Capacitors Flashcards
what is a capacitor
the amount of energy stored per unit potential difference
what components make up a capacitor
2 metal plates that are separated by an insulator called a dielectric. this is then connected to a cell.
what happens when a capacitor has not charge
the two plates have an equal amount of electrons
what happens when a capacitor has a working cell
- the electrons repel away from the negative cell into a plate and the positive charges flow toward the cell. this creates a difference in charge between the 2 plates therefore creating an electric field. when a path is created around the plates, all the energy very quickly moves to let the electrons flow away from the negative cell
what power supply can only be used for capacitors and why
can only be used by DC as AC changes the plates charges constantly so the electrons would never build up at a plate so no electric field is created
what are the differences in a capacitor and a battery
capacitors store energy as electrical potential energy versus chemical energy.
capacitors only store a small amount of energy but can release extremely quickly, whereas batteries can store a lot more energy but is released slower
what are the applications of a capacitor
-Camera flash
-Volatile memory on a computer
how do you use a experiment/graph to measure capacitance
plot a graph of voltage against charge. measure the gradient which is 1/C
voltage is measured with a voltmeter
use Q=It to find charge (current will need to be constant though)
why is current not constant when charging a capacitor
there aren’t fixed spaces for each electron so as more electrons are piled up at the plate the more repulsion that happens. the force of repulsion eventually reaches the EMF and the current stops
how can you keep current constant when charging a capacitor
by using a variable resistor in a circuit, set it initially high, to reduce the rate of flow of electrons. and slowly decrease the resistance to create a constant current making it easier to calculate current.
how is energy stored in a capacitor
through electrical potential energy, the electrons are being forced together when they want to repel, so the moment they are allowed to, all the energy is released
how do you calculate energy stored in a capacitor
the area under a voltage/charge graph
E=0.5 QV
what are the differences in energy stored for a capacitor and a battery
in a battery voltage is steady, whereas in a capacitor, half the energy is lost through heat when electrons move from one plate to another
what is a dielectric
an insulating layers that is put between the 2 metal plates in a capacitor
what does a dielectric do
prevents electrons jumping from plate to plate
what is relative permittivity
the ratio of the permittivity of a medium to the permittivity of free space
what’s another name for relative permittivity
dielectric constant
what is permittivity
a measure of how difficult it is to create a electric field in that medium
what is the breakdown voltage
how strong the electrical field need to be to allow electrons through the dielectric
how are polar molecules arranged when there’s no electric field
randomly arranged
how are polar molecules arranged when there is an electric field
they are arranged anti-parallel to the field. positive end of the molecules are attracted to negative plate and vice versa.
what happens to the electric field when the capacitor is charged
the polar molecules produce an electric field. due to the way its attracted, their own fields oppose the one created by the two plates.
what happens to capacitance when permittivity increases
if there’s a larger permittivity, the polar molecules are more polarised so there’s a larger opposing field. this means a weaker overall field so its easier for electrons to transfer plates. this reduces the pd, and further increases capacitance
what 3 properties effect capacitance
area of overlap- more area= more electron storage
permittivity of dielectric
distance between plates- decrease capacitance
how to calculate capacitance in a parallel circuit
add capacitance together
how to calculate capacitance in a series circuit
add 1/C together for all capacitors then flip fraction.
what does a current time graph look like (charging)
starts high and falls exponentially
what does a charge time graph look like (charging)
charge quickly builds up and then starts to plateau
what does a voltage time graph looks like (charging)
does the same as the charge but it peaks at the emf.
how does a resistor in a charging capacitor gain voltage
at the beginning, resistor and the charging circuit, but as time goes on the resistor will get a smaller share, until it reaches 0
how does discharging a capacitor work
when the capacitor is charged the electrons are close together, meaning the force is greater. so if it discharges the force between them decreases
what does a current time graph look like (discharging)
reduces exponentially after starting high
what does a charge time graph look like (discharging)
reduces quickly when current is high and slows as current slows
what does a voltage time graph look like (discharging)
voltage does the same as charge as they are proportional
what happens to a resistor in a discharging capacitor
all the voltage goes to the resistor as its the only other component, however because it flows in the opposite direction to charging it will be negative
how does voltage in a resistor and capacitor link in a discharging capacitor
Vc=-Vr
what are the discharging equations
all are the same exponential equation (check nugget)
what are the charging equations
current is exponentially decreasing but charge and voltage are exponentially increasing (check nugget)
what happens when there are two capacitors in a circuit (with switch)
in position 1 the first capacitor will be charged by the battery but when you change the positions the second capacitor will be charged by the fully charged first capacitor. voltage will eventually be the same and current will stop flowing
why do all capacitors in a circuit reach the same voltage
V=kQ/r
for a charging/discharging graph what are the features of the log version
intercept- V0
gradient - -1/RC
how do you find the time constant mathematically
sub in t=RC into exponential formula (0.37)
what does the time constant mean when charging and discharging
discharging- the value will be at 37% of its original value
charging- the value will be at 63% of its final value
how do you mathematically find time to halve
Q=0.5 x Q0
sub into exponential equation
T1/2 = 0.69 RC
