Specific Charge Experiments

• What is specific charge?

  Charge per unit mass of an object
• How is the specific charge of an electron expressed?

  $\frac{e}{m_e}$
• What methods can be used to determine specific charge?

  • A magnetic field only
  • A magnetic field and an electric field
  • An electric field only
• How can the radius of an electron beam's circular path be used?

  To calculate specific charge
• What is the magnetic force on a charge in a magnetic field?

  $F_B =$$Bev$
• How is work done on electrons related to their kinetic energy?

  $eVA =$$\frac{1}{2}m_ev^2$
• What is the formula for specific charge derived from kinetic energy?

  $\frac{e}{m_e} =$$\frac{2VA}{B^2r^2}$
• What apparatus did J.J. Thomson use to balance forces?

  Helmholtz coils
• What happens when the magnetic and electric forces are equal?

  The beam appears straight
• How can the electric field strength be varied?

  By changing the potential difference
• What is the expression for electron speed derived from electric field?

  $v =$$\frac{V}{Bd}$
• What happens when the electric field is switched off?

  The beam forms a circular path
• What is the combined expression for specific charge from Thomson's method?

  $\frac{e}{m_e} =$$\frac{Vr}{B^2d}$
• What is the specific charge of an electron?

  Magnitude of charge divided by mass
• What should you be prepared to do for the exam regarding specific charge methods?

  • Answer questions on all three methods
  • Perform calculations on all methods
  • Recall one method in detail