Describe a force field and calculate the strength of an electric field due to a point charge.
Calculate the force exerted on a test charge by an electric field.
Explain the relationship between electrical force (
F ) on a test charge and electrical field strength (
E ).
The information presented in this section supports the following AP® learning objectives and science practices:
2.C.1.1 The student is able to predict the direction and the magnitude of the force exerted on an object with an electric charge
q placed in an electric field
E using the mathematical model of the relation between an electric force and an electric field:
, a vector relation.
(S.P. 2.2)
2.C.1.2 The student is able to calculate any one of the variables – electric force, electric charge, and electric field – at a point given the values and sign or direction of the other two quantities.
(S.P. 2.2)
2.C.2.1 The student is able to qualitatively and semiquantitatively apply the vector relationship between the electric field and the net electric charge creating that field.
(S.P. 2.2, 6.4)
3.C.4.1 The student is able to make claims about various contact forces between objects based on the microscopic cause of those forces.
(S.P. 6.1)
3.C.4.2 The student is able to explain contact forces (tension, friction, normal, buoyant, spring) as arising from interatomic electric forces and that they therefore have certain directions.
(S.P. 6.2)
Contact forces, such as between a baseball and a bat, are explained on the small scale by the interaction of the charges in atoms and molecules in close proximity. They interact through forces that include the
Coulomb force . Action at a distance is a force between objects that are not close enough for their atoms to “touch.” That is, they are separated by more than a few atomic diameters.
For example, a charged rubber comb attracts neutral bits of paper from a distance via the Coulomb force. It is very useful to think of an object being surrounded in space by a
force field . The force field carries the force to another object (called a test object) some distance away.
Concept of a field
A field is a way of conceptualizing and mapping the force that surrounds any object and acts on another object at a distance without apparent physical connection. For example, the gravitational field surrounding the earth (and all other masses) represents the gravitational force that would be experienced if another mass were placed at a given point within the field.
In the same way, the Coulomb force field surrounding any charge extends throughout space. Using Coulomb's law,
, its magnitude is given by the equation
, for a
point charge (a particle having a charge
) acting on a
test charge
at a distance
(see
[link] ). Both the magnitude and direction of the Coulomb force field depend on
and the test charge
.
The Coulomb force field due to a positive charge
is shown acting on two different charges. Both charges are the same distance from
. (a) Since
is positive, the force
acting on it is repulsive. (b) The charge
is negative and greater in magnitude than
, and so the force
acting on it is attractive and stronger than
. The Coulomb force field is thus not unique at any point in space, because it depends on the test charges
and
as well as the charge
.
Questions & Answers
sound waves can be modeled as a change in pressure ,why is the change on in pressure used and not the actual pressure
I am a university student of Myanmar.I am first year,first semester.I want to learn about physics.
Maung
two charges qA and qB are separated by a distance x. if we double the distance between the charges and triple the magnitude of the charge A, what happens to the magnitude of the force that charge A exerts on charge B.
what happens to the magnitude of the force that charge B exerts on charge A
a reflected ray on a mirror makes an angle of 20degree with the incident ray when the mirror is rotated 15degree what angle will the incident ray now make with the reflected ray
Roofs are sometimes pushed off vertically during a tropical cyclone, and buildings sometimes explode outward when hit by a tornado. Use Bernoulli’s principle to explain these phenomena.
Insulators (nonmetals) have a higher BE than metals, and it is more difficult for photons to eject electrons from insulators. Discuss how this relates to the free charges in metals that make them good conductors.
