Transcript Review Question - Wellington High School

Review Question
Describe what happens to the lightbulb after the switch
is closed. Assume that the capacitor has large capacitance
and is initially uncharged, and assume that the lightbulb
illuminates when connected directly across the battery
terminals.
Charging a Capacitor in an RC circuit
 As charge builds up on the plates of the capacitor, the voltage across the
capacitor increases and the current flowing decreases.
 The rate of charging decreases as the plates of the capacitor begin to fill up with
charge.
V (t )  V0 (1  e
t

)
 Where V(t) is the potential difference across the capacitor
 V0 is the supply voltage
 t is the time elapsed since charging began
 τ (tau) is the time taken for the voltage to increase to 0.63 of the supply voltage.
It can be found by using the equation
  RC
 Where R is the resistance in the circuit and C is the capacitance in the circuit.
Charging a Capacitor
 After τ seconds, the capacitor
will have reached 63% of its
full amount of charge.
V(t)
V0
0.63V0
 Thus, the charging time can be
increased by increasing the
resistance or by increasing the
capacitance.
 τ is called the time constant
and is given by the equation:
V (t )  V0 (1  e
t

)
  RC
Where R is the resistance in the
circuit and C is the capacitance.
Current in a Charging Capacitor
 As the voltage across the
capacitor increases, it
becomes more difficult for
charge to flow.
 Thus the current will be
maximum at the start of
charging, and will be zero
when the capacitor is
charged to the supply
voltage.
Discharging a Capacitor
 As a capacitor is discharged
through a resistor, the voltage
across it drops.
 The current flowing will be
high initially (as the voltage is
initially large), but will
gradually slow.
 The voltage across the
capacitor will decrease as the
plates lose their charge.
V0
0.37V0
V (t )  V0 e
t

Capacitors Review
Conceptual Quiz: Explaining Capacitors
Conceptual Quiz: Capacitors
 The plates of a capacitor are connected to a battery. What happens
to the charge on the plates if the connecting wires are removed
from the battery? What happens to the charge if the wires are
removed from the battery and connected to each other?
 A farad is a very large unit of capacitance. Calculate the length of
one side of a square, air-filled capacitor that has a capacitance of 1
F and a plate separation of 1 m
 A pair of capacitors are connected in parallel while an identical
pair are connected in series. Which pair would be more dangerous
to handle after being connected to the same battery? Explain.
 If you are given three different capacitors C1, C2 and C3, how
many different combinations of capacitance can you produce?
Capacitors -- Concepts
 The sum of the charges on both plates of a capacitor is zero. What does a




capacitor store?
Because the charges on the plates of a parallel-plate capacitor are
opposite in sign, they attract each other. Hence, it would take positive
work to increase the plate separation. What type of energy in the system
changes due to the external work done in this process?
Explain why the work needed to move a charge Q through a potential
difference V is W = QV whereas the energy stored in a charged
capacitor is Ep = ½Q V.
If the potential difference across a capacitor is doubled, by what factor
does the energy stored change?
If you were asked to design a capacitor where small size and large
capacitance were required, what factors would be important in your
design?