REVIEW ELECTRIC FIELD The direction of the field is taken to be the direction of the force it would exert on a positive test charge. The electric field is radially outward from a positive charge and radially in toward a negative point charge.
REVIEW ELECTRIC FIELD • The direction of the field is taken to be the direction of the force it would exert on a positive test charge. • The electric field is radially outward from a positive charge and radially in toward a negative point charge.
Electric fields are used to help visualize how a charge influences the region around it. • Electric field is like gravity. The electric field from a positive charge points away from the charge; the electric field from a negative charge points toward the charge. Like the electric force, the electric field E is a vector. If the electric field at a particular point is known, the force a charge q experiences when it is placed at that point is given by:
If q is positive, the force is in the same direction as the field; if q is negative, the force is in the opposite direction as the field.
An electric field can be visualized on paper by drawing lines of force, which give an indication of both the size and the strength of the field. • Lines of force are also called field lines. • Field lines start on positive charges and end on negative charges, and the direction of the field line at a point tells you what direction the force experienced by a charge will be if the charge is placed at that point. • If the charge is positive, it will experience a force in the same direction as the field; if it is negative the force will be opposite to the field.
When there is more than one charge in a region, the electric field lines will not be straight lines; they will curve in response to the different charges. In every case, though, the field is highest where the field lines are close together, and decreases as the lines get further apart.
EXAMPLE 11: THE ELECTRIC FIELDS FROM SEPARATE CHARGES MAY CANCEL • Two positive point charges, q1 = +16uC and q2 = +4.0uC, are separated in a vacuum by a distance of 3.0m. Find the spot on the line between the charges where the net electric field is zero
Between the charges the two field contributions have opposite directions, and the net electric field is zero at the place where the magnitude of E1 equals that of E2. However, since q2 is smaller than q1 this location must be closer to q2 in order that the field of the smaller charge can balance the field of the larger charge. In the drawing, the cancellation spot is labeled P, and its distance from q1 is d.
THE PARARALLEL PLACE CAPACITOR • Two parallel metal plates. Each with an area (A). • A charge +q is spread uniformly over one plate, while a charge –q is spread uniformly over the other plate. In the region between the plates and away from the edges, the electric field points from the positive plate toward the negative plate and is perpendicular to both.
The field has the same value at all places between the plates. • The field does not depend on the distance from the charges, in distinct contrast to the field created by an isolated point charge. = 8.85 x 10^-12 C^2/(Nxm^2) (part of k constant)
18.7 ELECTRIC FIELD LINES • Map that gives the direction and indicates the strength of the field at various places. • Michael Faraday came up with a map of electric field lines. • Electric field: electric force per unit charge. • Electric field lines are sometimes called linesofforce. • Pg. 553
The electric field created by the charge +q is directed radially outward. • Field on the –q charge is inward because the force on a positive test charge is one of attraction. • Electric field lines are always directed away from positive charges and toward negative charges.
Where the electric field is stronger the lines are closer together. • At distances far from the charges, where the electric field is weaker, the lines are more spread out. • The number of lines per unit area passing perpendicularly through a surface is proportional to the magnitude of the electric field.
Electric dipole: • Two separated point charges that have the same magnitude but opposite signs. • The electric field of a dipole is proportional to the product of the magnitude of one of the charges and the distance between the charges. • Dipole moment: the product • Water and HCl have dipole moments. • The electric field vector at a point is tangent to the line at that point.
Electric field lines always begin on a positive charge and end on a negative charge and do not start or stop in midspace. • The number of lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge. • Ex. 100 lines are drawn leaving a +4uC charge, then 75 lines would have to end on a -3uC charge and 25 lines on a -1uC charge. • 100 lines leave the charge of +4uC and end on a total charge of -4uC. • The lines begin and end on equal amounts of total charge.
Absence of lines in the region between the charges. • Electric field is relatively weak between the charges.