6.1 - Capacitors

• Time constant

  Time taken for pd to drop by 37% of its initial value
• What is 1 farad

  1 coulomb of charge stored per unit volt
• Unit for capacitance

  Farads
• Factors that affect the amount of charge stored in a capacitor

  Area of plate - physical space for charge

  Distance between plates - this affects strength of field

  Permittivity - insulation
• Capacitance

  The amount of charge able to be stored per unit potential difference across a capacitor
• Safety when using capacitors

  Must be the correct polarity to avoid breakdown Voltage rating must be higher than battery Voltage rating in an electrolytic type must be low due to the strength of the electric field
• Adding capacitance in parallel

  C=C1+C2+....

  Capacitance is added
• Adding capacitance in series

  1/C=1/C1+1/C2+....

  Add reciprocals of the capacitance
• Investigating capacitors Required practical 9 may be assessed

  Record the change in potential difference in regular intervals to find time period record the time for a certain drop in potential difference for different capacitances to find the capacitance of an unknown capacitor
• The charges that are stored on each plate repel each other, creating a potential due to the electric field. A potential difference is created between the plates
• Describe the charge in a capacitor as the capacitor is charged

  The EMF of the cell transfers the charge from one plate to another As capacitor charges up, the p.d across the capacitor increases (V = Q/c) As it charges the resultant p.d reduces = EMF p.d of capacitor) This reduces the current and so the rate of flow of charge decreases Rate of charging decreases When the p.d of the capacitor equals the EMF of the cell, then charge stops flowing = no current and the capacitor is fully charged
• Discharging a capacitor 


  Source of EMF is removed causing the capacitor to become the circuits main source of EMF A current flows around the circuit in the opposite direction as the electrons from the negatively charged plate drift towards the positively charged plate due to the force of attraction The attraction pulls the electrons through the circuit back to the positive plate causing the potential difference to decrease Current decreases The capacitor has completely discharged when the potential difference between the plates is zero.
• Uses of capacitor

  Flash photography - high current Back up power supplies charge for short period of time Smoothing out p.d- maintains constant output of charging / discharging
• Dielectric insulators -increase the capacitance of the device by polarizing in the electric field and effectively increasing the charge stored on the plates
• The switch S is closed. How do the capacitor plates A and B acquire opposite charges?

  Electrons in the circuit move in a clockwise rotation, depositing electrons on plate B. An equal number of electrons are removed from plate A giving it a positive charge of equal magnitude.
• The capacitor consists of two parallel metal plates separated by air. The switch is closed to charge the capacitor. The switch is then opened and the separation between the charged plates is doubled. What happens to the energy stored by the capacitor ?

  The charge on each plate remains the same. As C = 80A/d, the capacitance is halved. E= 12 Q2/C Eo 1/C and so energy stored doubles.
• How can the time constant can be determined experimentally in the Iaboratory?

  Connect a voltmeter across the resistor or capacitor. Time from when the switch is opened and stop when the p.d. or the current to decreases to 37% of its initial value. The time constant is the time taken for the p.d. or the current to decreases to 37% of its initial value
• Capacitance formula:

  C = Q / V
• Energy stored (internal energy)= work done of the capacitor For a p.d - charge graph the energy stored is the area under the graph, this can be used to derive other energy equations