6.1 Capacitors Flashcards

1
Q

what is a capacitor?

A

a capacitor is a circuit component that stores energy by separating charges onto two electrical conductors (often called plates) with an insulator between them, one plate becomes positively charged and the other becomes negatively charged

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2
Q

what is the name of the insulating material between the plates of a capacitor?

A

dielectric

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3
Q

what is the circuit symbol for a capacitor?

A

two parallel lines connected with wires either side

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4
Q

how does a capacitor charge up? what happens to the voltage across the capacitor and the current when the capacitor is fully charged?

A
  • when a capacitor is connected to a source of emf such as a cell, charge cannot flow between the plates of the capacitor
  • electrons will be transferred from the negative terminal onto one plate, which becomes negatively charged and off the other plate, which becomes positively charged
  • this results in a potential difference across he plates of the capacitor
  • once the capacitor has become fully charged, no more charge will flow in the circuit, since the electrons on the negatively charged plate will repel any further electrons away
  • at this point, the potential difference across the capacitor will be equal to the emf of the cell, there will also be no current
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5
Q

what is the definition of capacitance?

A

the capacitance, C, of a capacitor is defined as the quantity of charge. Q, which can be stored per unit p.d, V. across the plates of the capacitor

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6
Q

what is the definition of capacitance?

A

the capacitance, C, of a capacitor is defined as the quantity of charge. Q, which can be stored per unit p.d, V, across the plates of the capacitor

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7
Q

what is the unit of capacitance?

A

farads, F

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8
Q

why do we not use farads when stating the capacitance of capacitors?

A

a farad is a huge unit, so usually capacitance is usually expressed as micro-farads, more appropriate unit

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9
Q

what is the rule for combining capacitance in parallel?

A

Ct = C1 +C2 +C3
because voltage across all capacitors is the same as the cell and and the charge adds up to total
(opposite to resistors same as springs)

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10
Q

what is the rule for combining capacitance in series?

A

1 / Ct = 1 / C1 + 1 / C2 + 1 / C3
because every plate has the same magnitude of charge and the voltage adds up to total
(opposite to resistors same as springs)

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11
Q

what does the area under a voltage-charge graph represent?

A

the energy stored by the capacitor, or the work done by the battery to separate the charges on the two plates

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12
Q

what are the three equations for energy stored by a capacitor?

A
E = 0.5 x QV (from graph)
E = 0.5 x CV^2 (from subbing Q = CV)
E = 0.5 x Q^2 / C (from subbing in V = Q / C)
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13
Q

what are some common uses of capacitors?

A

-flash photography
-backup power supplies
-smoothing out p.d
(page 136)

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14
Q

what are the three exponential equations for charging a capacitor?

A
Q = Qf (1-e^-t/RC)
V = Vf (1-e^-t/RC)
I = Io x e^-t/RC
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15
Q

what are the three exponential equations for discharging a capacitor?

A
Q = Qo x e^-t/RC
V = Vo x e^-t/R
I = Io x e^-t/RC
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16
Q

what does the graph look like for charge against time for a capacitor charging up?

A

charge starts at 0 and gradually increases upwards exponentially until it plateaus where charge = Qf (final charge)

17
Q

what does the graph look like for voltage against time for a capacitor charging up?

A

voltage starts at 0 and gradually increases upwards exponentially until it plateaus where voltage = Vf (final voltage)

18
Q

what does the graph look like for current against time for a capacitor charging up?

A

current starts at initial current, decreases exponentially downwards until plateaus

19
Q

what does the graph look like for charge against time for a capacitor discharging?

A

charge starts at initial charge (max charge if you like) the gradually decreases exponentially downwards

20
Q

what does the graph look like for voltage against time for a capacitor discharging?

A

voltage starts at initial voltage (max voltage if you like) the gradually decreases exponentially downwards

21
Q

what is the time constant, T (tau), dependent on? and what is the equation for the time constant?

A
  • the capacitance of the capacitor (C), this affects the amount of charge that can be transferred at a given voltage
  • the resistance of the circuit (R). this affects the current in the circuit

T = CR

22
Q

what is the definition for the time constant (in words)?

A

the time constant, T, is the time taken fro the charge remaining on a capacitor to decrease to a 1 / e (about 37%) of its initial value, measured in seconds

23
Q

outline an experiment to investigate the way p.d and current changes as a capacitor charges and discharges

A

(see page 137 for clear diagram)

  • set up a circuit with a capacitor and a voltmeter across it, connect an ammeter in series also and a resistor, use a switch with two positions to create two separates circuits, one with the cell and one with the bulb
  • close the switch to position 1 to allow the capacitor to charge up, record the current and p.d values every 15 seconds or use a data logger
  • then connect the switch to position 2 and monitor the current and p.d ad the capacitor discharges through the lamp
  • then plot graphs of how current flowing into the capacitor and the voltage across the capacitor vary as it charges and discharges
24
Q

how does the rate of charging/discharging change?

A

discharging: the more charge on the plates, the faster they can discharge, as more charge is transferred. discharge become slower due to electron repulsion
charging: the less charge on the plates, the faster they can charge up (initial charging at the start is when it is more fast), as the capacitor gets charged up, charging becomes slower because of electron repulsion