Electromagnetics Flashcards
electrostatic field
- results from static electric charge
- fields and forces determined by spatial relationships between charges
- relationships analyzed by vector analysis
net electric field
- superposition sum of all electric fields due to all sources
Coulomb Force Equation
- The force on a point charge, Q2 due to the electric field of another point charge. Q1
F2 = Q1 Q2 / (4 π ε ) ar 12
ar12 : a unit vector directed from point 1 to point 2
general force equation
The force on a test charge particle is
F= QE
E is the electric field exerting the force on the particle
the electric field exerting force may be
- point charge
- distributed charge field (sphere, line, sheet)
- sum of fields due to multiple sources
If medium is linear, net electric field is vector sum of fields due to all sources.
If field acts on distributed charge, force is integral of force over charge distribution
magnetic field strength, H
- measure of strength of magnetic field in free space
- independent of medium
- units are amperes per meter (A/m)
magnetic flux density, B
- includes magnetic response of material that field passes through
- dependent on medium
- units are teslas (T)
- in strongly magnetic material, B-field is greater than in free space
permeability, μ
- units are henrys per meter (H/m)
μ = μn μo
μr: relative permeability
μr approximately equals 1 –> unless currents can circulate in the material, in which case material is magnetic
μo is permeability of free space = 4 π x 10^-7 H/m
magnetic field lines
- imaginary lines drawn so that the direction of a line at any point is the direction of B at that point
- never terminate – each line eventually joins back on itself to form a loop
If an imaginary tube is bounded by B lines and end surfaces S1 and S2, the magnetic flux through S1 and S2 is the same
static magnetic field
- can change the direction of an electron
- does not perform work on the electron
changing magnetic field
- can perform work
- can induce voltage in a conductor, this causes electrons to move
static magnetic field
- can change the direction of an electron
- does not perform work on the electron
magnetic flux
analogous to electric flux
unit is the weber (Wb) Φ
magnetic poles
- exists only in pairs, called magnetic dipoles
- poles in pair have equal and opposite strength
- by convention, magnetic flux runs from north pole and south pole
ferromagnetic materials
examples: iron, nickel, cobalt
- divided into small “magnetic domains” (–> region with a magnetic material in which magnetization is in a uniform direction)
- within each domain, spin of atoms is aligned
- if material is unmagnetized, domains not aligned (no net magnetization)
- if placed in strong enough magnetic field, domains align (become magnetized)
hysteresis
- when magnetic field is applied to material, atoms migrate to domains that align with field, increasing magnetization of material
- if field is weak, process will reverse when field is removed (material returns to no net magnetization)
- if field is strong enough, magnetization persists when field is removed
- material become permanent magnet, retains magnetization until demagnetized by heat or magnetic field in opposite direction
magnetic field strength for an infinite wire
H = B/μ = IaΦ/ 2πr
- Magnetic field lines are circles around wire
For any point,
r is distance from center of wire to point
unit vector aΦ is cylindrical coordinate tangent to circle that intersects point