Physics Speedrun - Electric Field
Electric Charge
- Two types: positive, negative
- Quantized:
- Conserved.
Coulomb's Law
Premittivity of free space
Electric Field
For a point charge:
Notice that
Electric Dipoles
Combination of two equal charges of opposite charges is called an electric dipole.
Dipole moment:
- Total force
- Torque on the dipole
Field Lines
- The field point is in the direction tangent to the field line.
- Magnitude of
numbers of lines crossing unit area the lines. - Starts on + charges, ends on - charges.
- Never cross. Never closed.
Electric Flux
In uniform field:
General cases:
For a closed surface, define outward as positive, the net flux:
Gauss's Law
The Gauss's law:
Differential form:
Useful when the charge distribution is highly symmetric, may determine the magnitude of electric field by
On a proper Gaussian surface, there must be
Outside the shell:
Inside the shell:
Same for a uniformly charged conductor sphere
Outside the shell:
Inside the shell:
Conductor in external field
When in electorstatic equilibrium:
- Inside:
- On surface:
Charge tends to concentrate on surface with greater curvature.
Electric Potential
Coulomb force is conservative.
Potential energy
Electric potential:
Potential difference / voltage:
Usually let
: Same as point charge, : Same potential at any point,
Determine
Capacitors
Capacitance of capacitor:
Electric energy stored:
Energy per unit volume / Energy density:
Dielectrics
Dielectric: Insulating material in capacitors.
- Harder to break down: Increase
. - Less equivlant distance between plates.
- Increase the capacitance:
Dielectric constant
The permittivity of material:
Energy density in a dielectric: