Heisenberg Uncertainty Principle Explained | NEET Notes

 Heisenberg’s Uncertainty Principle (NEET – Structure of Atom)

Electron cannot have exact position and momentum at the same time due to Heisenberg’s Uncertainty Principle.

Heisenberg’s Uncertainty Principle (1927)

• Werner Heisenberg was a German physicist.
• In 1927, he proposed the Uncertainty Principle.
• It is a direct result of the dual nature of matter and radiation (wave–particle duality).

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Statement of the Principle

• It is impossible to measure simultaneously:

  • exact position of an electron
  • exact momentum (or velocity) of an electron

• If one is known more accurately, the other becomes less accurate.

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Mathematical Expression

\Delta x \cdot \Delta p_x \ge \frac{h}{4\pi}

OR

\Delta x \cdot \Delta v_x \ge \frac{h}{4\pi m}

Where:

• Δx = uncertainty in position
• Δpx = uncertainty in momentum
• Δvx = uncertainty in velocity
• h = Planck’s constant
• m = mass of particle (electron)

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Important Meaning of Formula

• If Δx is small (position known accurately)
  → Δv (velocity uncertainty) becomes large

• If Δv is small (velocity known accurately)
  → Δx becomes large

👉 Conclusion: We can never know both exactly at the same time.

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Physical Meaning

• Electron motion cannot be described as a fixed path.
• Instead, we get a blurred or fuzzy picture of electron position and velocity.
• This is a fundamental limit, not an experimental error.

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Example (Paper Thickness Analogy)

• If you measure thickness of paper using an unmarked scale, result is inaccurate.
• To improve accuracy, we need a smaller unit scale.

👉 Similarly:

• To locate an electron, we need radiation with very small wavelength.

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Effect of Measuring Electron

• To observe an electron, we use light (electromagnetic radiation).

• For accurate position measurement:

  • We need high energy photons (short wavelength)

• These photons have:

  • high momentum

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Problem with Observation

• High-energy photons collide with electron.
• This collision disturbs the electron’s velocity.

👉 Result:

• Position becomes known
• But velocity becomes uncertain

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Final Key Point (NEET Important)

• Uncertainty principle is not due to instrument limitation
• It is a fundamental property of nature

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Heisenberg’s Uncertainty Principle (NEET level):

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Heisenberg’s Uncertainty Principle (1927)
│
├── Scientist
│ └── Werner Heisenberg (German physicist)
│
├── Basis
│ └── Wave–particle duality of matter and radiation
│
├── Statement
│ └── Cannot measure simultaneously:
│ ├── Exact position (x) of electron
│ └── Exact momentum / velocity (p or v) of electron
│
├── Mathematical Form
│ ├── Δx · Δp ≥ h / 4π
│ └── Δx · Δv ≥ h / 4πm
│
├── Terms Meaning
│ ├── Δx = uncertainty in position
│ ├── Δp = uncertainty in momentum
│ ├── Δv = uncertainty in velocity
│ ├── h = Planck’s constant
│ └── m = mass of electron
│
├── Key Relationship
│ ├── Small Δx → Large Δv
│ └── Small Δv → Large Δx
│
├── Physical Meaning
│ ├── Electron has no fixed path
│ └── Only probability / fuzzy region of location
│
├── Reason
│ └── Observation disturbs electron (photon collision)
│
├── Measurement Method Issue
│ ├── Need high energy (short wavelength) light for position
│ └── High energy photons change electron velocity
│
└── Conclusion (Important)
├── Not experimental limitation
└── Fundamental law of nature

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1. Very Short Answer Questions (1 mark)

Q1. Who proposed the Uncertainty Principle?
Ans: Werner Heisenberg (1927)

Q2. What does Δx represent?
Ans: Uncertainty in position

Q3. Write the SI unit of momentum.
Ans: kg m s⁻¹

Q4. Is electron path fixed in atom?
Ans: No

Q5. What is the value of Planck’s constant (h)?
Ans: 6.626 × 10⁻³⁴ J s

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2. Short Answer Questions (2–3 marks)

Q1. State Heisenberg’s Uncertainty Principle.
Ans:
It states that it is impossible to determine simultaneously the exact position and exact momentum (or velocity) of an electron.

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Q2. Write mathematical expression of uncertainty principle.
Ans:
Δx · Δp ≥ h / 4π
or
Δx · Δv ≥ h / (4πm)

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Q3. Why is electron path uncertain?
Ans:
Because measurement of position disturbs its momentum due to interaction with high energy photons.

