Electric Fields

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 .

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

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

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

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

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

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

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

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)

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

Dipole solution:

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

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