March 13 AP Physics

In: An object of charge +q feels an electric force F E 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)–F E /2 b)–2F E c)–2qF E d)–2F E /q e)–F E /(2q)

Objective: To learn about static electricity: –Electric potential energy –Electric potential Success criteria: Understand the equations

Electrical potential energy Work

Electric potential energy The work done to bring a charge from infinity to a particular distance = - potential energy the charge has at that point. U E =-Work Also, the change in PE from one point to another = -work done to change the position. ΔU E =-W

In a field generated by a point charge: U E =kq 1 q 2 r In a uniform field: ΔU E =qEΔd

Both Gravitational potential and Electrical potential energy depend on position

Electric potential The energy per each individual charge. Measured in Volts V = U E 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 Equations Variablesymbolpoint source uniform field ForceF e unit FieldE unit EnergyU e unit PotentialV unit

Out: The figure shows the electric field lines for two point charges separated by a small distance. (a) Determine the ratio q 1 /q 2. (b) What are the signs of q 1 and q 2 ?