capacitors Flashcards
what does a capacitor do?
stores charge (& therefore energy on two plates when a p.d. is applied across it.
on a Q V graph, how would you find the capacitance and energy stored?
capacitance is gradient of the graph, Q=VC
energy is area under E=1/2 QV
what is a variable resistor used for in a charging circuit?
to keep current constant so that charge can be found in Q=It
Can a capacitors p.d. be the same as the battery’s ?
yes, once fully charged.
Capacitance definition.
amount of charge stored per potential difference used to store it.
what is a unit farad
1F is when 1 coulomb of charge is stored due to a potential difference of 1 V being applied across a capacitor.
Differences between battery and capacitor
Batteries store chemical energy, therefore can store much more but is much harder to convert
Capacitors store potential energy, therefore stores a lot less but charges and discharges faster
uses of capacitors
defibrillator, camera flash, ignition switch, energy backup
What happens as a capacitor charges?
- e- the plate connected to the negative terminal gains electrons and becomes negatively charged, as e- are repelled by negative charge of terminal.
the e- on the other side become repelled by negatively charged plate and attracted to positive terminal, leaving plate + charged .
Rate of charging decreases over time as charge builds up on plates makes it more difficult as like charges repel
how does the charged plates create energy?
charge built up on plates cause potential difference.
pd causes electrical field
therefore charged object has to do work to move across electrical field.
what is a dielectric?
A material of greater insulance than air so there is less chance of e- crossing
definition of permittivity
measure of how difficult it is to generate a uniform electric field in a certain material
definition of permittivity of free space
how difficult it is for air or a vacuum to generate an electric field. Eo, universal constant
definition of relative permittivity
how well a material can generate an electrical field compared to how well a vacuum can
Er=E/Eo =Q/Qo = C/Co
what does a dielectric breaking down mean
when e- can cross plates and therefore can no longer store charge
how can a capacitance of a capacitor be changed (explain)
by increasing the attraction between the two plates as long as e- cannot move between the two plates
how can the capacitance of a capacitor be changed (examples)
-decreasing plate separation
-making area of plates as large as possible
- adding a stronger insulator
what is a polarised molecule
a molecule with areas of positive and negative charge
Describe the position of polarised molecules in a dielectric in an uncharged capacitor
randomly orientated
describe the position of polarised molecules in a dielectric in a charged capacitor and how this increases capacitance
polar molecules rotate to attract to the opposite charges
molecules produce an electric field in opposite direction to capacitors
reduces overall electric field which makes it easier to to place more charge on each plate
in terms of particle interactions, explain how a dielectric with polar molecules increase capacitance
-Negative side of dielectric repels electrons back to the battery from the positive plate
-Positive side of dielectric attracts more electrons from the battery to the negative plate
can store more electrons (and therefore more charge) without interference from charge of electrons
explain what happens as a capacitor discharges
E- are held in place by terminals of the power supply
turn power supply off
Negative charge on negative plate pushes e- towards negative terminal
positive charge on positive plate attracts e- towards positive plate
rate of discharge decreases over time as like charges repel
RQ Practical, what is the resistor used for
to slow down charging and discharging so that it can be measured
what happens if capacitor is not aligned properly
dielectric will break down as electrons will cross
RQP how to decreases absolute uncertainty
have two people one measuring time the second recording voltage
Charging Capacitor equations
V=Vo(1-e^-t/RC)
Q=Qo(1-e^-t/RC)
I=Ioe6-t/RC
Discharging Capacitor equations
V=Voe^-t/RC
Q=Qo^-t/RC
I=Ioe^-t/RC
what is a time constant
time taken for a value (V/Q/I) to fall to 37% its original value
time taken for a capacitor to charge or discharge
5 time constants
why is using current the most accurate measure of capacitance
Voltage decreases as there is resistance in the circuit
