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Key Topics Electric Forces and General Electric Fields: Coulomb’s Law:
• General relationship describing the force between two charged particles • Equation:
Electric Field:
• The force exerted on a positive test charge by a charged particle • Equation:
Possible Scenarios -‐ How do we tackle the following?
• Array of discrete charges?
• Continuous, non-‐symmetric charge distributions?
• Symmetric charge distributions: Gauss’s Law:
• Relates the Electric flux through a closed Gaussian surface to the electric field • Equation:
• When can we use it? o
Type of Charge Distribution
Gaussian Surface (Draw them out!)
Surface Area Integral (Useful for Gauss’s)
Volume Integral (Useful if there is an
insulator)
N/A
Electric Potential
• Related to the amount of work required to move a charged particle through an electric field o Relative value. You need a reference point to quantify the potential at a given point!
• Continuous function! Keep this in mind for problems where you move through different regions of Electric fields → Potential must be continuous from one region to another!
o My advice: work from the outside inwards, constantly updating your reference point to the closest (in proximity) known value
o Often use infinity as the first reference point, with an electric potential of 0. • Equations:
Capacitance • General Equation:
• Capacitance of a plate:
• Equivalent Capacitance
o Series:
§ Common Element: § Equation:
o Parallel:
§ Common Element: § Equation:
• Dielectrics:
o Electrically polarizable material inserted between capacitor plates to lower the electric field between the plates
§ Results in greater capacitance of the system, therefore greater ability to store charge at a given voltage difference between the plates
o How do we account for it mathematically? §
DC Circuits General Equations and Concepts
• Ohm’s Law:
• Kirchoff’s Laws o
o
• Equivalent Resistance o Series:
o Parallel:
• Power:
RC and RL Circuits
• Equations for voltage and current
** Review how to derive these equations for RL and RC circuits! Transient behavior of Capacitors and Inductors T = 0 T = infinity C L
Charging Discharging RC
RL
Problems:
1. Find the Electric Field at the origin for the following diagram. The charge density of the rod is λ .
2. Consider a system of two cylinders and an inclined plane. Both cylinders are insulators with a constant charge density A, mass m, length l, and radius R. Cylinder 1’s position is fixed at the bottom of the incline, where cylinder 2 is free to roll up and down the plane. At equilibrium, they are a distance d apart from one another (along the surface of the plane).
a. Find the electric field produced by Cylinder 1. b. Determine the force of Cylinder 1 on Cylinder 2. c. Draw the force diagram for Cylinder 2 and write down Newton’s laws for the force
interactions on Cylinder 2. d. At what angle of the incline does cylinder 2 remain stationary? e. What happens as the incline angle approaches 0 degrees? 90 degrees?
3. A sphere of radius R has a total charge Q. The volume charge density within the sphere is given by p(r) = Cr/R, where C is a constant to be determined.
a. Find C in terms of Q and R. b. Find the electric field everywhere. c. Find the potential everywhere.
4. Two concentric metal spheres of radii R and 2R carry equal and opposite charges Q and –Q.
a. What is the electric field (magnitude and direction) between the spheres? b. Which sphere is at a higher electrostatic potential? c. What is the potential difference between the spheres? d. What is the capacitance of the device? e. If the charges on the sphere remain unchanged, but the space between them is filled
with a dielectric (K = 4), what is the new potential difference between the spheres?
Key Topics Magnetic Field Biot-‐Savart Law
• General relationship describing magnetic field exerted by moving charge • Equation for moving point charge:
• Equation for a wire current:
Properties of Magnetic Field
• It’s a __________ product! Get your right-‐hand rule right • Magnetic monopoles do NOT exist • Charges must be in _____________
Ampere’s Law
• Relates current through a closed Amperian loop to the magnetic field • Equation:
• How to use
o Draw closed Amperian loop to take advantage of symmetry § Parallel: § Perpendicular:
Faraday’s Law and Lenz’s Law Faraday’s Law
• Changing magnetic flux induces an E-‐field (current) • Magnetic flux changes when:
o Fixed Area: o Fixed B-‐field: o Fixed Area and B-‐field:
• Equation: Lenz’s Law
• Determines the direction of the induced E-‐field (current) • Definition: the direction of the induced current is such that the induced magnetic field
opposes the change in the flux • 4 steps to apply Lenz’s Law:
1. Determine direction of ___________________ 2. Determine the sign of ___________ 3. Determine direction of ___________________ 4. Determine direction of ___________________
B-‐field up and steady
B-‐field up and increasing B-‐field up and decreasing
B-‐field down and steady
B-‐field down and increasing B-‐field down and decreasing
Maxwell Equations Name Mathematical Expression Meaning
Gauss’s Law
Gauss’s Law for Magnetism
Faraday’s Law
Ampere-‐Maxwell Law
EM Wave Properties of EM Waves
• E-‐field and B-‐field are _____________ to direction of propogation • E-‐field and B-‐field are _____________ to each other • Wave speed in vacuum is: • At any point on the wave:
Poynting Vector
• Points in direction of wave propogation • Magnitude measures energy transfer per area • Equation:
Intensity
• Average energy transfer • Equation:
AC Circuits Properties of AC Circuits
• Kirchoff’s and Ohm’s Laws still apply! • Source voltage will be _______________ as opposed to being ____________ in DC Circuits
Euler’s Formula
• Equation: Complex Impedance
• Find the magnitude by treating all circuit components as resistors and solve them using resistor methods
o zR = o zC = o zL =
• Find phase (angle) by trigonometry o AC current of resistor ____________ resistor voltage by _________ o AC current of capacitor ____________ capacitor voltage by ________ o AC current of inductor ____________ inductor voltage by ________ o Diagram:
• Take real part of solution at the end
Problems:
1. Consider a ring of radius R lying in the xy-‐plane, centered on the origin O , and carrying a current I in the direction shown. Further consider an infinitely long straight wire lying in the xy-‐plane, which has a minimum distance from the origin D , carrying an equal current I in the direction shown.
a. What is the magnetic field (magnitude and direction) at the origin arising solely from the straight wire?
b. What is the magnetic field (magnitude and direction) at the origin arising solely from the loop?
c. What is the total magnetic field (magnitude and direction) at the origin?
2. A frictionless conducting bar of mass m , length L , and resistance R falls vertically under the influence of gravity on a slotted track. A horizontal magnetic field
!B exists all along the
track as shown.
a. What is the magnitude of the emf induced in the bar when the velocity is v ?
b. What is the direction of the emf in the bar?
c. When the magnetic force balances gravity, the bar achieves a terminal velocity vt . In terms of the quantities given and the acceleration due to gravity g , what is this velocity?
3.
a. Find the direction and magnitude of the electric field everywhere. b. Find the direction and magnitude of the magnetic field everywhere. c. Calculate the Poynting vector in the cable.
4. Consider the following parallel RLC-‐circuit with an alternating current.
Find the current and the phase.