capacitance Flashcards
definition of capacitance
Charge stored per unit potential difference of a capacitor. Unit = Farad (F) which is the same as C/V
what is a Dielectric?
Material that increases the capacity of a parallel plate capacitor to store charge when placed between the plates of a capacitor.
Electrically insulating material that increases the capacitance of the capacitor
Permittivity of free space
The charge per unit area in coulombs per square meter on oppositely charged parallel plates in a vacuum when the electric field strength between the plates in 1 volt per meter
Relative permittivity (aka dielectric constant)
Ratio of the charge stored by a parallel plate capacitor with dielectric filling the space between its plates to the charge stored without the dielectric for the same potential difference.
Exponential increase/decrease
Value increases/decreases by the same factor in a set amout of time
describe a capacitor
2 parallel plates placed near each other. When the plates are connected to a power supply, e- move through the battery and are forced to move from its negative terminal onto one of the plates. An equal number leave the other plate and return to the battery via the positive terminal.
And equal and opposite charge builds up on both plates
What happens when a dielectric containing unpolarised molecules is placed between the plates
The electrons are pulled slightly towards the positive plate. This is called charge separation. Induced dipoles form. Decreases the potential difference across the plates.
called charge separation.
What happens when a dielectric containing polar molecules is placed between the plates
The molecules rotate between the plates because the e- are attracted towards the positive plate.
why does a dielectric increase the charge that can be stored on the plates?
the positive side of the dielectric attracts more electrons from the battery to the negative plate
the negative side of the dielectric repels electrons from the positive plate
What happens when a dielectric containing polar molecules is placed between the plates
The molecules rotate between the plates because the e- are attracted towards the positive plate.
Why do dielectric allow more charge to be stored on the plate?
The positive side of the dielectric attract more e- from the battery to the negative plate
The negative side of the dielectric push more e- back to the battery from the positive plate
define exponential change
rate of change of “x” is proportional to the amount of “x”
changes by the same factor in equal time intervals
what is the time constant?
RC
when charging a capacitor, what would happen to the current, assuming the resistance of the resistor is constant?
current will decrease
as charge builds up on the plates, it becomes harder to add the next “bit” of charge. the negative plate will repel the negative electrons, so more energy is required to add more electrons to the plate
the positive plate will attract the negative electrons so more energy is required to remove more electrons
what fraction of energy from the cell is stored in the capacitor? what happens to the rest?
50%
the rest is transferred to the surroundings by resistance
when discharging a capacitor, what is the rime taken for the charge to fall to 0.37 of its original value?
RC
when we keep current constant, which equations cannot be used?
the exponential equations
when current is changing exponentially, which equations cannot be used?
Q=It, etc
a dielectric being placed between the plates of a parallel plate capacitor increases its capacitance. how can this be explained if the charge of the capacitor is fixed?
isolated so charge on plates stays the same
electric field between the plates causes charge separation
the induced diploes/polar molecules have an electric field pointing in the opposite direction to the field caused by the capacitor
therefore the overall strength of the electric field decreases.
C = Q/V
Q remains constant, V decreases, so C must increase
a dielectric being placed between the plates of a parallel plate capacitor increases its capacitance. How can this be explained if the voltage of the capacitor is fixed?
electric field between the plates causes charge separation.
this induced dipoles/polar molecules have an electric field that points in the opposite direction to the electric field of the plates.
overall electric field strength decreases
more charge can be added to the plates to maintain the same p.d
C = Q/V
Q increases, V remains the same so C must increase.
what does it mean if the charge on the capacitor is fixed / capacitor is isolated?
capacitor is not connected to a circuit
what does it mean if the voltage of a capacitor is fixed?
the capacitor is connected to a cell in a closed circuit.
I charge a capacitor, disconnect it from the circuit and then pull the dielectric out from in between the plates. What happens to Q, V, C and the energy stored.
(state equation for C in terms of relative permittivity’s, etc, and C = Q/V)
dielectric pulled out so εr decreases, so C decreases.
Q is fixed
C decreases so V must increase
E = 0.5QV so E increases
when a dielectric is pulled out from in between an isolated parallel plate capacitor, the energy stored by the capacitor increases. explain why.
work is done on the system to remove the polarised molecules from the electric field. overcome the force of attraction between the plates and the polarised molecules in the dielectric.
