1. Electric Fields

2. Definition: Electric Field? q
Force
Field Picture:
The “source” charge q produces an electric field in space. The field pushes on the “test” charge q0 .

3. Definition:
Force on tiny “test charge” q0
Electric Field =
charge q0
Units: N/C
A vector
Exists before test charge is introduced
Is produced by other charges (not q0)
q0 is small so as to not disturb the charges producing the field
it is a mathematical construct

4. q
Example:
Calculate the force on an alpha particle (q = +2e) if it is placed in the field.
Repeat for an electron.

5. E Fields are Produced by positive charges, and terminate on negative charges
Which way will a + or – charge move in this field?

6. E Fields between two charged plates
Which way will a + or – charge move in this field?

7. E Fields are Produced by positive Charges+q E
Coulomb:
But also:
Field of a positive point charge q
The field decreases in strength (magnitude) with distance

8. Several Charges:
is the (vector) sum of the fields produced by the individual charges:
What are the signs of
the 2 charges?

9. Find the electric field of the three charges at the origin:
400mm
300mm
x-axis
y-axis

10. A) +i (right) B) –i (left) C) +j (up) D) –j (down)
Quiz
What is the direction of at point B ? a x
B : (0,y)
A : (x,0) -q +q y
A) +i (right) B) –i (left) C) +j (up) D) –j (down)

11. Example: Electric Dipole
Derive an expression for at point B. y a x
B : (0,y)
A : (x,0) -q +q

12. Dipole solution:

13. For fun: find E at point A, and show that it is approximately proportional to x -3, at large distances x.

14. Summary Define: is force exerted on by is produced by other charges.
Point charge “q” produces: