1. Oregon State University PH 106, Lecture #15
Electric Fields
The continuing question: How can any object exert a force on another object at a distance—without contact??
Mass does it—via the gravitation force: FG = Gm1m2/r2
Charge does it—via the electrical force: FE = kq1q2/r2
(But what do we mean by contact, anyway?)
5/5/17
Oregon State University PH 106, Lecture #15

2. Force at a Distance: A Change in Space Itself
A field (more correctly called a force field) is a pattern of variation in some area of space—and that can be empty space; no matter (nothing with mass) needs to be located there.
What is it about space itself that can vary? Certain properties—namely, the force(s) that can be exerted on an object placed at a given point.
5/5/17
Oregon State University PH 106, Lecture #15

3. Oregon State University PH 106, Lecture #15
Examples of force fields:
Gravitational: Any object with mass placed in space near another mass will feel a gravitational force at that point in space.
Electrical: Likewise, any object with a net charge placed in space near another net charge will feel an electrical force at that point in space.
5/5/17
Oregon State University PH 106, Lecture #15

4. Oregon State University PH 106, Lecture #15
Field Lines
How can we think about these variations in the properties of space itself? What should we draw or write to represent them?
We draw “field lines” to show the direction (and magnitude) of the force that would be felt by an object placed in the field.
Consider a simple gravitational field.
5/5/17
Oregon State University PH 106, Lecture #15

5. Oregon State University PH 106, Lecture #15
Now consider an electrical field.
Field lines are represented as beginning at positive charges (emanating outward—with arrows pointing away from the positive charges).
And field lines are represented as terminating at negative charges (with arrows pointing toward those negative charges).
5/5/17
Oregon State University PH 106, Lecture #15

6. Electric Field Lines: The Paths to Store Energy
What good is this set of imaginary lines—lines we draw in space?
･ It tells us at every point how a charge will accelerate (i.e. what
force will be exerted on it) if it is placed there.
･ It tells us how much potential energy that charge will have (i.e. how
much work it will take to place that charge there).
Again, consider the analogy to gravity.…
5/5/17
Oregon State University PH 106, Lecture #15