Capacitors Flashcards
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Which side do electrons join’??
The same side of the cathode
What is definition of capcatiance?
The amount of charge per unit p.d across it
What are the three attributes of a series circuit of capacitors?
The charge is the same throughout all of them
The total capacitance equals, 1\C= 1\C1+ 1\C2…
The total p.d of the circuit is equal to the sum of p.d across components (Kirchoff’s second law)
Explain how a capacitor stores charge?
current .. fully charged
Current cannot flow across the capacitor because of the DIELECTRIC so it travels around
The electrons joins plate A, the net charge therefore becomes negative
The electrons therefore leaves plate B, the net charge becomes positive
Because the charge must be the same, there is a p.d across AB
When the p.d equals the EMF, current falls to zero and the capacitor is..
FULLY CHARGED
What is the equation for C?
C= Q\V
The three attributes of a parallel circuit of capacitors?
The p.d across them are the same
The total charge equals, Q=Q1+ Q2+ Q3…
What is the units for capacitance?
Farads
What are the five things to remember about charging capacitors with a resistor??
The p.d, R and I are related by V=IR
The p.d, Q and C are related by Q=CV
The current is decreasing exponentially I=I. e-t\CR
X= X. (1-e-t\CR)
V. = Vc +Vr
Describe interative modelling in 5 steps and what it equals
Start with a known value of initial charge Qo and a known value of CR
Choose a ∆t which is very small compared to CR
Calculate the charge leaving the capacitor, ∆Q, in a time interval ∆t using..
∆Q= ∆t\ CR x Q
Calculate charge left on capacitor at end of ∆t from previous change
Repeat this process
This equals how much charge is lost per the ∆t
The three discharging equations
X=Xo e-t/ CR
What is the time constant and how much does it represen
How long it takes to lost 37% of its initial value
Three uses of capacitors?
Camera flash
Smoothing voltages
Backup lights
Describe how smoothing works?
Rectifier circuit
KEY WORK: RIPPLE ( the top of the wave of the voltage)
The mains electricity is alternating, so the current like the voltage goes from positive to negative.
The diode let’s current in one direction
The capacitor smooths out and becomes almost DC
The ripples can become less if the time period of voltage is a lot smaller than the CR
The three equatios of power
W= 1\2 QV W= 1\2 CV^2 W= 1\2 Q^2\V
