20.1 Millikan’s Oil Drop Experiment
- Purpose: To measure the charge of an electron (e).
- Idea: Oil drops fall under gravity; by applying electric field, drop is balanced → net force = 0.
- Observation: Charge on oil drops is always an integer multiple of a smallest value = e = 1.6 × 10⁻¹⁹ C.
20.2 Motion of Electron Beam in Electric and Magnetic Fields
- In Electric Field: Electron deflects towards positive plate (since it is negatively charged).
- In Magnetic Field: Force acts perpendicular to velocity → electron moves in circular/helical path.
- Crossed Fields (E ⊥ B): At a certain velocity v=E/B, electron passes undeviated.
20.3 Thomson’s Experiment to Determine Specific Charge (e/m)
- Setup: Cathode ray tube with parallel electric and magnetic fields.
- Condition for no deflection: v=E/B.
- Deflection measured → gives velocity of electrons.
- Result: Specific charge (e/m) = 1.76 × 10¹¹ C/kg.
Short Questions and Answers in Short
Q1. What was concluded from Millikan’s oil drop experiment?
👉 Charge on electron is quantized (smallest unit = e).
Q2. Why does an electron move in circular path in magnetic field?
👉 Because force is always perpendicular to velocity.
Q3. Write condition for electron beam to pass undeviated in crossed fields.
👉 v=E/B.
Q4. What value of e/m was obtained by Thomson?
👉 1.76 × 10¹¹ C/kg.
Q5. What is the principle of Millikan’s oil drop experiment?
👉 Balance of electric force and gravitational force on oil drop.
Q6. Why did Millikan use oil drops instead of water drops?
👉 Oil has low vapor pressure → does not evaporate easily.
Q7. What does the quantization of charge mean?
👉 Charge exists only in integral multiples of e.
Q8. Write expression for radius of electron’s circular path in a magnetic field.
👉 r=mv/eB
Q9. What is the direction of force on an electron moving in magnetic field?
👉 Perpendicular to both velocity and magnetic field (given by Fleming’s Left-Hand Rule).
Q10. How can velocity of electron be measured using crossed fields?
👉 By adjusting fields until no deflection → v= E/B
Q11. Mention one limitation of Thomson’s method.
👉 It measures e/m, not the charge (e) or mass (m) separately.
Q12. Why is electron beam deflected more in electric field than magnetic field?
👉 Because electron is very light → acceleration in electric field is high.
Numerical Examples
✅ Formula Sheet (Quick Revision)
