Electric fields of charged objects

The electric field is defined by equation (18.2). Equation (18.2) says that the force F on a positive test charge q is equal to the charge multiplied by the electric field strength E at that point in space. The equation also defines the electric field E as the force F on the charged object divided by the amount of charge q. Read the text aloud
(18.2) F e =qE   or   E= F e q
E  = electric field (N/C)
Fe  = electric force (N)
q  = electric charge (C)
Electric field
and electric force
Left:  two positive charges. <br/>
Middle:  two negative charges. <br/>
Right:  one positive, one negative charge.
Consider the electric field around two point charges. At any position in space, the electric field E is the force on an imaginary positive test charge at that position. A positive test charge is repelled by both positive charges, so the field lines point outward from each positive charge. A positive test charge is attracted to both negative charges, so the electric field lines point toward each negative charge. Read the text aloud
Electric field of a dipoleThe net force on a test charge at any position is the sum of the individual forces exerted by each charge in the system. The electric field between a positive and negative charge generally points away from the positive charge and toward the negative charge. Between the charges the field points directly from positive to negative. At any point in space, the total electric field is the vector sum of the field from the positive charge and the field from the negative charge as if each were independent. Read the text aloud Show Strength of electric fields
Oppositely-charged parallel plates create a uniform electric field;What is the force on a positive test charge located between two oppositely charged parallel plates? The horizontal components of force cancel, leaving only a vertical force. Therefore, the electric field between parallel plates is purely vertical and points from the positive plate to the negative plate. The electric field is also constant in strength in the space between the plates. Why is it constant? Consider a positive test charge between the plates. When it moves away from the positive plate the repulsive force diminishes, but it simultaneously moves closer to the negative plate, causing the attractive force to increase. The two effects combine to create a constant electric field. Parallel plates are often used in experiments because they create a constant electric field and are found inside the capacitor, a commonly used device found in many electrical circuits. Read the text aloud Show Infinitely long, charged plates
What is the sign of the charge Q<sub>1</sub>?In the illustration at right, what is the charge Q1?
  1. positive
  2. negative
  3. neutral
  4. There is not enough information.

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