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.
Click here to go to the applet.
java@falstad.com
A far higher place must be assigned to Judaism among the competitors for the allegiance of Europe. The cosmopolitan importance at one time assumed by this religion has been considerably obscured, owing to the subsequent devolution of its part to Christianity. It is, however, by no means impossible that, but for the diversion created by the Gospel, and the disastrous consequences of their revolt against Rome, the Jews might have won the world to a purified form of their own monotheism. A few significant circumstances are recorded showing how much influence they had acquired, even in Rome, before the first preaching of Christianity. The first of these is to be found in Cicero’s defence of Flaccus. The latter was accused of appropriating part of the annual contributions sent to the temple at Jerusalem; and, in dealing with this charge, Cicero speaks of the Jews, who were naturally prejudiced against his client, as a powerful faction the hostility of which he is anxious not to provoke.330 Some twenty years later, a great advance has been made. Not only must the material interests of the Jews be respected, but a certain conformity to their religious prescriptions is considered a mark of good breeding, In one of his most amusing satires, Horace tells us how, being anxious to shake off a bore, he appeals for help to his friend Aristius Fuscus, and reminds him of217 some private business which they had to discuss together. Fuscus sees his object, and being mischievously determined to defeat it, answers: ‘Yes, I remember perfectly, but we must wait for some better opportunity; this is the thirtieth Sabbath, do you wish to insult the circumcised Jews?’ ‘I have no scruples on that point,‘ replies the impatient poet. ‘But I have,’ rejoins Fuscus,—‘a little weak-minded, one of the many, you know—excuse me, another time.‘331 Nor were the Jews content with the countenance thus freely accorded them. The same poet elsewhere intimates that whenever they found themselves in a majority, they took advantage of their superior strength to make proselytes by force.’332 And they pursued the good work to such purpose that a couple of generations later we find Seneca bitterly complaining that the vanquished had given laws to the victors, and that the customs of this abominable race were established over the whole earth.333 Evidence to the same effect is given by Philo Judaeus and Josephus, who inform us that the Jewish laws and customs were admired, imitated, and obeyed over the whole earth.334 Such assertions might be suspected of exaggeration, were they not, to a certain extent, confirmed by the references already quoted, to which others of the same kind may be added from later writers showing that it was a common practice among the Romans to abstain from work on the Sabbath, and even to celebrate it by praying, fasting, and lighting lamps, to visit the synagogues, to study the law of Moses, and to pay the yearly contribution of two drachmas to the temple at Jerusalem.335 Jeff’s hand was quietly coming down. “What happened to you?” begged Sandy. “Something new has come up, sir. I was waiting there by my ship a good while back, and I heard another one cruising and spiraling, shooting the field, I guess, because he came in and set down. My crate, just the way you ordered, was down by the grove, not in plain sight in the middle of the course. But Jeff set his ship down, left the engine running, and went off. I stayed hid to see what would happen, but when he didn’t come back, I thought I’d better go and find you—and see if it meant anything to you.” She stood alone, with the sticky, wet knife in her hand, catching her breath, coming out of the madness. Then she stooped, and pushing the branches aside felt about for her pistol. It lay at the root of a tree, and[Pg 80] when she had picked it up and put it back in the holster, there occurred to her for the first time the thought that the shot in the dead stillness must have roused the camp. And now she was sincerely frightened. If she were found here, it would be more than disagreeable for Landor. They must not find her. She started at a swift, long-limbed run, making a wide detour, to avoid the sentries, bending low, and flying silently among the bushes and across the shadowy sands. The year 1756 opened with menaces to England of the most serious nature. The imbecility of the Ministry was beginning to tell in the neglect of its colonies and its defences. France threatened to invade us, and a navy of fifty thousand men was suddenly voted, and an army of thirty-four thousand two hundred and sixty-three of native troops; but as these were not ready, it was agreed to bring over eight thousand Hessians and Hanoverians. To pay for all this it was necessary to grant excessive supplies, and lay on new duties and taxes. In presenting the money bills in the month of May, Speaker Onslow could not avoid remarking that there were two circumstances which tended to create alarm—foreign subsidies and foreign troops introduced, and nothing but their confidence in his Majesty could allay their fears, or give them confidence that their burdens would be soon reduced. There was, in fact, no chance for any such reduction, for wars, troubles, and disgraces were gathering around from various quarters. The first reverse came from the Mediterranean. MUCH to their amazement, the boys waked up the next morning in Nashville, and found that they had passed through the "dark and bloody ground" of Kentucky absolutely without adventure. After drawing and dividing the rations and cartridges. Si gave the boys the necessary instruction about having their things ready so that they could get them in the dark the next morning, and ordered them to disregard the bonfires and mirth-making, and lie down to get all the sleep they could, in preparation for the hard work of the next day. Then, like the rest of the experienced men, who saw that the campaign was at length really on, and this would be the last opportunity for an indefinite while to write, he sat down to write short letters to his mother and to Annabel. "Bully for the Wild Wanderers of the Wabash," Shorty joined in. "They're the boss regiment in the army o' the Cumberland, and the Army o' the Cumberland's the boss army on earth. Hooray for US Co. Le's have a speech. Where's Monty Scruggs?" "Bring a light, do—I can't abide this dark." Albert suddenly began to look uneasy. After all he was not really drunk, only a little fuddled. He walked straight, and his roll was natural to him, while though he was exceedingly cheerful, and often burst into song, his words were not jumbled, and he generally seemed to have a fair idea of what he was saying. "But I heard what the doctor said to you." "A purty accident—wud them stacks no more dry than a ditch. 'Twas a clear case of 'bustion—fireman said so to me; as wicked and tedious a bit o' wark as ever I met in my life." Calverley stept from the shadow of the cliff, and beheld a meteor in the sky, brightening and expanding, as the clouds opened, until it assumed the appearance of a brilliant star, of astonishing magnitude, encircled by dazzling rays, which, in a singular manner, were all inclined in one direction, and pointing to that part of the horizon where lay the rival of England—France. The foreman's face assumed a deeper hue than usual: he looked fiercely at the galleyman, but there was a determination in the weather-beaten face that made him pause ere he spoke. "Galleyman," he at length said, "you knew the business before you came: if you be so fond of saving old witches' lives, why didn't you say so, that I might not now be in this dilemma?" "No, no, not the boy," replied Merritt, rather impatiently.