NEET Physics 4 — Questions and Answers
Question 1: The work function of a metal is 2 eV. What is the threshold frequency for the photoelectric effect? (h = 6.6 × 10⁻³⁴ J·s)
- 4.83 × 10¹⁴ Hz (Correct answer)
- 3.0 × 10⁸ Hz
- 6.6 × 10¹⁴ Hz
- 2.4 × 10¹⁴ Hz
Correct answer: 4.83 × 10¹⁴ Hz
ν₀ = W/h = (2 × 1.6 × 10⁻¹⁹)/(6.6 × 10⁻³⁴) ≈ 4.83 × 10¹⁴ Hz.
Question 2: In a nuclear reaction, ²³⁵U absorbs a neutron and splits into ⁹⁰Kr and ¹⁴³Ba. How many neutrons are released?
- 3 (Correct answer)
- 2
- 1
- 4
Correct answer: 3
Mass number conservation: 235+1 = 90+143+n gives n = 3 neutrons released.
Question 3: The half-life of a radioactive element is 10 years. After 40 years, the fraction of the original sample remaining is:
- 1/16 (Correct answer)
- 1/8
- 1/4
- 1/32
Correct answer: 1/16
40 years = 4 half-lives, so the remaining fraction = (1/2)⁴ = 1/16.
Question 4: In a p-n junction diode under forward bias, what happens to the depletion region?
- It decreases (Correct answer)
- It increases
- It remains the same
- It disappears completely at low voltages
Correct answer: It decreases
Forward bias reduces the potential barrier and narrows the depletion region, allowing current to flow.
Question 5: The output of a NAND gate with inputs A=1 and B=1 is:
- 0 (Correct answer)
- 1
- Undefined
- High impedance
Correct answer: 0
NAND gate output = NOT(A AND B); when A=B=1, AND=1, so NAND output = 0.
Question 6: The binding energy per nucleon is highest for which element?
- Iron (Fe-56) (Correct answer)
- Uranium (U-238)
- Hydrogen (H-1)
- Helium (He-4)
Correct answer: Iron (Fe-56)
Fe-56 has the highest binding energy per nucleon (~8.8 MeV), making it the most stable nucleus.
Question 7: The de Broglie wavelength of an electron accelerated through a potential difference V is proportional to:
- 1/√V (Correct answer)
- 1/V
- √V
- V
Correct answer: 1/√V
Since λ = h/mv and kinetic energy eV = ½mv², we get λ = h/√(2meV), so λ ∝ 1/√V.
NEET Physics 4
The work function of a metal is 2 eV.
What is the threshold frequency for the photoelectric effect? (h = 6.6 × 10⁻³⁴ J·s)