NUMS — Medical & Dental Entry Test

NUMS Physics — Electronics, Modern and Nuclear Physics (Set 2)

Take the NUMS Physics — Electronics, Modern and Nuclear Physics (Set 2): 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. Which statement about a pure (intrinsic) semiconductor such as silicon is correct?
- A. Its resistance increases as it is heated
- B. It has equal numbers of free electrons and holes, and its conductivity rises with temperature ✓
- C. It has more holes than electrons at all temperatures
- D. It conducts only when doped with a metal
Answer: B. It has equal numbers of free electrons and holes, and its conductivity rises with temperature
Explanation: In intrinsic material each thermally broken bond releases one electron and leaves one hole; heating creates more pairs, so conductivity increases.
Q2. A silicon diode does not conduct appreciably in forward bias until the applied voltage exceeds about:
- A. 0.07 V
- B. 0.7 V ✓
- C. 0.3 V
- D. 7 V
Answer: B. 0.7 V
Explanation: The potential barrier of a silicon p–n junction is about 0.7 V (about 0.3 V for germanium); forward current rises sharply above this.
Q3. A p–n junction diode is reverse biased below its breakdown voltage. Why does a very small current still flow?
- A. Majority carriers cross the narrowed depletion layer
- B. Minority charge carriers are swept across the junction ✓
- C. Electrons tunnel through the metal contacts
- D. The depletion region disappears
Answer: B. Minority charge carriers are swept across the junction
Explanation: Reverse bias widens the depletion layer and stops majority carriers, but thermally generated minority carriers are pushed across by the field, giving a tiny leakage current.
Q4. A full-wave rectifier is fed from the 50 Hz mains. What is the frequency of the ripple in its output?
- A. 25 Hz
- B. 50 Hz
- C. 200 Hz
- D. 100 Hz ✓
Answer: D. 100 Hz
Explanation: A full-wave rectifier produces one output pulse for each half-cycle of the input, giving 2 × 50 = 100 pulses per second.
Q5. A light-emitting diode is made from a semiconductor with a band gap of 2.0 eV. What is the approximate wavelength of the light it emits? (hc = 1240 eV nm)
- A. 2480 nm
- B. 620 nm ✓
- C. 1240 nm
- D. 310 nm
Answer: B. 620 nm
Explanation: Photon energy ≈ band gap, so λ = hc/E = 1240/2.0 = 620 nm (red–orange light).

