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### Physics 2102

Jonathan Dowling

Exam 2: Review Session

Chapters 24.9-28.8 / HW04-06

Some links on exam stress:

http://appl003.lsu.edu/slas/cas.nsf/$Content/Stress+Management+Tip+1

http://wso.williams.edu/orgs/peerh/stress/exams.html

http://www.thecalmzone.net/Home/ExamStress.php

http://www.staithes.demon.co.uk/exams.html

Exam 2

- (Ch24) Sec.11 (Electric Potential Energy of a System of Point Charges); Sec.12 (Potential of Charged Isolated Conductor)
- (Ch 25) Capacitors: capacitance and capacitors; caps in parallel and in series, dielectrics; energy, field and potential in capacitors.
- (Ch 26) Current and Resistance: current, current density and drift velocity; resistance and resistivity; Ohm’s law.
- (Ch 27) Circuits: emf devices, loop and junction rules; resistances in series and parallel; DC single and multiloop circuits, power; RC circuits.
- (Ch 28) Magnetic Fields: F=vxB, Right Hand Rule, Circular Motion, Force on Wire, Magnetic Dipole.

Potential V of Continuous Charge Distributions

Straight Line Charge: dq=dx

=Q/L

Curved Line Charge: dq=ds

=Q/2R

Surface Charge: dq=dA

=Q/R2

dA=2R’dR’

Potential V of Continuous Charge Distributions

Curved Line Charge: dq=ds

=Q/2R

Straight Line Charge: dq=dx

=Q/L

Potential V of Continuous Charge Distributions

Surface Charge: dq=dA

=Q/R2

dA=R’dR’d

Straight Line Charge: dq=dx

=bx is given to you.

Capacitors

E = s/e0 = q/Ae0

E = V d

q = C V

C = e0A/d

Connected to Battery: V=Constant

Disconnected: Q=Constant

C=e0ab/(b-a)

Resistors and Capacitors

ResistorsCapacitors

Key formula: V=iR Q=CV

In series: same current same charge

Req=∑Rj 1/Ceq=∑1/Cj

In parallel: same voltage same voltage

1/Req=∑1/Rj Ceq=∑Cj

Capacitors and Resistorsin Series and in Parallel

- What’s the equivalent resistance (capacitance)?
- What’s the current (charge) in each resistor (capacitor)?
- What’s the potential across each resistor (capacitor)?
- What’s the current (charge) delivered by the battery?

Problem 25-21

When switch S is thrown to the left, the plates of capacitor 1 acquire a potential V0. Capacitors 2 and 3 are initially uncharged. The switch is now thrown to the right. What are the final charges q1, q2, and q3 on the capacitors?

Series

Seri-Q

Problem 26-56

A cylindrical resistor of radius 5.0mm and length 2.0 cm is made of a material that has a resistivity of 3.5x10-5Wm. What are the (a) current density and (b) the potential difference when the energy dissipation rate in the resistor is 1.0W?

Figure 27-33 shows five 5.00 resistors.

(Hint: For each pair of points, imagine that a battery is connected across the pair.)

Fig. 27-33

(a) Find the equivalent resistance between points F and H.

(b) Find the equivalent resistance between points F and G.

In an RC series circuit, E = 17.0 V, R = 1.50 M, and C = 1.80 µF.

(a) Calculate the time constant.

(b) Find the maximum charge that will appear on the capacitor during charging.

(c) How long does it take for the charge to build up to 10.0 µC?

F

B into blackboard.

Circular Motion:

Since magnetic force is perpendicular to motion, the movement of charges is circular.

r

In general, path is

a helix (component of v parallel to field is unchanged).

electron

Radius of Circlcular Orbit

.

C

r

Angular Frequency:

Independent of v

Period of Orbit:

Independent of v

Orbital Frequency:

Independent of v

In the figure below, a charged particle moves into a region of uniform magnetic field , goes through half a circle, and then exits that region. The particle is either a proton or an electron (you must decide which). It spends 160 ns in the region.

(a) What is the magnitude of B?

(b) If the particle is sent back through the magnetic field (along the same initial path) but with 3.00 times its previous kinetic energy, how much time does it spend in the field during this trip?

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