5 Common Mistakes in Modern Physics (and How to Avoid Them)
Identify and fix costly errors in Photoelectric Effect, Nuclear Physics, Logic Gates, and more with proven strategies.
Why These Mistakes Cost JEE Aspirants
Based on analysis of JEE Main & Advanced papers, these 5 mistakes account for over 70% of lost marks in Modern Physics section. Fixing them will give you:
- 3-8 marks secured in every JEE Physics paper
- Quick identification of trap options
- Confidence in numerical problems
- Time saved by avoiding common pitfalls
Confusing Work Function with Threshold Frequency
❌ The Common Error:
Students often mix up the relationships:
Wrong: $K_{max} = h\nu - \phi$ and $\phi = h\nu_0$ are used interchangeably
Wrong: Thinking work function depends on incident frequency
✅ The Correct Approach:
Key Relationships:
$K_{max} = h\nu - \phi$
$\phi = h\nu_0$
$\therefore K_{max} = h(\nu - \nu_0)$
Remember: Work function ($\phi$) is material property, threshold frequency ($\nu_0$) is the minimum frequency needed.
🎯 Quick Check Example:
Problem: Light of wavelength 300 nm falls on a metal with work function 2.5 eV. Find maximum KE of photoelectrons.
Solution:
Energy of photon: $E = \frac{1240}{300} = 4.13$ eV
$K_{max} = E - \phi = 4.13 - 2.5 = 1.63$ eV
Mass Defect and Binding Energy Formula Errors
❌ The Common Error:
Wrong: $\Delta m = Z m_p + (A-Z) m_n - M_{nucleus}$ (forgetting electron mass)
Wrong: Using atomic mass instead of nuclear mass inconsistently
✅ The Correct Approach:
Choose ONE consistent approach:
Method A (Using Nuclear Masses):
$\Delta m = [Z m_p + (A-Z) m_n] - M_{nucleus}$
Method B (Using Atomic Masses - RECOMMENDED):
$\Delta m = [Z \cdot M(^1H) + (A-Z) m_n] - M_{atom}$
Binding Energy: $BE = \Delta m \times 931.5$ MeV
🎯 Quick Check Example:
Problem: Find binding energy of $^4_2He$ with atomic mass 4.0026 u
Solution (Atomic Mass Method):
$\Delta m = [2 \times 1.007825 + 2 \times 1.008665] - 4.0026$
$\Delta m = 4.03298 - 4.0026 = 0.03038$ u
$BE = 0.03038 \times 931.5 = 28.3$ MeV
Logic Gate Truth Table Confusion
❌ The Common Error:
Wrong: Mixing up NAND and NOR gate outputs
Wrong: Forgetting that XOR gives 1 when inputs are different
Wrong: Confusing bubbled inputs with gate types
✅ The Correct Approach:
NAND Gate:
AND + NOT → Output 0 only when all inputs 1
A=0,B=0 → 1
A=1,B=1 → 0
NOR Gate:
OR + NOT → Output 1 only when all inputs 0
A=0,B=0 → 1
A=1,B=1 → 0
🎯 Memory Technique:
NAND: "NOT AND" - Opposite of AND
NOR: "NOT OR" - Opposite of OR
XOR: "eXclusive OR" - 1 when different
🚀 Exam Survival Strategies
For Photoelectric Effect:
- Always check if ν > ν₀ first
- Work function is constant for material
- Intensity affects current, not Kmax
- Use eV to J conversion: 1 eV = 1.6×10-19 J
For Nuclear Physics:
- Stick to one mass method consistently
- Remember 1 u = 931.5 MeV
- Check mass number conservation
- Verify atomic number in decays
Mistakes 4-5 Available in Full Version
Includes de Broglie wavelength errors and radioactive decay formula mistakes with detailed solutions
📝 Quick Self-Test
Identify the errors in these statements:
1. "Increasing light intensity increases the maximum kinetic energy of photoelectrons."
2. "For NAND gate, output is 1 only when all inputs are 1."
3. "Mass defect = sum of nucleon masses - atomic mass"
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