Lecture 31: MON 30 MARReview Session : Midterm 3

Physics 2113

Jonathan Dowling

EXAM 03: 8PM MON 30 MAR in Cox Auditorium

The exam will cover: Ch.26 through Ch.29The exam will be based on: HW07 – HW10.

The formula sheet for the exam can be found here:http://www.phys.lsu.edu/classes/fall2014/phys2113/downloads/FormulaSheetTest3.pdf

You can see examples of old exam IIIs here:http://www.phys.lsu.edu/faculty/gonzalez/Teaching/Phys2102/Phys2102OldTests/

Current and ResistanceCurrent and Resistancei = dq/dt

V = i RE = J ρ

ρ = ρ0(1+(TT0))

R = ρL/AJunction rule

Current and ResistanceCurrent and Resistance

Current and Resistance: Checkpoints, QuestionsCurrent and Resistance: Checkpoints, Questions

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

CircuitsCircuitsi = dq/dt

Junction rule

DC CircuitsDC Circuits

Single loop Multiloop

V = iRP = iV

Loop rule

Resistors vs CapacitorsResistors vs Capacitors

Resistors Capacitors

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

ResistorsResistorsin Series and in Parallelin Series and in Parallel

• What’s the equivalent resistance?• What’s the current in each resistor? • What’s the potential across each resistor?• What’s the current delivered by the battery?• What’s the power dissipated by each resisitor?

Series and Parallel

Series and Parallel

Series and Parallel

Circuits: Checkpoints, QuestionsCircuits: Checkpoints, Questions

Problem: 27.P.018. [406649]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.Slide Rules:

You may bend the wires but not break them.

You may slide any circuit element along a wire so long as you don’t slide it past a three (or more) point junction or another circuit element.

Problem: 27.P.046. [406629]

In an RC series circuit, E = 17.0 V, R = 1.50 MΩ, 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 16.0 µC?

Magnetic Forces and TorquesMagnetic Forces and Torques

L

vF

C

C

Top viewSide view

(28-13)

(28-14)

Ch 28: Checkpoints and QuestionsCh 28: Checkpoints and Questions

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

B into blackboard.

v

F

In general, path is a helix (component of v parallel to field is unchanged).

In general, path is a helix (component of v parallel to field is unchanged).

r

.

.electron

C

r

Radius of Circlcular OrbitRadius of Circlcular Orbit

Angular Frequency:Independent of v Angular Frequency:Independent of v

Period of Orbit:Independent of v Period of Orbit:Independent of v

Orbital Frequency:Independent of v Orbital Frequency:Independent of v

Problem: 28.P.024. [566302]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?

Highest Torque: θ = ±90° sinθ = ±1

Lowest Torque: θ= 0° & 180° sinθ = 0

Bθ = 180°–cosθ = +1

θ = 0°–cosθ = –1

Right Hand Rule: Given Current i Find Magnetic Field B 

Checkpoints/QuestionsCheckpoints/QuestionsMagnetic field?

Force on each wire due to currents in the other wires?

Ampere’s Law: Find Magnitude of ∫B∙ds?

The current in wires A,B,D is out of the page, current in C is into the page. Each wire produces a circular field line going through P, and the direction of the magnetic field for each is given by the right hand rule. So, the circles centers in A,B,D are counterclockwise, the circle centered at C is clockwise. When you draw the arrows at the point P, the fields from B and C are pointing in the same direction (up and left).

Right Hand Rule: Given Current i Find Magnetic Field B 

A length of wire is formed into a closed circuit with radii a and b, as shown in the Figure, and carries a current i.

                                           

(a) What are the magnitude and direction of B at point P?                                            

(b) Find the magnetic dipole moment of the circuit.                                                

μ=NiA

Right Hand Rule & Biot-Savart: Given i Find B