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Magnetism
Day 2Charged Particles Moving Through Magnetic Fields
When a charged particle travels through a magnetic field, it “might” experience a force.
When will it?
When won’t it?
When a component of the magnetic field is perpendicular to the particle’s velocity.
When the velocity of the particle and the velocity are either PARALLEL or ANTI-PARALLEL.
Right Hand Rule #1 (or RHR-1):
Finding the direction of the force exerted on a positive charge moving through a magnetic field
Charged particle’s sign: +
Motion direction: East
Dir. of Magnetic Field: North
Dir. of Magnetic Force
Charged particle’s sign: -
Motion direction: East
Dir. of Magnetic Field:
Dir. of Magnetic Force South
Charged particle’s sign: -
Motion direction: North
Dir. of Magnetic Field: Downward
Dir. of Magnetic Force
Charged particle’s sign:
Motion direction: South
Dir. of Magnetic Field: West
Dir. of Magnetic Force Upward
Some Examples
More ExamplesCharged particle’s sign: -
Motion direction: West
Dir. of Magnetic Field: East
Dir. of Magnetic Force
Charged particle’s sign: +
Motion direction: Upward
Dir. of Magnetic Field: West
Dir. of Magnetic Force
Charged particle’s sign:
Motion direction: South
Dir. of Magnetic Field: Upward
Dir. of Magnetic Force West
Charged particle’s sign: +
Motion direction:
Dir. of Magnetic Field: South
Dir. of Magnetic Force East
OK, so RHR-1 gives us the direction of the force. But since force is a
vector, can we find the MAGNITUDE of the force?
qvBFmagnetic
Magnetic Field Strength (measured in Teslas, T)
Velocity of the particle (m/s)
Charge of the particle (C)
NOTE: This equation is only valid if B and v are perpendicular to each other!!!
What exactly is a Tesla?
• 1 T = _____________
• 1T is REALLY big. So we often us prefixes like mT, T, or nT.
• The earth’s magnetic field is soooooo small that we often use a term called a GAUSS (G) to measure it.
• 1 G = 0.1mT (and the earth’s magnetic field is ~ .1-.3 G)
€
1N /(C⋅ ms )
But what if v and B are NOT perpendicular?
sinqvBFmagnetic
Magnetic Field Strength (measured in Teslas, T)
Velocity of the particle (m/s)
Charge of the particle (C)
The angle between v
and B
sinqvBFmagnetic
What if = 0? F = ____
What if = 90? F = ____
What if = 180? F = ____0
0
qvB
What would happen if ……
What would you have to do to make the path helical?
+ -
qB
mvr
r
mvqvBFF
r
mvFqvBF
cmag
cmagnetic
2
2
Mass Spectrometer
Only ions with a certain speed make it through….thus, if an ion makes it through, it must have the same speed as all the other ions that make it through.
Use the radius of the motion to find the mass.
qB
mvr
