This applet is a magnetostatics
demonstration which displays the magnetic field in a number of situations. You
can select from a number of fields and see how particles move in the field if
it is treated as either a velocity field (where the particles move along the
field lines) or an actual force field (where the particles move as if they were
little magnets). This helps you visualize the field. You can also view the vector
potential (A).
When you start the applet, you will
see 500 particles moving in the field of a current line. By default the particles
are treating the field as a velocity field, which means that the magnetic
field vectors determine how fast the particles are moving and in what direction.
In this case, the particles follow the field lines around the current line.
Of course in real life, particles do not follow the field lines; magnetized
particles will align themselves with the field lines and then move closer to
the source of the field, whereas moving charged particles are accelerated in
a direction perpendicular to the field (and their velocity).
The Field Selection popup
will allow you to select a vector field. The choices are:
- current line: This is the
field of an infinitely long line of current.
- current line double: This
is two lines of current moving in the same direction.
- cur line double + ext:
This is two lines of current moving in the same direction, plus a uniform
external field.
- current line dipole: This
is two lines of current moving in opposite directions.
- cur line dipole + ext:
This is two lines of current moving in opposite directions, plus a uniform
external field. (In fluid dynamics this field is called the Lamb dipole.)
- uniform field: A uniform
magnetic field; the direction is adjustable by modifying the two angles theta
and phi.
- moving charge: The field
of a moving point charge.
- fast charge: The field
of a point charge moving close to the speed of light. The ratio between the
speed of the particle and the speed of light is adjustable.
- moving charge double: The
field of two moving point charges.
- moving charge dipole: The
field of two point charges moving in opposite directions.
- current loop: Current moving
in a circular loop. This field is equivalent to that of a flat disc magnet;
the north pole is at the top by default.
- loop pair: Current moving
in two circular loops (or two flat disc magnets). The size of the loops and
the separation are both adjustable. Also you can introduce a vertical offset
between the two loops.
- loop pair opposing: Two
circular loops with current moving in opposite directions. (Or two opposing
flat disc magnets.)
- loop pair stacked: Current
moving in two stacked circular loops. (Or two stacked disc magnets.)
- loop pair stacked, opp.:
Two stacked circular loops with current moving in opposite directions. (Or
two stacked disc magnets with their north poles pushed together.)
- concentric loops: Two concentric
circular loops with the current moving in opposite directions. The field is
the same as that of a flat ring magnet.
- solenoid: A coil of wire
with current running through it. (In real life the coil would need to be attached
to something in order to have current running through it, of course.) The
diameter of the coil, the height, and the number of turns are adjustable.
The field is the same as that of a magnetic rod (assuming a large number of
turns).
- toroidal solenoid: A coil
of wire wrapped around a torus (donut). Outside the torus, the field is fairly
weak.
- horseshoe electromagnet:
A coil of wire wrapped around half a torus.
- square loop: Current moving
in a square.
- corner: Current rounding
a corner.
- magnetic sphere: The field
of a magnetized sphere, with the north pole at the top. This is similar to
the earth's magnetic field, except upside down, because the earth's north
pole (even the "magnetic north" as opposed to the geographic north) is actually
its magnetic south pole.
- monopole attempt: This
is a simulation of what would happen if you tried to make a magnetic monopole
(a magnet with only a north pole and no south pole) by taking a bunch of square
magnets and forcing them into a cube with their south poles in the center.
Instead of getting a monopole, you would find that the gaps between the magnets
would act as south poles, and the field there would be much stronger than
on the faces of the magnets. If you could force them together perfectly with
no gaps, there would be no field at all anywhere because all the currents
would cancel out. (This field is so slow to compute that we display field
vectors by default instead of particles.)
The Display popup will allow
you to select how the field is displayed:
- Display: Particles (Vel.)
means particles will move through the field, with the magnetic field vectors
(B) determining their velocity. Note that the particles are only a educational
device intended to show what the field looks like; in real life, particles
would not move in this manner.
- Display: Particles (A Field,
Vel.) means particles will move, with the vector
potential (A) determining their velocity.
- Display: Field Vectors
shows you the field vectors at an array of locations.
- Display: Field Vectors (A)
shows you the vector potential at an array of locations.
- Display: Field Lines shows
you the field lines. The Line Density slider controls how many lines
to draw. The color indicates the field strength.
- Display: Parts (Magnetic)
means that magnetized particles (little current loops) will move through the
field in a realistic fashion. Particles are displayed as little arrows; their
north pole is at the head of the arrow and the south pole is at the tail.
(So, the arrow represents the magnetic moment vector.) They align themselves
with the field lines and then move closer to the source of the field. In the
case of a uniform field; they don't move; they just align themselves with
it. A fair amount of damping is used so that the particles don't oscillate
very much if they are out of alignment with the field.
- Display: Mag View Film
simulates the behavior of magnetic
viewing film. This requires slicing to be on.
The Mouse popup controls
what happens when you click on the box. If you set it to Adjust Angle
or Adjust Zoom, you can adjust the orientation or size of the 3-d view
by clicking and dragging on the box.
The Slice popup allows you
to look at planar slices of the box rather than looking at the contents of the
entire box. If the popup is set to No Slicing, you view the entire box.
Otherwise you will see the box sliced in one of three directions. The location
of the slice can be adjusted by dragging the line running along the sides of
box near the slice.
The Stopped checkbox will
stop the particles.
The Reverse checkbox will
reverse the direction of all the field vectors.
The Reset button can be used
to reset the positions of all the particles to random values.
The Field Strength slider
makes the field stronger or weaker, and also adjusts the brightness of the field
vectors if you have Display: Field Vectors selected.
The Vector Density slider
controls the number of vectors present if you have Display: Field Vectors
selected. It controls the resolution of the viewing paper if you have Display:
Mag View Film selected.
The Number of Particles slider
allows you to reduce the number of particles, which can be useful if you want
to watch the behavior of just a few of them. Also it might speed things up if
you have fewer particles.
A few additional field-specific
sliders may be present, depending on the field you have selected.
Autor
d'aquesta pàgina: Paul Faslstad. http://www.falstad.com.