CAPACITORS

1
CAPACITORS
2
A capacitor is a device that can store charge (it
has a capacity). It is basically made of two
plates
Charge builds up on these plates and the voltage
between them increases until it reaches the
supply voltage.
3
How Capacitors Work

• The simplest capacitors are big plates of metal
  close to each other but not touching.
• Extra electrons build up on the negative plate.
• The plate becomes more negative and so becomes
  less attractive.
• Flow of electrons gradually reduces which means
  the current gradually reduces.
• The electrons in the other plate are repelled by
  the build up of electrons in the first plate.

4

• As charge builds up, so does the pd across the
  plates Charged stored is directly proportional to
  the potential difference across the plates.
• 
  
• is
  constant
• So..
• C is capacitance because it is the charge stored
  per unit pd across the plates - the capacity of
  the plates to store charge.
• Capacitance is measured in farad, F. (Which is
  impractically big!!!!)1F 1 C V-1 (A
  capacitance of 1 farad will mean a charge of 1
  coulomb can be stored for each volt across the
  plates).

5
Warning

• When you use capacitors in practical work you
  must connect them the right way round or they
  have a tendency of exploding, (not a pleasant
  occurrence and quite dangerous). There is
  normally an arrow on the case of the capacitor
  which points from high voltage to low, to show
  you which way around it should go.

6
Charging and discharging capacitors
7
Charging and discharging a capacitor
P.d.
Time
P.d.
Time
Time
8
Time Constant

• T RCWhere
• T time constant
• R resistance in the circuit (?)
• C capacitance of the circuit (F)So the factor
  that governs how quickly the charge drops is a
  combination of the capacitance of the capacitor
  and the resistance it is discharging through.
• In practice it takes 0.69 x RC (ln2 x RC) for
  the charge to be half its original value. In this
  time the discharge current also drops to half its
  original value too.

9
(No Transcript)