Electric Field

Review… Magnitude Direction

You Try it… + - r= 1x m Q p =1.6x C Q e =-1.6x C

You Try it…

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 + + F QpQp r= 1x m q0q0 Test charge Source 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.electric fieldpoint charges vector sum

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

Your turn… Which charge is positive and which charge is negative? How do you know?

Your turn… What is the ratio of charges? Q A : Q B

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, 17 p Problems #14, 29, 30, 32, 34 Do 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.