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, Second Edition by Fawwaz T. Ulaby and Michel M. Maharbiz, © NTS
Press, Used with Permission by the Publisher
F/m
Capacitors: Where is the current?
Current = flow of charges
I = dq/dt
Air (no charges)
Circuits, Second Edition by Fawwaz T. Ulaby and Michel M. Maharbiz, © NTS
Press, 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, Second Edition by Fawwaz T. Ulaby and Michel M. Maharbiz, © NTS
Press, 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 T.
airUlaby
space,
because
Circuits, Second Edition by Fawwaz
and
Michel itM. Maharbiz, © NTS
is Publisher
an open circuit, R = ∞
Press, Used with Permission by the
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.
+ 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
Capacitors: What happens when you
first turn on the voltage?
Meanwhile it starts to
Charges flow
from voltage
The
voltage
starts
out
as
source
It can’t change
towards
capacitor
+
+
+
build a voltage.
vs and then grows.
instantly.
a
+
+
b
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?
Meanwhile, this starts to create
+ 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
What does a capacitor do to the
current ?
18
What does a capacitor do to the
current ?
19
What does a capacitor do to the
current ?
20
What does a capacitor do to the
current ?
Vs/R
36% at
t=RC
21
What does a capacitor do to the
voltage ?
22
Vs
66% at
t=RC
23
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
24
Charging and Discharging
http://lwn.net/Articles/250967/
25
How can we use capacitors?
• Energy Storage / Release
26
How can we use capacitors?
• Stabilize power (reduce ripple)
http://www.beavisaudio.com/techpages/Caps/
27
Low Pass filter
Noise Filter
28
Integrator
http://www.piclist.com/images/www/hobby_elec/e_ckt1.htm
29
High Pass Filter
DC Block
30
Differentiator
31
http://www.piclist.com/images/www/hobby_elec/e_ckt1.htm
Introduction to Electrical & Computer Engineering
Antelope Island, Great Salt Lake, Utah
Dr. Cynthia Furse
University of Utah
32