Electric Fields Flashcards
How electric field patterns are mapped out
Using electric field lines
Properties of electric field patterns/field lines
- Never start or stop in empty space, only form charge or at “infinity”
- Never touch/cross
- Have direction
- Density of lines indicates strength of e field
- Parallel and equally spaced lines indicate field of constant field strength
Direction
-Tangent to electric field line gives direction which a positive point charge placed at point would move
Density
Closely spaced lines indicates greater e field strength
Flux
measure of number of field lines passing through an area
Electric flux
number of electric field lines penetrating a surface/area
Flux formula
Φ = normal component of the field E x area A
what is normal
line perpendicular to object/plane
Guess’ Law
Electric flux through any closed surface is proportional to enclosed electric charge
Guess’s Law Formula
Φ = qₑₙ𝒸/ε₀
E
- vector
- points in direction of net electric force on positive charge
- units are N/C
F
-vector force on test charge
q
- test charge
- scalar
E and F
-only parallel if test charge is positive
Application of electric fields
Electrostatic dust particle scrubber
Electrostatic dust particle scrubber
- particulate collection device
- removes particles from flowing gas
- using force of induced electrostatic charge
Common way to produce uniform E field
w/ a parallel plate capacitor
Parallel plate capacitor
- Two flat, metal, parallel plates
- One charged neg, one pos
- except for some fringing on edges, field is nearly uniform inside
- everywhere inside capacitor has about same magnitude + direction
work done by fields eg
- To lift a positive charge q in a downward field E, requires a force qE
- Positive work done in lifting charge, field does negative work
work done on charge
-can go into kinetic e, waste heat, or potential e.
-If there is no friction + acceleration, work goes into change of potential energy:
ΔU = qEΔx
Change in potential energy for charge in uniform electric field
ΔU = qEΔx
-sign of Δx determined sign of ΔU
work done by field and change in potential energy
-Work done by field is opposite of change n potential energyL
W𝒻ᵢₑₗ𝒹 = -ΔU
Electric potential
V, is defined to be electric potential energy per unit charge
Electric potential notes
- Constant at any given distance from charged surface in uniform field
- Independent of charge
Electric potential formula
V = Ed = U/q
where d is the distance from the charged surface and U is the potential energy
SI units for potential
the volt (V)
1 Volt =
1 J/C
Moving in an electric field
- Electric fields are conservative fields
- when charge moves about, any ΔU is independent of path
-ΔV is independent of path
ΔV is independent of path
-If charge takes straight line path from A to B, the change in potential is the same as if it took a long, curvy path to B.
They gain the same amount of J of potential energy
