Chapter 11 - Capacitors Flashcards
Explain the composition of a capacitor?
Insulating material (dialetric) in between two metal plates, with wires going from each metal plate
Explain how a capacitor is charged? (3)
1) connect up so positive side of capacitor is connected to the positive terminal
2) when switched on, negative terminal pushes electrons onto negative plate
3) tf electrons are repelled from other plate so it becomes positively charged
Note about plates?
Equal and opposite charge
Area under a current-time graph?
Charge
Capacitance equation?
C=Q/V
Capacitance unit and si unit?
F - farad
CV^-1
Define capacitance?
The charge stored per unit potential difference across a capacitor
What happens if you exceed the safe working voltage?
Charge may leak across plates
Energy stored in a capacitor equation?
W = 0.5CV^2
Investigating E stored by capacitor?
Learn diagram from notes
1) capacitor charges
2) capacitor discharges through joulemeter
How to derive W=0.5QV?
1) area under p.d. against charge graph = work done W=QV
2) since it requires more energy for every additional electron, V-Q graph for capacitor is a straight line so W=0.5QV
3) Q=CV tf W=0.5CV^2
Capacitance, charge and voltage totals in parallel?
Ctotal = C1 + C2 Qtotal = Q1 + Q2 Vtotal = V1 = V2
Capacitance, charge and voltage totals in series?
1/Ctotal = 1/C1 + 1/C2
Qtotal = Q1 = Q2
Vtotal = V1 + V2
Comparing capacitors and resistors table?
Notes
Capacitor discharge diagram and steps?
Check notes
1) no resistance tf capacitor charges fast
2) flip switch, discharges through resistor
3 things that change as charge leaves capacitor and note?
Charge stored by capacitor
Current in circuit
P.d. across capacitor and resistor
ALL DECAY EXPONENTIALLY WITH TIME
Define exponential decay graph?
A decaying graph that has a constant-ratio property for a given interval of time
5 equations for Q, I and V for a discharging capacitor?
Initial charge: Q0 = V0C Initial current: I0 = V0/R Q=Q0e^(-t/CR) I=I0e^(-t/CR) V=V0e^(-t/CR)
Time constant τ = ?
CR
What is the gradient and area under a Q-t graph?
Gradient = current Area = total charge that has flowed from capacitor
Define time constant?
The time taken for the current, charge stored or p.d. to fall to 1/e (approx. 37%) of its initial value
3 uses of capacitors?
Camera flashes
Lasers in nuclear fusion experiments
Computers to give them time to save data
What do capacitors do?
Charge up in normal use, then gradually discharge when power fails
