Download Lecture 5-2 Capacitor Intro

Introduction to Electrical & Computer Engineering
Capacitors
Dr. Cynthia Furse
University of Utah
Dr. Cynthia Furse
University of Utah
1
Capacitors
•
•
•
•
What is Capacitance ?
How does it relate to current & charges?
Explain the effect of ɛ, A, d
What does it do to a voltage and current?
2
Capacitors
Passive element that stores energy in electric field
Parallel plate capacitor
C 
A
d
-12
Ɛo = 8.854 x 10
Ɛr = 1 for air
= 80 for water
Circuits, FTUlaby, MMMaharbiz,CMFurse, © NTS Press 2015, Used with
Permission by the Publisher
F/m
Capacitors: Where is the current?
Current = flow of charges
I = dq/dt
Air (no charges)
Circuits, FTUlaby, MMMaharbiz,CMFurse, © NTS Press 2015, Used with
Permission by the Publisher
Capacitors: Where is the current?
Charges flow
from voltage
source
towards
capacitor
Current = flow of charges
I = dq/dt
+
+
+
Air (no charges)
Circuits, FTUlaby, MMMaharbiz,CMFurse, © NTS Press 2015, Used with
Permission by the Publisher
Capacitors: Where is the current?
Current = flow of charges
I = dq/dt
+ Charges
accumulate
on the top
plate
+
+
+
+
+
+
+
+
+ + +
Note: This
creates a
voltage
+
a ++
++
Air (no charges)
b
The charges can’t cross
the air space,
because
it
Circuits, FTUlaby, MMMaharbiz,CMFurse,
© NTS
Press 2015,
Used with
is an open circuit, R = ∞
Permission by the Publisher
Capacitors: Where is the current?
Current = flow of charges
I = dq/dt
+ Charges
accumulate
on the top
plate
+
+
+
+
+ + ++
- +
++ + +
Note: This
creates a
voltage
+
a ++
++
Air (no charges)
b
The + charges on top push away the + charges
on the bottom. They flow towards the source
and leave behind – charges on the bottom plate
Conduction = Displacement Current
Conduction Current = flow of charges
I = dq/dt
Note: This
+ Charges
creates a
voltage
accumulate
on the top
plate
+
+
+
+
+
+
+ + ++
a
Displacement Current
+
+
= appearance+
of the
flow of charges,
- induced by time
b
varying
electric
field
+
+ + + +Conduction Current = flow of charges
Air (no charges)
The + charges on top push away the + charges
on the bottom. They flow towards the source
and leave behind – Icharges
on the bottom plate
= dq/dt
Capacitors: Where does this mean?
C = ɛA/d
+
+
+
+
+ + ++
Larger ɛ
- -increases
attraction
between
+
+ + + + the
charges
Greater A can store
more charges
Larger d
reduces
attraction
between the
charges on the
Air (no charges)
two plates
How to ‘Build with this’
C = ɛA/d
Two pieces of metal with foam between.
Squish them (d)
Slide them (A)
Wet the foam (ɛ)
http://www.chenyang-ism.com/CapaSensorPosi.htm
10
Capacitors
• What is Capacitance ?
• How does it relate to current & charges?
• Explain the effect of ɛ, A, d
11
Electrical Properties of Capacitors
• Capacitance – current, charge, voltage,
power, energy
• What happens at DC?
• Series and Parallel
12
Capacitors: Current & Charge
Current = flow of charges
I = dq/dt
At DC there is no change with time (d/dt = 0)
So…I=0 >>>> OPEN CIRCUIT
Air (no charges)
Circuits, Second Edition by Fawwaz T. Ulaby and Michel M. Maharbiz, © NTS
Press, Used with Permission by the Publisher
Capacitors: What happens when you
first turn on the voltage?
Charges flow Current immediately starts to
from voltage
It
is
largest
at
the
start.
source
towards
capacitor
flow.
+
+
+
Air (no charges)
Circuits, Second Edition by Fawwaz T. Ulaby and Michel M. Maharbiz, © NTS
Press, Used with Permission by the Publisher
Capacitors: What happens when you
first turn on the voltage?
Current immediately starts to flow.
But then the plates start to get ‘full’, so it
decreases
with time, and eventually goes to 0.
+ Charges
accumulate
on the top
plate
+
+
+
+
+ + ++
- +
++ + +
Air (no charges)
The + charges on top push away the + charges
on the bottom. They flow towards the source
and leave behind – charges on the bottom plate
What does a capacitor do to the
current ?
21
What does a capacitor do to the
current ?
22
What does a capacitor do to the
current ?
23
What does a capacitor do to the
current ?
Vs/R
36% at
t=RC
t
24
Capacitors: What happens when you
first turn on the voltage?
Meanwhile it starts to build a voltage.
Charges flow
from
voltage
The
voltage
starts
out
as
0
and
then
grows
to
source
It can’t change instantly.
towards
capacitor
+
+
+
a
Vs.
+
+
b
Air (no charges)
Circuits, Second Edition by Fawwaz T. Ulaby and Michel M. Maharbiz, © NTS
Press, Used with Permission by the Publisher
What does a capacitor do to the
voltage ?
26
Vs
27
Vs
66% at
t=RC
t
28
What does a capacitor do to a
voltage and current?
Current changes
Instantly
At ‘steady state’
Current =0
At ‘steady state’
Voltage is Vs
Voltage changes
slowly
• t=0: SHORT CIRCUIT
• t=∞: OPEN CIRCUIT
29
RC Charging and Discharging
http://lwn.net/Articles/250967/
30
How can we use capacitors?
• Energy Storage / Release
31
How can we use capacitors?
• Stabilize power (reduce ripple)
http://www.beavisaudio.com/techpages/Caps/
32
Low Pass filter
Noise Filter
33
Integrator
http://www.piclist.com/images/www/hobby_elec/e_ckt1.htm
34
High Pass Filter
DC Block
35
Differentiator
36
http://www.piclist.com/images/www/hobby_elec/e_ckt1.htm
Capacitors
•
•
•
•
What is Capacitance ?
How does it relate to current & charges?
Explain the effect of ɛ, A, d
What does it do to a voltage and current?
37
Introduction to Electrical & Computer Engineering
Antelope Island, Great Salt Lake, Utah
Dr. Cynthia Furse
University of Utah
38