The Time Constant

• What is the time constant of a capacitor discharging through a resistor?

  A measure of discharge duration

  
• How is the time constant defined for a discharging capacitor?

  Time to decrease to 37% of original value
• What does 37% represent in the context of a discharging capacitor?

  0.37 or 1/e of original value
• What symbol represents the time constant?

  The Greek letter tau, τ
• In what units is the time constant measured?

  Seconds (s)
• What does the time constant allow us to compare?

  Rate of change of charge, current, and p.d.
• What is the equation for the time constant?

  τ = RC

  
• What do R and C represent in the time constant equation?

  R = resistance, C = capacitance

  
• How can you find the time constant from a voltage-time graph?

  Calculate 0.37V and find corresponding time
• What is the time to half (half-life) for a discharging capacitor?

  Time to reach half of initial value
• How is the half-life related to the time constant?

  t_1/2 = 0.69τ
• If a capacitor has a capacitance of 7 nF and a time constant of 5.6 × 10^-3 s, how do you calculate R?

  Use τ = RC to find R

  
• What should you check in an exam question regarding capacitors?

  Whether the capacitor is charging or discharging

  
• What does the time constant refer to for a charging capacitor?

  Time to reach 63% of maximum value
• What does the time constant refer to for a discharging capacitor?

  Time to discharge to 37% of initial value