵U and produces a steady thermal power of 100 MW. About what mass of ²³⁵U undergoes fission per day? (1 MeV = 1.60 × 10⁻¹³ J; M = 235 g mol⁻¹; N_A = 6.02 × 10²³ mol⁻¹)
Q24. A radiation worker's tissue absorbs 0.10 mGy of alpha radiation (radiation weighting factor 20). What equivalent dose does this represent?
Q25. Carbon-11 decays by positron emission, as used in PET scans. What is the daughter nucleus, and what is detected by the scanner?
Q26. Carbon-14 has a neutron-rich nucleus (6 protons, 8 neutrons) compared with stable carbon-12. By which process does it decay?
Q27. The resistance of an intrinsic semiconductor (such as a thermistor) is measured at 20 °C and again at 60 °C. What is observed, and why?
Q28. A silicon solar cell gives an open-circuit voltage of about 0.60 V. How many such cells must be connected in series to produce about 21.6 V?
Q29. The human body surface at 310 K radiates most strongly at what wavelength? (Wien's law: λ_max T = 2.898 × 10⁻³ m K)
Q30. Two 4-bit binary numbers 1011 and 0110 are added. What is the result in binary?