Chapter 12.2 - Charged particles in an external magnetic field

Magnetic force

An attraction or repulsion felt by an appropriate object in a magnetic field

• “Action at a distance” force —> no - contact force

• Vector; use 3rd hand rule to find direction

The inficed magnetic field AROUND THE moving particle will interact with the external magnetic field, resulting in a _____

Force

• a charge that moves perpendicular to a magnetic field will experience a force

Motion, field and force are all ______

Mutually perpendicular

A magnetic field

A charged particle moving _______ to an external magnetic field will experience a PERPENDICULAR magnetic force.

Perpendicular

Third hand rule

Indicates the direction of the FORCE when a CHARGED particle enters a field

Fingers

• direction of the external magn

To determine the deflecting force on a charged particle in an external magnetic field:

Fm = qvBsin(theta)

• Fm = deflecting force FROM the magnetic field (N)

• q = charge of moving particle (C)

• v = velocity of moving particle (m/s)

• B = magnetic field strength (T)

What if a charge is stationary?

It will NOT experience a force (v=0)

What if the initial motion of a charged particle is parallel to the external magnetic field?

There is NO effect

What if the initial motion of the particle is PERPENDICULAR to the external magnetic field?

The charge is deflected in a circle

What if the initial motion is at an angle to the external magnetic field?

The deflection is a helical path

Opposite charges deflect in _______ directions

Opposite

If it deflects in a circle:

Fm = Fc

qvB = mv² / r

Velocity selector

A device for measuring the speed of a charged particle

• if the magnetic and electric forces are EQUAL AND OPPOSITE, the net force on the charged particle is 0, resulting in the particle moving UNDEFLECTED

To determine the velocity of a charged particle in a velocity selector

Fe=Fb

Eq = qvB

v = E

Mass spectrometer

Makes use of the combination of centripetal motion and magnetic deflection of charged particles

• Separates particles according to their masses

• The lighter the isotope, the smaller the circle

3 parts of mass spectrometer

1) Ion generator and accelerator

2) Velocity Selector

3) Ion Separator/Detector

Ion generator and accelerator

Atoms are ionized either by extreme heating or by electrical discharge. The ions are accelerated thought a potential difference where they gain kinetic energy

• Ek=qV

Velocity Selector

Composed of a uniform (B) and an electric field which are PERPENDICULAR to each other. The ion experience a Fm and a Fe in OPPOSITE directions.

If forces are balanced:

• Fe = Fm

  Eq= qvB

• v=E/B

Ion separator

Deflects the ions in a circular path

• the radius of the circular path can be measured, and the mass of the isotope is calculated

• Fm = Fc

  qvB = mv²/r