Transcript Capacitors_Review - Beverley High School

Capacitors Review
6V
Remember
Q=VC
A
Calculate the
charge stored when
the capacitor is fully
charged here.
200μF
Q
The charge
stored is the
area under this
graph
What does
the gradient
of this
graph
represent?
t
V
I
t
Energy Stored
Q=VC implies that graphs of Q against V must be straight line graphs
Q
The
gradient
here is C
E =1/2 QV
V
Energy Stored
V
The
gradient
here is 1/C
E =1/2 QV
Q
Charging and discharging with
constant current
Charging and discharging with
constant
current
In these situations
6V
it is possible to use
Q=It to get the
value of the charge
A constant current of
0.5milliamps flows into
this capacitor for 10s
A
A)What charge is
stored?
B) What is the
capacitance of the
capacitor
It is possible to charge
or discharge a capacitor
keeping the current
constant by using a
variable resistor
V
Look for the key words constant current in questions
I
t
Charging and discharging with
constant
current
In these situations
6V
it is possible to use
Q=It to get the
value of the charge
A
200μF
V
I
What is the
numerical value of
the area under this
graph?
t
Charging and discharging with
constant
current
6V
A
A constant current of 0.4 milliamps
flows for 10s. Calculate
a) the charge stored on the
capacitor
b) The capacitance of the
capacitor
V
I
The area under
an I/t graph is
the charge that
was stored
t
Charging and discharging with
constant current
A
200μF
V
Alternatively they may give you an average value for the
current when the capacitor charges or discharges.Use
this average value with Q=It to calculate the charge
stored
An average current
of 3 x 10-4A flows for
5 seconds calculate
the final p.d. across
the capacitor.
The effect of a resistance on the
charging and discharging
1.5KΩ
6V
• Putting a resistor in
series with the
capacitor increases
the charging time
• and increases the
discharging time.
2 200μF
What is the time constant for this
arrangement?
Time constant = RC
Exponential decay
Charge μC
Io
1
I   Io
e
I = 0.368Io
A capacitor of 1000μF is in series
with a 1kΩ resistor.
0.368Io
It originally has a charge of
0.05C.How long will it take
the charge to fall to
(0.368)2Io
(0.368)3Io
a)
RC
2RC
Time s
0.368 of its value?
b) b) 0.0068C
3RC
The time it takes the current to fall by a factor of 1/e is a constant.
That time interval is RC the time constant
Exponential decay of charge through a capacitor in series with a fixed
resistance
Q  Qo e
t
CR
Qo
Charge Coulombs
A capacitor and resistor in series has
a time constant of 10s.
The capacitor when fully charged has
a charge of 0.5C.
Calculate the remaining charge 15s
into the discharge.
Time s