Transcript If the electric field is pointing up and the magnetic field is

Midterm Review
Chapters 17 through 23
Midterm Exam
 ~ 1 hr, in class
 15 questions
 6 calculation questions


One from each chapter with Ch. 17 and 18 combine
Require use of calculator
 9 conceptual questions


One from each chapter plus some extra
Don’t need calculator, just need brain
Conceptual vs. Calculation
 This is a calculation
question:
“Given that a bulb is a 2
meters away, how long
does it take light from the
bulb to reach me?”
 This is a conceptual
question:
“If the electric field is
pointing up and the
magnetic field is pointing
right, which direction is the
EM wave moving?”
Not on Exam
 What NOT to Study!
 Also
 Potential to Kinetic Energy, and vice versa
 Electric or Magnetic Energy density
 Complicated 3-Branch circuits
 Torque
 Intensity and Polarization
Charge and Field Lines
q1q2
q1qF
2 
F k 2
2
4

r
r
o
Electric Flux and Gauss’s Law
 Electric flux measures electric field penetrating any
surface
 Gauss’s law gives an easy way to calculate electric
flux through a closed surface
E 
q
o
Conductors vs. Insulators
 Conductors
 Charge is free to move
around
 Interior is shielded

In equilibrium, Ein = 0
 Most metals are
conductors
 Insulators
 Charge stays where it is
placed
Polarization and Induction
 Polarization
 Charges in material align with external electric
field
 Object remains with no net charge
 Occurs in insulators
 Induction
 Charge moves about object
 Flow of charge followed by “a separation” induces
a net charge on the object
 Occurs with conductors
Equipotential Surfaces
 Surfaces are perpendicular
to electric field
 Moving between surfaces
changes energy of system
 Moving along a surface
requires no work
Summary
Magnetic Fields
 Moving charges create
magnetic fields
 Field lines point from north to
south poles
 No isolated poles have been
discovered
 Field due to a wire can be
calculated by Ampere’s Law
and Right-hand Rule 1
Ampere’s Law and Right-hand Rule 1
 Ampere’s law relates magnetic field along a closed
path to current penetrating the enclosed surface
B
L  μo Ienclosed
closed
path
 For a current wire, direction is given by Right-hand
Rule 1
Magnetic forces and Right-hand Rule 2
 Magnetic fields exert a force on isolated charges
 FB = q v B sin θ
 And on current wires
 Fwire = I L Bext sin θ
 Direction is given by Right-hand Rule 2
Magnetic Flux and Faraday’s Law
 Magnetic Flux is similar to electric flux, but for
magnetic fields
 ΦB = B A cos θ
 Faraday’s Law relates change in flux to induced
voltage
 B
ε
t
 Direction of induced current given by Lenz’s Law

“The magnetic field produced by an induced current always
opposes any changes in the magnetic flux”
Electromagnetic Radiation
 E and B field oscillate
 E and B are perpendicular to each other and to the
direction of propagation
 E, B, and Propagation related by Right-hand Rule 2
 Travels at the “speed of light” in vacuum, and at
slower speeds in material
 Electromagnetic Spectrum
 Runs from Radio Waves (long wavelength) to Gamma
Rays (short wavelength)
Current
 Involves flow of charge
 Indicates direction of flow of positive charge carriers
q
I
t
 Flows from high potential to low potential
 Microscopically, involves drifting charges
 I = - n e A vd
Batteries and Ohm’s Law
 Batteries supply a potential difference to push
charge in circuit
 Know as emf or voltage
 Ohm’s law relates current in a component to voltage
difference “across” component
 Ohm’s law is very general
 In capacitors and inductors, other effects must be
accounted for
I
V
R
Kirchhoff’s Rules, Summary
 Kirchhoff’s Loop Rule
 The total change in the electric potential around any closed
circuit path must be zero
 Kirchhoff’s Junction Rule
 The current entering a circuit junction must be equal to the
current leaving the junction
 These are actually applications of fundamental laws of
physics
 Loop Rule – conservation of energy
 Junction Rule – conservation of charge
 The rules apply to all types of circuits involving all types
of circuit elements
Section 19.4
Resistors and Power
 Resist the flow of charge
 Resistance can be calculated from material and
geometric properties
L
R
A
 Only resistors dissipate power
 P = I V = I² R = V² / R
 Ideal capacitors and inductors store and release
power without dissipation
Capacitors
 Store energy and charge
PEcap
1
1
2
 QV  C  V  
2
2
 Capacitance can be calculated from geometric and material
properties
 For parallel-plate capacitors
C
o A
d
 With the inclusion of a dielectric
( parallel  plate capacitor )
Series vs. Parallel
 Current is same through
 Voltage is same across
different components in
series
 For resistors in series,
different components in
parallel
 For resistors in parallel,
 For capacitors in series,
 For capacitors in parallel,
1
Cequiv

1
1
1



C1 C2 C3
Inductors
 Oppose changes in current
 (Self-) Inductance can be calculated from material
and geometric properties
 For a long solenoid
 Inductors store energy in magnetic field
 PEind = ½ L I2
DC Circuits
 For RC circuit,
 τ = RC
 For RL circuit,
 τ=L/R
AC Circuits
LRC Circuits and Impedance
 In an LRC Circuit
 Energy performs simple harmonic motion between
capacitor and inductor
 Resistor damps motion
 Voltage source drives motion
 Impedance characterizes circuit
 Impedance is a “sum” of component “resistances”

1 
Z  R 2   2π ƒL 

2
π
ƒC


2
Resonance
 Resonance amplifies current in circuit
 Occurs when reactance of inductor matches
reactance of capacitor
 Characteristic resonant frequency is
ƒres
1

2π LC
Transformers
 Used to “step-up” (increase) or “step-down”
(decrease) voltage
 Power in must equal power out
 The relation between voltage in and voltage out is
Vout
Nout

Vin
Nin