Capacitance Flashcards
define capacitance
the charge stored per unit p.d across it
what is a dielectric
material made up of polar molecules
factors that affect the capacitance for dielectric action in a capacitor
-relative permittivity
-effective area of the plates
-separation of the plates
what happen when an electric field is applied across a dielectric
the polar molecules align with the field
what is dielectric constant
relative permittivity
how to find Er
E of the material /E of free space
why is the resultant electric field reduced when dielectric is used
each polar molecule in the dielectric have its own electric field which opposes the field from the plate
what happen when an electric field is applied across a dielectric
polar molecules aligned with the field and in opposing direction(antiparallel)
why is the capacitance increased with a dielectric
p.d needed to push the charge onto the plate is reduced as electric field is weaker(C=Q/V)
what happens if capacitor made from a sheet of dielectric disconnected from the power supply
charge and voltage remains unchanged
dielectric is removed from the capacitor whilst it remains disconnected, what happen to the energy and the charge stored
charge remains unchanged but energy increases as work is done to remove the dielectric(voltage increases)
current that charges the capacitor falls as charging it, why
-increase in charges increases the p.d. across the capacitor
-as its p.d. increases the p.ds across the resistor falls
-so current falls
when discharging capacitor, what is the effect on the initial current with using a resistor with larger resistance
lower, I=V/R
when discharging capacitor, what is the effect on the initial charge with using a resistor with larger resistance
same, as Q=CV
when discharging capacitor, what is the effect on the time taken for p.d. across the capacitor to halve with using a larger resistance
longer as T= RC ln(2)
the formulas for energy stored in a capacitor
E=0.5QV,0.5CV^2,0.5Q^2/C
work done by the cell
W=QV
