Field Forces

Magnetic Fields and Force on a Moving Charge

By Rachel Wampler

Multiple Choice

A particle with a charge of -1.6×10^-19 C and mass 9.11x10^-31 kg moves along the positive x-axis from left to right.  It enters a 3 T B-field is in the x-y plane and points at 45° above the positive x-axis.

What is the magnitude of the force on the particle?

• 7.86x10^-17 N

• 923.2 N

• 3.58x10^-17 N

• 465.7 N

Magnetic Field

• Around a magnet is a magnetic field (B-field)

  • At every point in space there is a magnetic force

  • Earth's magnetic field can be visialized with a compass

  • Unit for magnetic field: Tesla (T)

• Magnets can be visualized with field lines​.

  • Start at N pole and ends at S pole

  • More lines in one area means a stronger field. ​

Magnetic Field

Right Hand Rule

• Direction of a force on a positive charge can be determined with the right hand rule

  • Fingers point in the direction of the B-field

  • Thumb in the direction of v

  • Palm faces direction of F on the positive charge.

• Force will be zero if B and v are parallel, so a moving charge will be unaffected. ​

Motion of moving charged particle in a Uniform B-field

Applications

• Bubble Chamber

  • Used to determine relative mass to speed and charge ratios of particles.​

• Mass Spectrometer

  • Determines relative quantities of various masses in a substance by taking very small particles of the substance and charging them. 

  • The charged particles are sent through a uniform magnetic field. 

  • The radius of their path tells you their mass.​

Multiple Choice

A particle with a charge of -1.6×10^-19  C and mass 9.11x10^-31 kg moves along the positive x-axis from left to right.  It enters a 3 T B-field is in the x-y plane and points at 45° above the positive x-axis.

What is the direction of the force on the particle?

• -Y

• +X

• +Y

• -Z

Multiple Choice

A particle with a charge of -1.6×10^-19 C and mass 9.11x10^-31 kg moves along the positive x-axis from left to right.  It enters a 3 T B-field is in the x-y plane and points at 45° above the positive x-axis.

After it has been in the B-field, the particle moves in a circle. If the radius of its path is 2x10^-10 m, what is the speed of the particle?

• 3.0x10⁸ m/s

• 105.4 m/s

• 731.9 m/s

• 4.31 m/s