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Chapter 23 Capacitors (23.2 (Energy in a cloud (Earth positive,…

- - - - Capacitor provides own pd as it charges- eventually reaches max.
      - Capacitor pd will oppose pd of whatever is charging it (electrons repel)
      - So it charges quickly at first but then slows down
      - Stops charging when capacitor's pd fully counteracts source pd (may not be fully charged)
    - - To get constant current must use variable resistor to balance effect of dropping pd
      - Can then calculate charge of capacitor at any given time using Q=It
    - - Capacitance
        
        Charge is proportional to pd
        
        Plot Q on y axis and V on x axis = straight line through origin gradient C
        
        Capacitance- charge stored per unit pd
- - - - RC= time constant= time taken to discharge to 37% original (bigger RC longer overall discharge time)
    - - RC= time constant= time taken to charge to 63%
- - - - More charge stored
        
        1) +ve side of dielectrics attracts more electrons from battery onto -ve plate
        
        2) -ve side of dielectric repels more electrons back to the battery from the +ve plate
    - - Relative Permittivity
        
        Ɛr- also known as dielectric constant
        
        Always greater than 1 as Q is greater than Q0
      - Alternating fields effects
        
        If alternating field applied to capacitor, polar dipole will rotate and the non-polar dipole alternates the way it rotates as the field strength increases and decreases
        
        Low freq. 3 polarisation mechanisms alternate in phase as freq. increases, each mechanisms ceases to work due to inertia
        
        Mass of particles dictates which mechanism ceases to work 1st
        
        Greater mass= greater inertia so ceases to work at lower frequency
    - - Increase capacitance by
        
        1) Make area as large as possible
        
        2) make distance small
        
        3) large Ɛr