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Current :
Charge is mobile and can flow freely in certain materials, called
conductors. Metals and a few other elements and compounds are
conductors. Materials that charge cannot flow through are called
insulators. Air, glass, most plastics, and rubber are insulators,
for example. And then there are some materials called
semiconductors, that seemed to be good conductors sometimes but much
less so other times. Silicon and germanium are two such materials.
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There is a force between them, the potential for work, and thus a
voltage. Now we connect a conductor between them, a metal wire. On
the positively charged sphere, positive charges rush along the wire
to the other sphere, repelled by the nearby similar charges and
attracted to the distant opposite charges. The same thing occurs on
the other sphere and negative charge flows out on the wire. Positive
and negative charges combine to neutralize each other, and the flow
continues until there are no charge differences between any points
of the entire connected system.
There may be a net residual charge
if the amounts of original positive and negative charge were not
equal, but that charge will be distributed evenly so all the forces
are balanced. If they were not, more charge would flow. The charge
flow is driven by voltage or potential differences. After things
have quieted down, there is no voltage difference between any two
points of the system and no potential for work. All the work has
been done by the moving charges heating up the wire.
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Fig - 1 :
Two spheres with opposite
charges are connected by a conductor, allowing charge to flow
The flow of charge is called electrical current. Current is measured
in amperes (a), amps for short (named after another French scientist
who worked mostly with magnetic effects).
An ampere is defined as a
flow of one Coulomb of charge in one second past some point. While a
Coulomb is a lot of charge to have in one place, an ampere is a
common amount of current; about one ampere flows through a 100 watt
incandescent light bulb, and a stove burner or a large motor would
require ten or more amperes. On the other hand low power digital
circuits use only a fraction of an ampere, and so we often use units
of 1/1000 of an ampere, a milliamp, abbreviated as ma, and even
1/1000 of a milliamp, or a microamp, µa . |
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The currents on the RoboBoard are generally in the milliamp range, except for the
motors, which can require a full ampere under heavy load. Current
has a direction, and we define a positive current from point A to B
as the flow of positive charges in the same direction. Negative
charges can flow as well, in fact, most current is actually the
result of negative charges moving. Negative charges flowing from A
to B would be a negative current, but, and here is the tricky part,
negative charges flowing from B to A would represent a
positive current from A
to B .
The net effect is the same: positive charges flowing to
neutralize negative charge or negative charges flowing to neutralize
positive charge; in both cases the voltage is reduced and by the
same amount.
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