Capacitor charge and discharge Flashcards Preview

A2 Physics F > Capacitor charge and discharge > Flashcards

Flashcards in Capacitor charge and discharge Deck (14):
1

Why does current decrease the capacitor is discharged?

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

 

2

How is charging investigated>?

  •  

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3

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

4

Draw the graphs or describe for charging for:

  • current
  • pd
  • charge

all against time

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5

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

6

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

7

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

 

8

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

9

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

10

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

11

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

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

12

What are the changes for discharging?

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13

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

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14

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