April 7April 7

AP PhysicsAP Physics

AP Practice Exam

April 28 ACT day

1 pm

Not optional

In:Two 1 kg spheres each carry a charge of magnitude 1C.

How does FE, the strength of the electric force between the spheres, compare to FG, the strength of their gravitational attraction?

a)FE< FG

b)FE=FG

c)FE>FG

d)If the charges on the spheres are of the same sign, then FE>FG; but if the charges on the spheres are of the opposite sign, then FE<FG

e)Cannot be determined without knowing the distance between the spheres.

Objective: To learn about

static electricity:– Millikan Oil Drop

Experiment– Electric potential

energy– Electric potential– Capacitance– Parallel plate

capacitors

Homework Check

18, 20, 24, 30

Robert Millikan

Oil Drop experiment

• Measured

charge to mass

ratio of electron

• Published in 1913

Nobel prize 1932

Millikan oil-drop experiment

qE = mg

q/m=g/E

qe= -1.6x10-19

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

Equipotential lines

Equipotential “surfaces”

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

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

Capacitance

A capacitor is a device that stores energy in the form of charges.

C = Q

ΔV

Parallel Plate capacitor

Stores energy by building up opposite charges on each plate.

C = ЄoA

d

Energy stored in a capacitor

Uc = ½ QV= ½ CV2

Equivalent Capacitanceparallel circuits

C=Q/V V1=V2=Ve Q1+Q2=Qe

so…C1+C2=Ce

Equivalent CapacitanceSeries circuits

C=Q/V V1+V2=Ve Q1=Q2=Qe

so…1/C1+1/C2=1/Ce

Out:What is the equivalent capacitance

of the circuit between a and b?