1P3 Electromagnetics Flashcards
What is Gauss’ law?
The total electric flux of D through any closed surface S equals the charge Q enclosed by the surface.
What is the difference between E and D and when should they be used?
E is electric field strength (V/m) and is directly related to voltage.
D is electric flux density (C/m^2) and is continuous accross the boundaries between dielectrics.
D = eE
Define an equipotential.
A line or surface of constant potential where we could move a point charge without having to do work against electric fields.
How do equipotential lines related to electric field lines?
They are perpendicular.
How does the method of images work?
A point charge above a conducting plane, the plane can be replaced with a point charge equidistant on the other side of the plane. This is because the plate is a conductor it represents an equipotential.
How do you model a point charge in the corner of two conducting plates?
BY adding three more virtual charges, two of opposite signs, one of the same sign. Forms four quadrants. Both plates represent equipotentials.
How do you derive the energy stored in an electric field?
Voltage = q/C
P=VI = q/C x (dq/dt)
dw = Pdt
dw = q/c x dq
How do you find the force on a dielectric as it enters a capacitor?
Take the x direction as the direction the dielectric moves in.
How do you apply virtual work?
Of two arbitrary conductors of +q and -q.
Attractive force F, battery keeps the potential V.
Apply a counter force F to the +q conductor to keep it in place.
Move conductor dx towards the other.
W = Fdx
dC = dq/V
Mechanical work is performed(-Fdx)
Battery does work (Vdq)
Net change in electrostatic energy of 1/2 Vdq
Therefore -Fdx + Vdq = 1/2 Vdq
Define magnetic flux.
The surface integral of magnetic flux density dotted with the normal vector to the surface.
The component of B perpendicular to the surface multiplied by the area.
How do you find the direction of B?
Cross product dlxr
Where dl is a small element of the wire, and r is the vector joining the wire and the point
What is Ampere’s law? What is an Amperian path?
The circulation of B around an arbitrary closed path equals relative permeability x current enclosed by the path.
The closed path is an amperian path
What is Faraday’s law of electromagnetic induction?
The emf induced is proportional to therate of change of the magnetic flux linking the circuit
What is self inductance and how is it derived?
L = flux linkage/current
V = L dI/dt
How do you derive the energy stored within an inductor?
dw/dt = P = I1V1 + I2V2
=I(LdI/dt + MdI/dt) ….
=1/2 L d(I^2)/dt +…+ M d(II)/dt
W = 1/2 LI^2 … + MII
