22 - Electric Fields Flashcards
Define electric field
region in space in which a charge would experience a force acting on it
Define electric field strength + units
force per unit positive charge measured in NC^-1
Electric field strength equation
E=F/Q
e. field strength = force/charge
Direction of field lines for +ve point charge
away from centre
Direction of field lines for -ve point charge
towards centre
Direction of field assumes direction __ charge would move
+ve charge
Arrows on electric field show…
direction of electric field
Relationship between field lines and conductor
field lines at right angles (perpendicular) to conductor
how is a stronger field shown
field lines closer together
how is a uniform field shown
field lines are parallel and equally spaced from each other
Point charges and uniformly charges spheres both produce…
radial fields
Define one coulomb of charge
The amount of charge passing a point in one second when the current is 1A
Outcome of Coulomb’s isolated charges experiment (Coulomb’s Law)
The force between 2 electric charges Q and q is proportional to Qq and inversely proportional to the square of their separation, r
F ∝ Qq/r^2 > F=kQq/r^2
k constant of proportionality is equal to … therefore F =
k=1/4πε0
> F=Qq/4πε0r^2
Investigating Coulomb’s law - top pan balance
Coat two balls in conductive paint (+ve charge) or touch each to +ve electrode oh high tension supply and charges ball lowers to create mass reading on balance. x by 9.81 to get force.
- balls are on insulating rods
- closer initial distance, higher reading, higher force
new equation for E. field strength
E=F/Q and F=Qq/4πε0r^2
so > E=Q/4πε0r^2
negative force means it is…
attractive force
similarities between elec. and grav. fields
both produce radial fields
differences between grav. and elec. fields
property that creates fields - mass vs charge
field - always attractive, direction of field always towards object vs repulsive (away) and attractive (towards) fields
diff. field stregth equations F/m vs F/Q (more detailed)
elec. field between two parallel plates e.g. capacitor plates
is uniform - charge will experience constant force wherever it is placed between the plates
new equation for elec. field strength (V)
E=V/d
units for E are Vm^-1
derivation using E=F/Q W=Fd W=QV
capacitance on parallel plate capacitor depends on…
overlap area of the plates (A) and separation of plates (d)
C∝A
C∝1/d
C = εA/d
ε = εrε0 εr>relative permittivity
Millikan’s oil drop experiment (process)
atomsier fires oil droplets into chamber, friction in tube causes some to be charges (can use x ray source to ionise droplets), droplets fall due to grav. and some fall between plates due to top plate hole, field turned on and -ve particles attract to top +ve plate, pd adjusted for oil to be in equlibrium + stationary
Millikan’s oil drop experiment (analysis + conclusion)
stationary - Fe = Fg
field off and measurements taken at terminal velocity - Fdrag = Fg
conclusion - Charge is quantised (all multiples of smallest charge recorded, e)
motion of electron moving in direction of field
experience deceleration, in opposite direction experiences constant acceleration due to constant electrostatic force. attracrs +ve and repels -ve electrode
shape of motion for particle entering field at right angle
parabolic path
motion for particle entering field at right angle - horizontal motion
no acceleration, velocity constant
time in field > t=d/s
motion for particle entering field at right angle - vertical motion
acceleration given by F=ma > a=F/m > a=EQ/m
initial velocity is 0
final velocity:
v = u + at
v = 0 + EQ/m x d/v
v = EQd/mv
a=EQ/m
E=V/d
so a =VQ/md
area under force separation graph
work done to separate charges or change in electric potential energy
why is energy negative
external energy is required to pull the charges apart
define electric potential
the work done in bringing a unit charge from infinity to a point.
or work done per unit charge using V=E/Q
potential energy at infinity is
zero
capacitance of a charged sphere equation
C=Q/V and V=Q/4πε0r
so C=4πε0r
what are equipotential lines
lines linking points of equal electric potential in an electric field
