Capacitor charge and discharge

Question 1

Why does current decrease the capacitor is discharged?

Answer 1

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

Question 2

How is charging investigated>?

Answer 2

*

Question 3

What happens as the capacitor is charged?

Answer 3

• 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

Question 4

Draw the graphs or describe for charging for:

• current
• pd
• charge

all against time

Answer 4

Question 5

How is a capacitor discharged?

Answer 5

• 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

Question 6

What happens when a capacitor discharges?

Answer 6

• 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

Question 7

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

Answer 7

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

Question 8

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

Answer 8

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

Question 9

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

Answer 9

• 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

Question 10

explain these equations for charging?

Answer 10

• 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

Question 11

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

Answer 11

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

Question 12

What are the changes for discharging?

Answer 12

Question 13

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

Answer 13

Question 14

What is time to halve?

Answer 14

• 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