capacitors Flashcards
(40 cards)
what is capacitance and its units?
the amount of charge an object is able to store per unit potential difference across it. measured in farads (F)
what’s a capacitor?
an electrical component that can store electrical charge.
made up of two electrical conducting plates separated by an electrical insulator (dielectric)
what does the voltage rating on a capacitor show?
the maximum potential difference that can be safely put across it
how can you investigate the relationship between potential difference across and the charge stored on a capacitor?
set up a circuit, voltmeter in parallel, switch, battery, variable resistor and ammeter in series with the capacitor.
after closing the switch, constantly adjust the variable resistor to keep the charging current constant for as long as possible
record pd at regular time intervals until it equals battery pd
plot a graph fixed charging current against time to work out Q
what does a Q-V graph look like for a capacitor?
straight line, positive gradient through origin. they are directly proportional
can capacitors store large amounts of charge?
no, relatively small amounts
how long do most capacitors provide power for?
a short amount of time, its tricky to prolong discharge time and the voltage through the circuit decreases as the capacitor discharges
how are capacitors useful?
they can store charge until needed then discharge very quickly.
are capacitors dangerous, why?
yes they are, they discharge all their charge very quickly which could kill you if it goes through your heart
example of a use of a capacitor
a camera flash, the entire charge of the capacitor is dumped onto the flash almost instantly which allows the camera flash to be very bright for a short amount of time.
ultracapacitors can be used in back up supplies to provide reliable power
they can smooth out variations in d.c voltage supplies
what happens to the two plates on a capacitor when it charges?
one becomes negatively charged while the other becomes positively charged, the charges are being force together against their will so energy is required
what does the area under a pd-Q or Q-pd graph represent?
the energy stored by the capacitor
what does a pd-Q graph look like for a capacitor and why?
straight positive line through origin
because pd is directly proportional to charge for the capacitor
how does the energy stored by the capacitor differ to energy supplied by the power source?
energy stored by the capacitor is half of the energy supplied by the power source - loss due to the resistance of capacitor and internal resistance of battery
what are dielectrics?
they are insulators
what does altering the properties of a capacitor do?
changes its capacitance
why does changing the dielectric material separating the two conducting plates change the capacitance of a capacitor?
because different materials have different relative permittivities
what is permittivity?
a measure of how difficult it is to generate a electric field in a medium
the higher the permittivity of a material, the more charge is needed to generate a electric field
what is relative permittivity?
the ratio of the permittivity of a material to the permittivity of free space also known as the ‘dielectric constant’
what is relative permittivity?
the ratio of the permittivity of a material to the permittivity of free space also known as the ‘dielectric constant’
when no charge is being stored by a capacitor, will there be an electric field?
no there won’t be
what do polar molecules in the dielectric look like before its charged and after charge is applied?
before its charged the molecules are aligned randomly
after its charged the negative end of the molecules are attracted to the positive charged plate and vice versa. this causes the molecules to rotate and align themselves anti-parallel to the electric field generated
how does the size of permittivity effect the size of the electric field?
the larger the permittivity, the larger the opposing is so this reduces the overall electric field between the parallel plates, which reduces pd needed to transfer a given charge to the capacitor so the capacitance increases
how can you investigate capacitance?
by setting up two parallel plates separated by a dielectric and connecting the plates to a capacitance metre
then you can alter how much the two plates overleapt change the effective area of the capacitor, or use different materials as the dielectric to vary permittivity. stacking multiple layers of the same material allows to to test how plate separation affects capacitance
