Click here to go to the applet.

This applet is an electrostatics demonstration which displays the electric 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 charged particles). This helps you visualize the field.

When you start the applet, you will see 500 particles moving in a point charge field. By default the particles are treating the field as a velocity field, which means that the field vectors determine how fast the particles are moving and in what direction. In this case, the particles just move toward the center. The velocity of all the particles at a certain point on the grid is always the same. If the field is a force field, then the field vectors determine the acceleration of the particles, but their velocity may vary depending on where they started.

The Field Selection popup will allow you to select a vector field. The choices are:

• point charge: This is the field of a single point of charge. The Reverse checkbox will change the sign of the charge.
• point charge double: Two point charges, with an adjustable separation between them.
• dipole: Two points with opposite charges, with an adjustable separation between them.
• quadrupole: Two positive charges and two negative charges.
• charged line: Charge evenly distributed along an infinitely long line.
• line charge double: Two charged lines.
• dipole lines: Two lines of opposite charge.
• quad lines: Two positive lines and two negative lines.
• finite line: A charged line of finite length. The length is adjustable.
• finite line pair: Two charged lines of finite length.
• finite line dipole: Two oppositely charged lines of finite length.
• conducting plate: A conducting plate, finite in the x direction but infinitely long in the y direction. The plate size (in the x direction) is adjustable.
• charged plate: An evenly charged plate, finite in the x direction but infinitely long in the y direction. The plate size (in the x direction) is adjustable.
• charged plate pair: Two evenly charged plates with opposite charges. The size and separation of the two sheets are adjustable.
• infinite plane: An infinite plane, evenly charged.
• conducting sphere + pt: A conducting sphere near a point charge. The size of the sphere, the separation between it and the point charge, and the potential of the sphere are all adjustable. By default the sphere is grounded.
• charged sphere + pt: A charged sphere near a point charge. This is provided to show the difference between a charged sphere and a conducting sphere. (The main difference is that the electric field lines are always perpendicular to the surface of the conducting sphere, whereas this is not true with a charged sphere. This is easier to see with a Y Slice.) By default the sphere has no charge, but this can be adjusted to a positive or negative value.
• cyl + line charge: A conducting cylinder near a line charge.
• conducting sphere in field: A grounded conducting sphere in a uniform external field.
• dielec sphere in field E: This is the electric field of a dielectric sphere in a uniform external field. The size of the sphere and the dielectric strength are adjustable. A dielectric is an insulating material whose atoms are polarized in response to an external field; this causes the field to be weaker inside the dielectric.
• dielec sphere in field D: This is the electric displacement vector of a dielectric sphere in a uniform external field. The size of the sphere and the dielectric strength are adjustable.
• cylinder in field: A grounded conducting cylinder in a uniform external field.
• dielec cyl in field E: This is the electric field of a dielectric cylinder in a uniform external field. The size of the cylinder and the dielectric strength are adjustable.
• dielec cyl in field D: This is the electric displacement vector of a dielectric cylinder in a uniform external field. The size of the cylinder and the dielectric strength are adjustable.
• dielec boundary E: This is the electric field of a point charge near a dielectric boundary. The point charge is located outside of the dielectric by default; so the dielectric is the area below the boundary plane. The location of the point charge and the dielectric strength are adjustable.
• dielec boundary D: This is the electric displacement vector of a point charge near a dielectric boundary. The location of the point charge and the dielectric strength are adjustable.
• conducting plane + pt: This is the electric field of a point charge near a conducting boundary.
• fast charge: This is the electric field of a point charge moving at a rate comparable to the speed of light. This causes the field to be distorted. The ratio between the speed of the particle and the speed of light is adjustable.
• charged ring: This is an evenly charged ring. The size of the ring is adjustable.
• charged ring pair: This is two evenly charged rings.
• charged ring dipole: Two oppositely charged rings
• slotted conducting plane: This is a grounded conducting plane with a rectangular slot cut in it. The plane is placed in a uniform external field. The slot is infinitely long, but finitely wide, and the width is adjustable.
• conducting planes w/ gap: Two infinite conducting planes, one at positive potential and one at negative potential, with a gap between them.

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 field vectors 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 (Force) means charged particles will move through the field, and the field vectors determine their acceleration.
• Display: Field Vectors shows you the field vectors 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: Equipotentials shows you equipotential surfaces. By default slicing is turned on because the equipotentials are easier to see that way, and you can see more than one at a time. Red means negative potential, green means positive, gray means ground. If you turn slicing off, you can only see one equipotential at a time; the Potential slider allows you to determine which one to view. If the slider is set too high or too low then the corresponding equipotential surface may not be visible or may not exist, so just play around with the slider a bit if you don't see anything.

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 Kick button can be used to give all the particles a random acceleration in some direction. This is only allowed if the particle movement is set to "Force". It can be useful if none of the particles are moving, or if they are all moving in the same direction.

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.

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.

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