The other 25 questions

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Q6. In a transistor, the base current is 50 μA and the current gain β is 100. What is the emitter current?
- A. 5.05 mA
- B. 4.95 mA
- C. 5.00 mA
- D. 0.50 mA
Q7. When a transistor is used as an electronic switch, between which two states is it operated?
- A. Active region only
- B. Zener and avalanche regions
- C. Forward and reverse breakdown
- D. Cut-off and saturation
Q8. A non-inverting op-amp amplifier uses a feedback resistor of 90 kΩ and a resistor of 10 kΩ from the inverting input to ground. What is its voltage gain?
- A. +10
- B. −10
- C. −9
- D. +9
Q9. A two-input NOR gate gives an output of 1 for which input combination?
- A. A = 0, B = 1
- B. A = 0, B = 0
- C. A = 1, B = 1
- D. A = 1, B = 0
Q10. Which Boolean expression simplifies to A?
- A. A·B̄ + B
- B. A + B
- C. A + A·B
- D. A·B
Q11. A metal has a work function of 2.48 eV. What is its threshold wavelength for photoelectric emission? (hc = 1240 eV nm)
- A. 500 nm
- B. 250 nm
- C. 620 nm
- D. 1000 nm
Q12. A graph of stopping potential against frequency of incident light is plotted for a photocell. What does its slope equal?
- A. Work function/e
- B. e/h
- C. h/e
- D. Threshold frequency
Q13. A diagnostic X-ray photon has a wavelength of 0.10 nm. What momentum does it carry? (h = 6.63 × 10⁻³⁴ J s)
- A. 6.6 × 10⁻²⁴ kg m s⁻¹
- B. 2.0 × 10⁻¹⁵ kg m s⁻¹
- C. 6.6 × 10⁻³³ kg m s⁻¹
- D. 6.6 × 10⁻¹⁴ kg m s⁻¹
Q14. A proton and an alpha particle (mass 4mₚ, charge 2e) are each accelerated from rest through the same potential difference. What is the ratio of their de Broglie wavelengths, λₚ : λα?
- A. 4 : 1
- B. 2√2 : 1
- C. 2 : 1
- D. 1 : 2√2
Q15. In Compton scattering, at what scattering angle is the increase in X-ray wavelength the greatest?
- A. 45°
- B. 90°
- C. 0°
- D. 180°
Q16. In PET scanning, a positron emitted by a tracer meets an electron and both are annihilated. What is produced?
- A. One gamma photon of 0.51 MeV
- B. An alpha particle
- C. Two gamma photons of about 0.51 MeV each, moving in opposite directions
- D. A neutron and a neutrino
Q17. The X-ray spectrum from a tube shows sharp peaks on top of a continuous background. What produces these sharp characteristic lines?
- A. Nuclear decay of the target metal
- B. Electrons in the target atoms falling into vacancies in inner shells
- C. Heating of the target to a high temperature
- D. Deceleration of electrons by target nuclei
Q18. A rod 10 m long in its own rest frame moves past an observer at 0.8c along its length. What length does the observer measure?
- A. 10 m
- B. 8 m
- C. 6 m
- D. 16.7 m
Q19. In a nuclear reaction the total rest mass of the products is 2.0 mg less than that of the reactants. How much energy is released? (c = 3.0 × 10⁸ m s⁻¹)
- A. 1.8 × 10¹⁴ J
- B. 1.8 × 10⁸ J
- C. 1.8 × 10¹¹ J
- D. 6.0 × 10² J
Q20. According to Wien's displacement law, if the absolute temperature of a black body is doubled, what happens to the wavelength at which its emission peaks?
- A. It becomes one sixteenth
- B. It doubles
- C. It halves
- D. It is unchanged
Q21. Laser action requires a metastable state. What is special about such an energy level?
- A. Electrons stay in it much longer than in ordinary excited states
- B. It emits light only by spontaneous emission
- C. It lies below the ground state
- D. Electrons cannot enter it
Q22. How many neutrons are present in a nucleus of uranium-235 (Z = 92)?
- A. 235
- B. 327
- C. 143
- D. 92
Q23. The mass defect of a helium-4 nucleus is 0.0304 u. What is its binding energy per nucleon? (1 u ≡ 931.5 MeV)
- A. 28.3 MeV
- B. 14.2 MeV
- C. 7.08 MeV
- D. 0.0076 MeV
Q24. Fluorine-18, a PET tracer, decays by positron (β⁺) emission. What happens to the atomic number and mass number of the nucleus?
- A. Z decreases by 2; A decreases by 4
- B. Both Z and A are unchanged
- C. Z decreases by 1; A is unchanged
- D. Z increases by 1; A is unchanged
Q25. A nucleus emits a gamma ray. What change occurs in its atomic number and mass number?
- A. A decreases by 4
- B. Z increases by 1
- C. Neither changes; the nucleus only loses energy
- D. Z decreases by 1 and A is unchanged
Q26. An archaeological bone contains 25% of the carbon-14 found in living bone. How old is it? (half-life of ¹⁴C = 5730 years)
- A. 11 460 years
- B. 5730 years
- C. 22 920 years
- D. 17 190 years
Q27. Roughly how much energy is released when one uranium-235 nucleus undergoes fission?
- A. About 200 MeV
- B. About 20 keV
- C. About 2 MeV
- D. About 2000 MeV
Q28. A tissue receives an absorbed dose of 0.1 Gy of alpha radiation, whose radiation weighting (quality) factor is 20. What is the equivalent dose?
- A. 2 Sv
- B. 20 Sv
- C. 0.005 Sv
- D. 0.1 Sv
Q29. A radiographer moves from 1 m to 2 m away from a small gamma source. Ignoring absorption in air, by what factor does the intensity she receives change?
- A. It falls to one half
- B. It is unchanged
- C. It falls to one eighth
- D. It falls to one quarter
Q30. What is the role of cadmium or boron control rods in a nuclear reactor?
- A. They absorb neutrons to control the rate of the chain reaction
- B. They slow down fast neutrons
- C. They emit neutrons to start fission
- D. They transfer heat to the turbines