March 13March 13

AP PhysicsAP Physics

In:An object of charge +q feels an electric force FE when

placed at a particular location in an electric field, E. Therefore, if an object of charge –2q were placed at the same location where the first charge was, it would feel and electric force of

a)–FE/2

b) –2FE

c) –2qFE

d) –2FE/q

e) –FE/(2q)

Objective: To learn about

static electricity:– Electric

potential energy– Electric

potential

Success criteria:

Understand the equations

Electrical potential energyWork

Electric potential energy

The work done to bring a charge from infinity to a particular distance = - potential energy the charge has at that point.

UE=-Work

Also, the change in PE from one point to another = -work done to change the position.

ΔUE=-W

In a field generated by a point charge:

UE=kq1q2

r

In a uniform field:

ΔUE=qEΔd

Both Gravitational potential and Electrical potential energy

depend on position

Electric potential

• The energy per each individual charge.

• Measured in Volts

V = UE

q

Electric potential: uniform field

V = -Ed

Electric potential: Point charge

V = kq

r

A uniform electric filed of 250 V/m is directed in the positive x direction. A 12 μCoulomb charge moves from the origin to the point (20cm, 50 cm).

• What was the change in potential energy of the charge?

• Through what potential difference did the charge move?

Equipotential lines

Equipotential “surfaces”

• No work is done moving around on an equipotential surface.

• Work is done moving from one equipotential surface to another.

Electrostatics EquationsVariable symbol point source uniform

field

Force Fe

unit

Field Eunit

Energy Ue

unit

Potential Vunit

Out: The figure shows the electric field lines for two point charges separated by a small distance. (a) Determine the ratio q1/q2. (b) What are the signs of q1 and q2?