Capacitors

• In order to store charge effectively, need two plates very close to eachother
• Capacitor is two conducting plates very close together but not touching. takes electrons from one side of the circuit to the other (other plate) one positively charged plate (lost electrons) and one negatively charged (gained electrons)
• When positive and negative charge builds up on the plates, it creates a uniform electric field
• capacitance is the charge per unit potential , measured in Farads
• As the charges across the plates increase, potential difference increases but electron flow decreases (due to force on electrostatic repulsion also increasing. So current - e flow - decreases and tends to 0)
• product of resistance and capacitance RC is the time constant. This is the value of time taken for discharge a capacitor to 1/e ~~ 0.37 of its initial value
• 1/e ~~ 0.37
• RC to charge a capacitor to (1- 1/e ) ~~ 0.63 of initial value
• Can find time constant on graphs by finding when value is 0.37 initial value for discharging
• The area of an I - t graph is charge
• Tangent of Q-t graph is the current
• to derive T1/2 (time taken for I,Q,V to half initial value) by subbing 0.5xinitial value into discharge equation
• Currents discharging equation is replacing I with Q in the discharging formula
• Currents charging equation is replacing I with Q in the discharging equation
• Dielectric constant = permittivity of medium/ permittivity of free space
• Area of a Voltage- Q graph is the energy stored (at that p.d)
• The gradient of a V-Q graph gives capacitance
• Capacitance is measured in Farads
• The energy stored is only half as the other half is lost as heat due to resistance in the circuit
• dialectrics are non conductive as you don’t want charge to leak to other plate (won’t be stored)
• In parallel, when same capacitance, total capacitance = c1+c2
• In parallel, when same capacitance, voltage is the same and charge is n x the number of capacitors
• In series, when same capacitance, voltage is shared and charge is the same
• In series, when same capacitance, 1/Ct = 1/c1 + 1/c2
• Bigger area means more capacitance
• higher capacitance means more charge is stored
• Smaller distance between plates means higher capacitance
• If power supply is still connected when a variable changes, then voltage is constant.
• If a power supply is no longer connected when a variable changes, then charge is constant (stored)