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3. Long Answer Questions (5 marks)

Q1. Explain Heisenberg’s Uncertainty Principle with example.
Ans:

• Proposed by Werner Heisenberg in 1927
• States that position and momentum cannot be measured simultaneously with accuracy
• Mathematical form: Δx · Δp ≥ h/4π
• If position is known accurately, momentum becomes uncertain
• If momentum is known accurately, position becomes uncertain
• Example: Measuring electron using high-energy light disturbs its motion
• Conclusion: Electron does not follow a fixed path, only probability region exists

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Q2. Explain effect of measuring electron using light.
Ans:

• To observe electron, high-energy photons are used
• These photons have short wavelength and high momentum
• When they collide with electron, they change its velocity
• Thus position is known but velocity becomes uncertain
• This proves uncertainty principle

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4. MCQs (Multiple Choice Questions)

Q1. Uncertainty principle is related to:
A. Bohr model
B. Wave nature only
C. Wave-particle duality
D. Nuclear stability
Ans: C

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Q2. If position is known accurately, then momentum will be:
A. Accurate
B. Zero
C. Uncertain
D. Constant
Ans: C

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Q3. Mathematical form of uncertainty principle is:
A. Δx + Δp ≥ h
B. Δx · Δp ≥ h/4π
C. Δx = Δp
D. p = mv
Ans: B

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Q4. Who discovered uncertainty principle?
A. Bohr
B. Rutherford
C. Heisenberg
D. Planck
Ans: C

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Q5. The uncertainty principle is a result of:
A. Classical mechanics
B. Quantum mechanics
C. Newton’s laws
D. Thermodynamics
Ans: B

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5. Assertion and Reason Questions

Q1.
Assertion (A): Electron follows a fixed circular path in atom.
Reason (R): Position and velocity of electron can be measured simultaneously.

Ans:
A is false, R is false

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Q2.
Assertion (A): Electron has no fixed trajectory.
Reason (R): It is impossible to measure position and momentum simultaneously.

Ans:
A is true, R is true, R correctly explains A

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Q3.
Assertion (A): Using high-energy photons improves accuracy of position.
Reason (R): High-energy photons disturb electron motion.

Ans:
A is true, R is true, but R does not explain A correctly

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6. Fill in the Blanks

1. Uncertainty principle was proposed by __________.
   Ans: Werner Heisenberg

2. Δx represents uncertainty in __________.
   Ans: position

3. Δp represents uncertainty in __________.
   Ans: momentum

4. Uncertainty principle is based on __________ nature of matter.
   Ans: wave-particle dual

5. Electron has __________ path in atom.
   Ans: no fixed

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7. Match the Column

Column A	Column B
1. Δx	a. Momentum uncertainty
2. Δp	b. Position uncertainty
3. Heisenberg	c. Principle
4. h	d. Planck’s constant

Ans:
1-b
2-a
3-c
4-d

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8. Statement Based Questions

Q1. Statement 1: Electron position and momentum cannot be known together accurately.
Statement 2:** Measurement disturbs the electron.

Ans: Both are correct and related

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Q2. Statement 1: Electron follows a fixed orbit like planets.
Statement 2:** Uncertainty principle allows exact trajectory.

Ans: Both are incorrect

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9. Case Study Based Question

Passage:
An experiment is performed to locate an electron using high-energy radiation. The photon collides with electron and changes its velocity.

Questions:

Q1. Why is high-energy radiation used?
Ans: To improve accuracy of position measurement

Q2. What happens after collision?
Ans: Electron velocity changes

Q3. What principle is illustrated?
Ans: Heisenberg’s Uncertainty Principle

Q4. Can both position and velocity be known accurately?
Ans: No

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Heisenberg’s Uncertainty Principle (NEET Notes)

Introduction

• Proposed by Werner Heisenberg in 1927.
• Based on dual nature of matter and radiation.
• Important concept in Structure of Atom.

Statement

• It is impossible to measure simultaneously the exact position and exact momentum (or velocity) of an electron.
• If one is known accurately, the other becomes uncertain.

Mathematical Form

Δx × Δpₓ ≥ h / 4π
OR
Δx × Δvₓ ≥ h / (4πm)

• Δx = uncertainty in position
• Δpₓ = uncertainty in momentum
• Δvₓ = uncertainty in velocity
• h = Planck’s constant
• m = mass of electron

Important Points

• If position is measured accurately (Δx small), velocity becomes uncertain (Δv large).
• If velocity is known accurately (Δv small), position becomes uncertain (Δx large).
• Electron motion cannot be fixed in a definite path.
• We get a “fuzzy” picture of electron location.

Example (Analogy)

• Measuring paper thickness with an unmarked scale gives inaccurate results.
• Similarly, measuring electron position requires very small wavelength radiation.

Observation Effect

• High energy photons are used to locate electrons.
• These photons disturb the electron during measurement.
• So position is known but velocity changes.

Conclusion

• Uncertainty principle is a fundamental law of nature.
• It is not due to experimental error.
• It sets a natural limit on measurements at atomic level.