Charge and Discharge Graphs

• What is used to charge capacitors?

  A power supply (e.g., a battery)
• What happens to electrons when a capacitor is charged?

  Electrons are pulled from the positive terminal plate

  
• What charge does the plate nearest the positive terminal acquire?

  It becomes positively charged
• What happens to electrons as they travel around the circuit?

  They are pushed onto the negative terminal plate
• What occurs as negative charge builds up on the negative plate?

  Fewer electrons are pushed onto the plate
• When does the charging of a capacitor stop?

  When no more electrons can be pushed onto the plate
• What is a parallel plate capacitor made of?

  • Two conductive plates
  • Opposite charges build up on each plate

  
• How does the current behave at the start of charging a capacitor?

  The current is large and gradually falls to zero

  
• How does the current decrease during charging?

  It decreases exponentially

  
• What does the potential difference between the plates do during charging?

  It slowly increases until it matches the power supply
• What happens to the charge of the plates during charging?

  It slowly increases until maximum charge is reached
• What are the key features of charging graphs for capacitors?

  • p.d. and charge graphs are identical
  • Current graph is an exponential decay curve
  • Initial current starts on the y-axis and decreases exponentially
  • Initial p.d. and charge start at 0 and increase to maximum

  
• How are capacitors discharged?

  Through a resistor with no power supply
• What occurs when there are equal numbers of electrons on each plate?

  No potential difference exists between them
• How is charging and discharging commonly achieved?

  • By moving a switch
  • Connects capacitor between power supply and resistor

  
• What happens to the current at the start of discharge?

  The current is large and falls to zero
• How do current, p.d., and charge behave during discharging?

  All decrease exponentially

  
• What happens to electrons during discharging?

  They flow back from negative to positive plate
• What are the key features of discharge graphs for capacitors?

  • Current, p.d., and charge graphs are identical
  • Each graph shows exponential decay curves
  • Initial values start on the y-axis and decrease exponentially

  
• What affects the rate of capacitor discharge?

  The resistance of the circuit
• What happens if the resistance is high during discharge?

  Current decreases and discharge is slower
• What happens if the resistance is low during discharge?

  Current increases and discharge is faster
• What is the formula for charge in terms of current and time?

  ΔQ = IΔt

  
• What does the area under a current-time graph represent?

  The charge stored in the capacitor

  
• What does the gradient of the charge-time graph represent?

  The current at that time

  
• How do you calculate the gradient of a curve?

  Draw a tangent and calculate its gradient
• What is the process to find the current at a specific time on a discharge graph?

  1. Draw a tangent at the desired time
  2. Calculate the gradient of the tangent