Capacitor charge and discharge Flashcards Preview

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Flashcards in Capacitor charge and discharge Deck (14):

Why does current decrease the capacitor is discharged?

  • PD decreases
  • resistance remains constant
  • PD does the same



How is charging investigated>?


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What happens as the capacitor is charged?

  • switch closed
    • current flow
    • electrons flow onto plate connected to -ve terminal of the dc power supply
    • -ve charge builds up
  • build up of -ve charge repels electrons off the plate connected to the +ve terminal of the power supply
    • makes the other plate positive
    • these electrons are attacted to the +ve terminal of the batter
  • equal and opposite charge builds up on each plate
    • causes PD
  • as charge builds up, electrostatic repulsion makes it harder for more electrons to be deposited
    • when the pd across the capacitor is equal to the pd across the power supply, current falls to 0


Draw the graphs or describe for charging for:

  • current
  • pd
  • charge

all against time

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How is a capacitor discharged?

  • remove power supply and close switch
  • data logger records pd and current over time 
  • when the current through the ammeter and pd across the plates are zero, the capacitor is fully discharged


What happens when a capacitor discharges?

  • current flow in the opposite direction from the charging current
  • as the PD decreases, the current decreases as well
  • when a capacitor is discharging, the amount of charge on and potential difference between the plates falls exponentially with time
    • that means it always takes the same length of time for the charge or potential difference to halve, no matter what value it starts at


What does the time taken to charge or discharge depend on?

  • capacitance
    • affects the amount of charge that can be transferred at a given pd
  • resistance
    • affects current



Why is the unit of the time constant of RC seconds?

  • R = V / I
    • C = Q / V
      • RC = V / I x Q / V = Q / I = t


What is the time constant for the circuit when discharging and charging?

  • τ, time taken for the charge of the pd or current of a discharging capacitor to fall to 37% of its value when charged
  • also time taken for the charge or pd of a charging capacitor to rise to 63% of its value when fully charged


explain these equations for charging?

Q image thumb

  • V and Q
    • growth rate of the amount of charge on and pd across the plates shows exponential decay (over time, they increase more and more slowly)
  • charging current is different
    • decreases exponentially


Why does charge always take the same length of time to halve?

  • charge left on plates falls exponentially with time as a capacitor charges


What are the changes for discharging?

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How do you find the time constnat from Log-linear graphs?

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What is time to halve?

  • 0.69RC xD
  • time taken for the charge, current or pd of a discharging capactiro to reach half of the vlaue it was when it was fully charged