Current Electricity
Electric Potential Difference
Perhaps one of the most useful yet taken-for-granted
accomplishments of the recent centuries is the development of electric
circuits. The flow of charge through wires allows us to cook our food, light
our homes, air-condition our work and living space, entertain us with movies
and music and even allows us to drive to work or school safely. In this unit of
The Physics Classroom, we will explore the reasons for why charge flows through
wires of electric circuits and the variables that affect the rate at which it
flows. The means by which moving charge delivers electrical energy to
appliances in order to operate them will be discussed in detail.
One of the fundamental principles that must be
understood in order to grasp electric circuits pertains to the concept of how
an electric field can influence charge within a circuit as it moves from one location to another. The concept of
electric field was first introduced in the unit on Static Electricity. In that unit, electric force was described as a non-contact force. A
charged balloon can have an attractive effect upon an oppositely charged
balloon even when they are not in contact. The electric force acts over the
distance separating the two objects. Electric force is an action-at-a-distance
force.
Action-at-a-distance forces are sometimes
referred to as field forces. The concept of a field force is
utilized by scientists to explain this rather unusual force phenomenon that
occurs in the absence of physical contact. The space surrounding a charged
object is affected by the presence of the charge; an electric field is
established in that space. A charged object creates an electric field - an
alteration of the space or field in the region that surrounds it. Other charges
in that field would feel the unusual alteration of the space. Whether a charged
object enters that space or not, the electric field exists. Space is altered by
the presence of a charged object; other objects in that space experience the
strange and mysterious qualities of the space. As another charged object enters
the space and moves deeper and deeper into the
field, the effect of the field becomes more and more noticeable.
Electric field is a vector quantity whose
direction is defined as the direction that a positive test charge would be
pushed when placed in the field. Thus, the electric field direction about a
positive source charge is always directed away from the positive source. And
the electric field direction about a negative source charge is always directed
toward the negative source.
Electric fields are similar to gravitational fields - both
involve action-at-a-distance forces. In the case of gravitational fields, the
source of the field is a massive object and the action-at-a-distance forces are
exerted upon other masses. When the concept of the force of gravity and energy was
discussed in Unit 5 of the Physics Classroom, it was
mentioned that the force of gravity is an internal or conservative force. When
gravity does work upon an object to move it from a high location to a lower
location, the object'stotal amount
of mechanical energy is conserved. However, during
the course of the falling motion, there was a loss of potential energy (and a
gain of kinetic energy). When gravity does work upon an object to move it in
the direction of the gravitational field, then the object loses potential
energy. The potential energy originally stored within the object as a result of
its vertical position is lost as the object moves under the influence of the
gravitational field. On the other hand, energy would be required to move a
massive object against its gravitational field. A stationary object would not
naturally move against the field and gain potential energy. Energy in the form
of work would have to be imparted to the object by an external force in order
for it to gain this height and the corresponding potential energy.
The important point to be made by this gravitational analogy
is that work must be done by an external force to move an object against nature
- from low potential energy to high potential energy. On the other hand,
objects naturally move from high potential energy to low potential energy under
the influence of the field force. It is simply natural for objects to move from
high energy to low energy; but work is required to move an object from low
energy to high energy.
In a similar manner, to move a charge in an
electric field against its natural direction of motion would require work. The
exertion of work by an external force would in turn add potential energy to the
object. The natural direction of motion of an object is from high energy to low
energy; but work must be done to move the object against nature. On the other hand, work would not be required to move an object from a
high potential energy location to a low potential energy location. When this
principle is logically extended to the movement of charge within an electric
field, the relationship between work, energy and the direction that a charge
moves becomes more obvious.
Consider the diagram above in which a positive
source charge is creating an electric field and a positive test charge being
moved against and with the field. In Diagram A, the positive test charge is
being moved against the field from location A to location B. Moving the charge
in this direction would be like going against nature. Thus, work would be
required to move the object from location A to location B and the positive test
charge would be gaining potential energy in the process. This would be analogous
to moving a mass in the uphill direction; work would be required to cause such
an increase in gravitational potential energy. In Diagram B, the positive test
charge is being moved with the field from location B to location A. This motion
would be natural and not require work from an external force. The positive test
charge would be losing energy in moving from location B to location A. This
would be analogous to a mass falling downward; it would occur naturally and be
accompanied by a loss of gravitational potential energy. One can conclude from
this discussion that the high energy location for a positive test charge is a
location nearest the positive source charge; and the low energy location is
furthest away.
The above discussion pertained to moving a positive
test charge within the electric field created by a positive source charge. Now
we will consider the motion of the same positive test charge within the
electric field created by a negative source charge. The same principle
regarding work and potential energy will be used to identify the locations of
high and low energy.
In Diagram C, the positive test charge is
moving from location A to location B in the direction of the electric field.
This movement would be natural - like a mass falling towards Earth. Work would
not be required to cause such a motion and it would be accompanied by a loss of
potential energy. In Diagram D, the positive test charge is moving from
location B to location A against the electric field. Work would be required to
cause this motion; it would be analogous to raising a mass within Earth's
gravitational field. Since energy is imparted to the test charge in the form of
work, the positive test charge would be gaining potential energy as the result
of the motion. One can conclude from this discussion that the low energy
location for a positive test charge is a location nearest a negative source
charge and the high energy location is a location furthest away from a negative
source charge.
As we begin to discuss circuits, we will apply
these principles regarding work and potential energy to the movement of charge
about a circuit. Just as we reasoned here, moving a positive test charge
against the electric field will require work and result in a gain in potential
energy. On the other hand, a positive test charge will naturally move in the
direction of the field without the need for work being done on it; this
movement will result in the loss of potential energy. Before making this
application to electric circuits, we need to first explore the meaning of the
concept electric potential.