Capacitors (Unit 4) Flashcards Preview

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Flashcards in Capacitors (Unit 4) Deck (9)
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1
Q

Capacitance Definition

A

Charge stored per unit potential difference

2
Q

Capacitance graphs (Q against V; V against Q)

A

Charge, Q, against Potential Difference, V
gradient is capacitance : C=Q/V
area under is energy stored by capacitor
Y = Q, X = V, Straight line proportional through the origin
Potential difference, V, against Charge, Q
gradient is 1/capacitance : 1/C = V/Q
area under is energy stored by capacitor
Y = V, X = Q, Straight line proportional through the origin

3
Q

Derivation of E=½QV

A

W = V Q explained (1)
energy stored = total work done in charging = charge x average voltage (1)
energy stored = work done (= ½QV) (1)

4
Q

Time constant definition

A

Time taken for the voltage (or charge or current) to fall to 1/e of its initial value.

5
Q

Finding time constant from a graph of Potential Difference (V) against time (t)

A
  1. Calculate 1/e of initial voltage, V0
  2. Locate value on graph.
  3. Time taken is time constant.
6
Q

Finding time constant from a graph of ln (V) against time

A
  1. ln(V) - ln(V0) =-t/RC
    5.ln(V) =-(1/RC)t + ln(V0)
  2. is in form y=mx+c. Gradient is -1/RC and intercept is ln(V0)
    Time constant (RC) = - (t/gradient)
7
Q

Graphical representations of energy stored against voltage

A

Energy = ½CV2

Energy proportional to V2

8
Q

Graphical representations of charging a capacitor against time

A

V against t
I against t
Q against t

9
Q

Graphical representations of discharging a capacitor against time

A

V against t
I against t
Q against t