Magnetic Fields Flashcards
(42 cards)
What happens when a current flows through a wire?
It produces its own magnetic field.
What is the motor effect?
A wire carrying a current in an external magnetic field experiences a force.
What is the equation for the motor effect force?
F = BIL
Where: F: Force, B: Magnetic flux density (T), I: Current (A), L: Length of wire in the magnetic field (m)
What is magnetic flux density and its unit?
Magnetic field strength, measured in tesla (T).
Alternate unit: N A⁻¹ m⁻¹
Under what condition does the equation F = BIL apply directly?
Only when the current and magnetic field lines are perpendicular.
What happens if the current is parallel to the magnetic field lines?
The wire experiences no force.
How do you determine the direction of the force on a current-carrying wire?
Use Fleming’s Left Hand Rule:
Thumb: Force
First finger: Magnetic Field (N to S)
Middle finger: Current
How can you measure the size of the force in the motor effect practically?
Place the magnet on a balance and measure the change in mass (due to Newton’s Third Law), then calculate force.
How can you calculate the magnetic flux density in a setup?
Measure current, wire length, and force (from mass), then use B = F / (IL).
What is the force on a free charged particle in a magnetic field?
F = Bqv
What is the equation for force on an electron or proton moving perpendicular to a magnetic field?
F = Bev
How is Fleming’s Left Hand Rule used with charged particles?
Use middle finger for velocity.
Flip the direction for negative charges (e.g., electrons).
What motion does a charged particle exhibit in a magnetic field?
Circular motion, since the force is always perpendicular to the velocity.
What is the equation linking magnetic force and circular motion?
Bqv = mv² / r or Bqv = mω²r
What is the expression for the radius of a particle’s circular path?
r = mv / (Bq)
Radius ∝ mass and speed; inversely ∝ magnetic flux density and charge.
How is frequency related to the particle’s motion in a magnetic field?
v = ωr
ω = 2πf
Thus: f = (Bq) / (2πm)
What does f = (Bq) / (2πm) imply about the motion of particles?
The frequency is independent of radius; all particles with the same mass and charge will orbit at the same frequency in a given B field.
What is a cyclotron and what is it used for?
A machine that uses F = Bev to accelerate particles like protons for applications like medical therapy.
Describe the structure of a cyclotron.
Two hollow metal D-shaped electrodes with a magnetic field perpendicular to the plane of the D’s and a variable PD across the gap.
How are particles accelerated in a cyclotron?
Magnetic field causes circular motion.
PD across the D’s accelerates particles across the gap.
PD reverses every half-cycle so particles keep accelerating.
Why does the radius of the particle’s path increase in a cyclotron?
As particles gain speed from repeated acceleration, r = mv / (Bq) increases.
Why must the PD of the D’s alternate at a specific frequency?
So it matches the particle’s orbital frequency f = (Bq) / (2πm), ensuring particles are accelerated correctly each half turn.
What happens to the particle beam in a cyclotron?
The particles spiral outward as their speed and radius increase, forming a high-energy beam.
Why doesn’t the cyclotron frequency need to be double the particle frequency?
Because one full cycle involves two flips; particles cross the gap twice per cycle, matching the flipping frequency.
