Capacitance Flashcards
What is Capacitance?
The amount of charge that can be stored per unit Potenial difference
What equation is the definition of Capacitance?
Capacitance = Charge / Potential Difference C = Q/V
What are the Units and Symbol of Capacitance?
Symbol: C
Units: Farad (F)
Why are Capacitors generally measured in μF ?
Because one Farad is a very large amount
What is the total charge of a Capacitor?
Zero
Conservation of charge means that the opposite chrges must be created in equal quanity
What is Relative Permittivity?
The ratio of a materials permittivity to that of free space
What is Permittivity?
How difficult it is to generate an electric field in a medium
What can Relative Permitivitty also be known as?
Dielectric Constant
What is a Dielectric?
insulator
How do Dielectric relate to Capacitors?
Dielectrics are placed between the two plates of a capacitor
What is the equation of Relative Permittivity?
ε(r) = ε(of material) / ε(0)
How do Polar Molecules reduce an E-field?
Tge E-field of a polar molecules aligns against the larger E-field. Reducing the resultant E-field
What is the equation for Capacitance with a Dielectric?
Capacitance = Area x ε(0) x ε(of material) / Distance of seperation C= Aε(of material)ε(0)/d
What equation should be used if a Capacitor is disharged at a constant current?
CV= It
What are uses of Capacitors?
Measuring small time intervals
Quickly discharging charge in a defibrilator
Tuning circuits
Smoothing AC → DC power supplies
What are the equations for Energy in a Capacitor?
Energy = 0.5 x Charge x p.d = 0.5 Capacitance p.d² = 0.5 x Charge²/Capacitance E = 0.5QV = 0.5CV² = 0.5Q²/C
What is the Gradient if pd is on th X axis and Q is on the Y axis?
Capacitance
What is the Energy on a Q-V graph
The area between the line and Charge axis
What is the Gradient if pd is on th Y axis and Q is on the X axis?
1/Capacitance
Why is only half the energy of the circuit in the Capacitor?
The other half is lost thermally due to resistance
What is present in a Chargeing Circuit of a Capacitor?
Power Supply (battery, mains, cell) Resistor Capacitor Ammeter Voltmeter
What is present in a Discharging Circuit of a Capacitor?
Resistor (to create load)
Capacitor
Ammeter
Voltmeter
What shape is Capacitor Current-time, Charge and Discharge graph?
Charge:
Exponetial decay from maximum current
Discharge:
Exponential decay towards zero from negative maximum current
What shape is Capacitor V(r)-time, Charge and Discharge graph?
Charge:
Exponetial decay from EMF
Discharge:
Exponential decay towards zero from negative EMF
What shape is Capacitor V(c)-time, Charge and Discharge graph?
Charge:
1-e curve toward maximum EMF
Discharge:
Exponential decay from maximum positive EMF
What shape is Capacitor Q(c)-time, Charge and Discharge graph?
Charge:
1-e curve toward maximum Q
Discharge:
Exponential decay from maximum positive Q
What is the gradient of a Charge-Time graph?
Current
What is the area under a Current-Time graph?
Charge
What is the Shape of a graph on the combined p.d of both the Resistor and Capacitor?
Charge:
Constant at EMF
Discharge:
Constant at 0 (as no overall p.d )
What happens to the Discharge time if supply p.d is doubled?
Discharge time is unaffected
What happens to the Discharge time if the Capacitor was doubled?
Discharge time is doubled
twice as long to remove twice the amount of charge
What happens to the Discharge time if the Resistance was doubled?
Discharge time is doubled
as half the current, half the rate of charge
What is the equation for Charge on a Discharging Capacitor as a function of time?
Q=Q(0) x e^(-t/RC)
Q(0) = maximum charge
What is the equation for p.d on a Discharging Capacitor as a function of time?
V=V(0) x e^(-t/RC)
V(0) = maximum p.d
What is the equation for Current on a Discharging Capacitor as a function of time?
I=I(0) x e^(-t/RC)
I(0) = maximum I
What is the equation for Charge on a Charging Capacitor as a function of time?
Q = Q(0) x (1-e^(-t/RC)) Q(0) = Maximum Charge
What is the equation for p.d on a Charging Capacitor as a function of time?
V = V(0) x (1-e^(-t/RC)) V(0) = Maximum p.d
What is the equation for Current on a Charging Capacitor as a function of time?
I = I(0) x (1-e^(-t/RC)) I(0) = Maximum Current
What is the Time Constant?
The time taken for the Charge(or Current or p.d) to reduce to 1/e of its initial amount
What is 1/e roughly equal to?
0.37
What is the relationship between Half Life and Time Constant?
T(0.5) ≈ 0.69τ
What is the Units and Symbol for Time Constant?
Symbol: τ
Units: ΩF or s
