Fundamentals
Electric fields and forces
The force and conservation laws are
only two aspects of electromagnetism, however. Electric and magnetic forces
are caused by electromagnetic fields. The term field denotes a property of
space, so that the field quantity has a numerical value at each point of
space. These values may also vary with time. The value of the electric or
magnetic field is a vector--i.e., a quantity having both magnitude and
direction. The value of the electric field at a point in space, for example,
equals the force that would be exerted on a unit charge at that position in
space.
Every charged object sets up an
electric field in the surrounding space. A second charge "feels" the
presence of this field. The second charge is either attracted toward the
initial charge or repelled from it, depending on the signs of the charges.
Of course, since the second charge also has an electric field, the first
charge feels its presence and is either attracted or repelled by the second
charge, too.
The electric field from a charge is
directed away from the charge when the charge is positive and toward the
charge when it is negative. The electric field from a charge at rest is
shown in Figure 1
Electric fields. (Left) Field of a positive electric
charge; (right) field of a...for various locations in space. The arrows
point in the direction of the electric field, and the length of the arrows
indicates the strength of the field at the midpoint of the arrows.
If a positive charge were placed in
the electric field, it would feel a force in the direction of the field. A
negative charge would feel a force in the direction opposite the direction
of the field.
In calculations, it is often more
convenient to deal directly with the electric field than with the charges;
frequently, more is known about the field than about the distribution of
charges in space. For example, the distribution of charges in conductors is
generally unknown because the charges move freely within the conductor. In
static situations, however, the electric field in a conductor in equilibrium
has a definite value, zero, because any force on the charges inside the
conductor redistributes them until the field vanishes. The unit of electric
field is newtons per coulomb, or volts per metre.
The electric potential is another useful field. It provides an alternative
to the electric field in electrostatics problems. The potential is easier to
use, however, because it is a single number, a scalar, instead of a vector.
The difference in potential between two places measures the degree to which
charges are influenced to move from one place to another. If the potential
is the same at two places (i.e., if the places have the same voltage),
charges will not be influenced to move from one place to the other. The
potential on an object or at some point in space is measured in volts; it
equals the electrostatic energy that a unit charge would have at that
position. In a typical 12-volt car battery, the battery terminal that is
marked with a + sign is at a potential 12 volts greater than the potential
of the terminal marked with the - sign. When a wire, such as the filament of
a car headlight, is connected between the + and the - terminals of the
battery, charges move through the filament as an electric current and heat
the filament; the hot filament radiates light.
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