An electron moving in a straight line enters into a non-zero magnetic field and goes undeviated. The angle between the direction of its motion and magnetic field is

The correct answer is 0º.

Key Points

• An electron moving in a straight line in a magnetic field remains undeviated if the direction of motion is parallel to the magnetic field.
• The angle between the velocity vector of the electron and the magnetic field vector is 0º.
• When the motion of the charged particle is parallel to the magnetic field, the magnetic force acting on it becomes zero.
• Magnetic force is given by the equation: F = q(v × B), where F is the magnetic force, q is the charge, v is the velocity, and B is the magnetic field.
• The cross product in the equation implies that the force is zero when the angle between v and B is 0º or 180º.

Additional Information

• Magnetic Force on Charged Particles:
  • The magnetic force acts perpendicular to both the velocity of the charged particle and the magnetic field.
  • If the angle between the velocity vector and magnetic field is 90º, the force is maximum, causing circular or helical motion.
  • If the angle is 0º or 180º, the force is zero, and the particle moves in a straight line.
• Right-Hand Rule for Magnetic Force:
  • The direction of the magnetic force can be determined using the right-hand rule: point your fingers in the direction of velocity, curl them towards the magnetic field, and the thumb points to the direction of the force.
  • This rule helps visualize the perpendicular nature of the force.
• Cross Product in Physics:
  • The cross product of two vectors gives a vector perpendicular to the plane containing the two original vectors.
  • In the case of magnetic force, the cross product ensures that the force is zero when the two vectors are parallel or anti-parallel.
• Lorentz Force:
  • The total force acting on a charged particle moving in an electromagnetic field is called Lorentz force.
  • Lorentz force is the sum of electric force (F = qE) and magnetic force (F = q(v × B)).
  • This force governs the motion of charged particles in combined electric and magnetic fields.