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physics paper 2 > capacitors > Flashcards

capacitors Flashcards

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1
Q

what is capacitance and its units?

A

the amount of charge an object is able to store per unit potential difference across it. measured in farads (F)

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2
Q

what’s a capacitor?

A

an electrical component that can store electrical charge.
made up of two electrical conducting plates separated by an electrical insulator (dielectric)

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3
Q

what does the voltage rating on a capacitor show?

A

the maximum potential difference that can be safely put across it

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4
Q

how can you investigate the relationship between potential difference across and the charge stored on a capacitor?

A

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

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5
Q

what does a Q-V graph look like for a capacitor?

A

straight line, positive gradient through origin. they are directly proportional

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6
Q

can capacitors store large amounts of charge?

A

no, relatively small amounts

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7
Q

how long do most capacitors provide power for?

A

a short amount of time, its tricky to prolong discharge time and the voltage through the circuit decreases as the capacitor discharges

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8
Q

how are capacitors useful?

A

they can store charge until needed then discharge very quickly.

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9
Q

are capacitors dangerous, why?

A

yes they are, they discharge all their charge very quickly which could kill you if it goes through your heart

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10
Q

example of a use of a capacitor

A

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

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11
Q

what happens to the two plates on a capacitor when it charges?

A

one becomes negatively charged while the other becomes positively charged, the charges are being force together against their will so energy is required

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12
Q

what does the area under a pd-Q or Q-pd graph represent?

A

the energy stored by the capacitor

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13
Q

what does a pd-Q graph look like for a capacitor and why?

A

straight positive line through origin
because pd is directly proportional to charge for the capacitor

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14
Q

how does the energy stored by the capacitor differ to energy supplied by the power source?

A

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

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15
Q

what are dielectrics?

A

they are insulators

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16
Q

what does altering the properties of a capacitor do?

A

changes its capacitance

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17
Q

why does changing the dielectric material separating the two conducting plates change the capacitance of a capacitor?

A

because different materials have different relative permittivities

18
Q

what is permittivity?

A

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

19
Q

what is relative permittivity?

A

the ratio of the permittivity of a material to the permittivity of free space also known as the ‘dielectric constant’

19
Q

what is relative permittivity?

A

the ratio of the permittivity of a material to the permittivity of free space also known as the ‘dielectric constant’

20
Q

when no charge is being stored by a capacitor, will there be an electric field?

A

no there won’t be

21
Q

what do polar molecules in the dielectric look like before its charged and after charge is applied?

A

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

22
Q

how does the size of permittivity effect the size of the electric field?

A

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

23
Q

how can you investigate capacitance?

A

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

24
Q

will a current continue to flow through a capacitor when it’s fully charged?

A

no once fully charged, current stops

25
Q

how is a capacitor charged?

A

-electrons flow from from the negative terminal of the supply onto a plate connected to it so a negative charge builds up on that plate
-at the same time, electrons flow from the other plate to the positive terminal of the supply, making the plate positive
-these electrons are repelled by the negative charge on the negative plate and attracted to the positive terminal of the supply
-equal and opposite charge builds up on each plate causing a pd between them

26
Q

what is separating the two plates in a capacitor?

A

an insulator, dielectric

27
Q

why can no charge directly flow between plates on a capacitor?

A

because they are separated by an insulator

28
Q

what is the current in a circuit charging a capacitor like initially?

A

high current

29
Q

why does current fall to zero as a capacitor becomes more charged?

A

as charge builds up on the plates, electrostatic repulsion makes it harder and harder for more electrons to be deposited
when pd across the capacitor is equal to the pd across the supply, the current will fall to zero as the capacitor is fully charged

30
Q

what does a current-time graph look like for a capacitor being charged through a fixed resistor?

A

decreasing exponentially

31
Q

what does a pd-time and charge-time graph look like for a capacitor being charged through a fixed resistor?

A

increasing exponentially

32
Q

how do you discharge a capacitor?

A

take out the battery and reconnect the circuit
when a charged capacitor is connected across a resistor, the pd drives a current through the circuit. this current flows in the opposite direction from the charging current
capacitor is fully discharged when pd across the plates and current in the circuit are both 0

33
Q

how to investigate capacitors discharging RP9

A

-open the switch and remove the power source and add a voltmeter and ammeter and data logger connected to computer
1. close the switch and allow the capacitor to discharge through the resistor
2. when the reading through the ammeter reaches 0, use the computer to calculate the charge on the capacitor over time
3. the computer can then plot a variety of graphs showing how the current, pd and charge vary over time

34
Q

what do the current-time, pd-time and charge-time graphs look like for a capacitor discharging through a fixed resistor?

A

all decreasing exponentially

35
Q

what does the time taken for a capacitor to charge and discharge depend on?

A
  1. the capacitance of the capacitor- this effects the amount of charge that can be transferred at a given voltage
  2. the resistance of the circuit- this effects the current in the circuit
36
Q

what is the time constant?

A

RC- the time taken for the charge on a discharged capacitor to fall to about 37% of initial charge and the time for the charge of a charging capacitor to rise to about 63% of initial charge

37
Q

how do you calculate time taken to halve for a capacitor?

A

0.69RC or you can read it off a graph

38
Q

what was Rutherfords scattering experiment?

A

-fired a stream of alpha particles from a radioactive source at very thin gold foil
-when the alpha particles strike a fluorescent circular screen a tiny visible flash of light is produced
-shows that alpha particles scattered at any angle can be detected
-most alpha particles were seen to pass straight through
-occasionally some alpha particles were scattered at angles greater then 90 degrees

39
Q

what were the conclusions made from Rutherford scattering?

A

-shows atoms must have small positively charged nucleus that contains most of the atoms mass
-the atom must be mostly empty space because most alpha particles travel straight through

physics paper 2 (5 decks)

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