Physics 2113 Jonathan Dowling Lecture 31: MON 30

Physics 2113 Jonathan Dowling Lecture 31: MON 30 MAR Review Session : Midterm 3 EXAM 03: 8PM MON 30 MAR in Cox Auditorium

The exam will cover: Ch.26 through Ch.29 The 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 Resistance i = dq/dt Junction rule

R = L/A = 0(1+(TT0)))) V=iR E=J

Current and Resistance Current 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 m. What are the (a)) current density and (b)) the potential difference when the energy dissipation rate in the resistor is 1.0W? Circuits i = dq/dt

Junction rule DC Circuits Loop rule V = iR P = iV

Single loop Multiloop Resistors vs Capacitors Resistors

Key formula: V=iR Capacitors Q=CV In series: same current Req=Rj

same charge 1/Ceq= 1/Cj In parallel: same voltage 1/Req= 1/Rj same voltage

Ceq=Cj Resistors in Series and in Parallel Whats the equivalent resistance? Whats the current in each resistor?

Whats the potential across each resistor? Whats the current delivered by the battery? Whats the power dissipated by each resisitor? Series and Parallel Series and Parallel

Series and Parallel Circuits: 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 dont 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 M, and C = 1.80 F. (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 C?

Magnetic Forces and Torques v r= F

L mv qB Top view

t net =iAB sin q Side view CFnet =0 C

(28-13) U =m B U = m B (28-14)

Ch 28: Checkpoints and Questions Circular Motion: v F r

B into blackboard. Since magnetic force is perpendicular to motion, the movement of charges is circular. 2

out centrifugal F v =ma=m

a =mrw =m r 2 in Fmagnetic =qvB

FB =FC 2 mv qvB = r

mv Solve : r = qB In general, path is a helix (component of v parallel to field is unchanged).

. electron C

. r mv r= qB v qB

w= = r m 2pr 2pmv 2pm T = =

v qBv qB 1 qB f = T 2pm

Radius of Circlcular Orbit Angular Frequency: Independent of v Period of Orbit:

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?

= m Bcosq B Highest Torque: q = 90 sinq = 1

Lowest Torque: q= 0 & 180 sinq = 0 q = 180 cosq = +1 q = 0

cosq = 1 Right Hand Rule: Given Current i Find Magnetic Field B Checkpoints/Questions Magnetic field?

Force on each wire due to currents in the other wires? Amperes Law: Find Magnitude of Bds? Right Hand Rule: Given Current i Find Magnetic Field B 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 & Biot-Savart: Given i Find 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? m 0 if B= 4pR (b)) Find the magnetic dipole moment of the circuit.

m=NiA

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