Electric Field

Review…

•We use Coulomb’s Law to find the force between two charges, q1 and q2, separated by a distance r:• k=9x109 Nm2/C2

• Opposite charges attract, like charges repel

Magnitude

Direction

You Try it…

• What is the magnitude of the force on the proton due to the electron in hydrogen? • k=9x109 Nm2/C2

• F= 2.3x10-8 N

• What is the direction?

+ -

r= 1x10-10 m

Qp=1.6x10-19C Qe=-1.6x10-19C

You Try it…

• Calculate the force on the +2μC charge due to the other two charges.• • k=9x109 Nm2/C2

Electric Field

• Charged particles create electric fields in the space around them.• Any other charged object that comes

into this space will interact with this electric field.• Direction of the E-field is the same as

for the force that a + charge would feel at that location.

+

Electric Field

• The strength of the electric field is measured by placing a positive “test charge” at a spot near the source charge and measuring the force on the “test charge”

+ +

F

Qp r= 1x10-10 m

q0

Test chargeSource charge

Electric Field

• Electric Field is defined as the electric force per unit charge.

• Units are N/C• E is a vector. The direction is taken as

the direction of the force it would exert on a POSITIVE point charge.

Electric Field

• The electric field from multiple point charges can be obtained by taking the vector sum of the electric fields of the individual charges.

Your Turn…

•What is the direction of the electric field at point A?

A

C

Electric Field

• It is the surrounding charges that create an electric field at a given point.• Any charge (positive or negative) placed

at the point interacts with the field and experiences a force.• Note: positive charges placed in an E-field

will move in the direction of the field. Negative charges will move in the opposite direction.

Electric Force vs. Electric Field

• Electric Force (F) is the force felt by a charge at some location• Electric Field (E) is found for a specific

location (any location) and tells what the electric force would be if a positive charge were located there• F = Eq• Both are vectors, with magnitude and

direction

Electric Force vs. Electric Field

• F = Eq• Note, we can calculate the net E-field for all

the fixed charges and then use this value to find the force on other charges • Examples: ions/electrons in neurons, heart

tissue, and cell membranes.

Electric Field

• The electric field is radially outward from a single positive point charge. Why?• What direction would E be for a single

negative point charge?• The circles represent where the magnitude

of E is the same (later we will see that these are equipotential surfaces)

Electric Field Lines

• We use Electric Field Lines to convey information about an E-field• Closeness (or density) of lines shows the field

strength. Note: lines NEVER cross• The number of lines entering or leaving a charge

is proportional to the magnitude of the charge.• The arrow gives the direction of the E-field

(start on +, end on – charge)

Electric Field Lines

Electric Field Lines

Your turn…

•Which charge is positive and which charge is negative? How do you know?

Your turn…

•What is the ratio of charges? QA: QB

Your turn…

•Where is the E-field stronger: at point X or point Y? How do you know?

Which one is correct?

Assignment

p. 563 Focus #8, 9, 12, 13, 17p. 564-565 Problems #14, 29, 30, 32, 34Do Focus tonight. We will do the Problems on white boards tomorrow!

E-Field inside a conductor

• In a conductor, electrons are free to move.• Therefore, if electrons feel an electric force,

they will move until the feel no more force (F=0).• Since F=Eq, if F=0 then E=0 inside of a

conductor. ALWAYS!• For a conductor at equilibrium, any excess

charge resides on the surface of the conductor.

E-Field inside a conductor

•A conductor shields any charge within it from electric fields created outside the conductor.