Stoke's Law

• What is the purpose of Millikan's experiment?

  To determine the charge of oil droplets
• Why was the mass of each oil droplet required in Millikan's experiment?

  To calculate charge using stationary conditions
• What happens to oil droplets when the electric field is turned off?

  They fall at terminal velocity
• What does terminal velocity indicate about the forces acting on the oil droplet?

  Weight is balanced by drag forces

  
• How did Millikan determine the speeds of the oil drops?

  By timing them as they passed a window
• What is the significance of the resultant force at terminal velocity?

  Resultant force is zero
• What does Stokes' Law allow us to calculate?

  Drag force on an object through a fluid
• What are the conditions for applying Stokes' Law?

  • The object is small
  • The object is spherical
  • The speed is low
• What is the formula for viscous drag force according to Stokes' Law?

  F = 6πηrv
• What is the relationship between drag force and weight at terminal velocity?

  Drag force equals weight
• How can mass be expressed in terms of radius for a sphere?

  m = ρV = ρ$\frac{4\pi r^3}{3}$
• What does the equation QV/d = mg represent in Millikan's experiment?

  Condition for a stationary oil drop
• What pattern did Millikan observe in the charges of different oil droplets?

  Charges were integer multiples of $1.60 \times 10^{-19} C$
• What does the quantization of electric charge imply?

  • Each droplet carries an integer number of electrons
  • Charge is quantized into multiples of $1.60 \times 10^{-19} C$
• How can the mass of a droplet be expressed using Stokes' Law and other variables?

  m = $\frac{6\pi \eta v}{g}$
• What is the final expression for the droplet's radius derived from Stokes' Law?

  r = $\frac{9\eta v}{2\rho g}$
• What are the steps to derive the mass of a droplet using Stokes' Law?

  1. Write Stokes' law: F = 6πηrv
  2. Set mg = 6πηrv at terminal velocity
  3. Express mass in terms of radius and density: m = $\frac{4\pi r^3 \rho}{3}$
  4. Substitute and rearrange to eliminate radius
  5. Final expression: m = $\frac{6\pi \eta v}{g}$
• What should you be cautious about in "Show that" questions?

  Be thorough and logical in working out