Plasma on the surface of the sun traces out the immense magnetic field lines.
The electromagnetic field has two parts, the electric field and the magnetic field (also called the B field).
We can begin to understand these fields through their respective forces.
The electric field is an interaction between charged particles.
The magnetic field is an interaction between moving charged particles.
Moving charged particles create a magnetic field. Moving through a magnetic field changes the motion of charged particles.
The force exerted on a charged particle by a magnetic field is called the Lorentz force.
A magnetic field affects the motion of charged particles, which in turn affects the magnetic field,
which affects the motion of the particles, which affects the field, which affects the particles...
A single charged particle is called a monopole and has radial electric field lines,
where a pair of oppositely charged particles is a dipole with dipole E field lines.
Magnetic field lines give information about the magnetic field similar to electric field lines.
Field lines in both cases are mathematic constructs illustrating the properties of the field.
The field is stronger where the lines are closer together, and the lines are labeled with arrows to indicate
the direction of the vector field.
Note that the direction of the magnetic field of a ferromagnet like this bar magnet point from North to South
outside the magnet.
Iron filings around a bar magnet show the magnetic field lines.
Compases are just magnets that are free to turn to align with the magnetic field.
Magnetic monopoles do not exist. Every magnetic field has at least a dipole structure.
The magnetic field of two bar magnets is similar to two electric dipoles placed near each other.
Earth's magnetic field arises from the rotation of its molten metal outer core. Although it is actually very complex,
we often approximate it as a bar magnet. Note that the north magnetic pole is actually the south pole of the "bar magnet."
The magnetic field of a permanent magnet, like a bar magnet,
is caused by magnetic domains in which the spins of electrons are aligned.
Lorentz force
A magnetic field exerts a force on a charged particle that is perpendicular to both the velocity of the particle
and the direction of the magnetic field. The Lorentz force is a cross product, and so obeys the right-hand rule.
A simple motor can illustrate the effect of the Lorentz force.
Can you explain what makes this work?
Sample questions
1. What would be the direction of the force exerted on a proton moving as shown, by this magnetic field?
A. up
B. down
C. right
D. into the page
E. out of the page
2. What would be the direction of the force exerted on a proton moving as shown, by this magnetic field?
A. up
B. down
C. right
D. into the page
E. out of the page
3. What would be the direction of the force exerted on an electron moving as shown, by this magnetic field?
A. up
B. down
C. right
D. into the page
E. out of the page
4. What would be the direction of the force exerted on a proton moving as shown, by this magnetic field?
A. up
B. down
C. right
D. left
E. out of the page
5. What would be the direction of the force exerted on a proton moving as shown, by this magnetic field?
A. up
B. down
C. right
D. left
E. out of the page
Cyclotron motion
A charged particle moving in a plane perpendicular to a magnetic field feels a Lorentz force.
The Lorentz force is always perpendicular to the velocity, so it constantly deflects the particle sideways.
The particle moves in a circle at constant velocity. The force is pointed radially inward.
Since the force is perpendicular to the velocity, we can simply write the force as qvB, and relate it to centripetal force.
This allows us to easily find the radius of the circular path of a particle, and its frequency.
The radius and frequency depend on the ratio q/m, which is a good identifier for a specific type of particle.
Measuring the radius or frequency of a praticle in a cyclotron is a powerful tool in studying atomic and subatomic particles.
If the velocity of the particle also has a component parallel to the magnetic field, it will move in a spiral pattern.
