Basic Electric Circuits Session 15A

2Basic Electric Circuits – © 2020 Mouli Sankaran Email: mouli.sankaran@yahoo.com 2
Capacitors

Basic Electric Circuits – © 2020 Mouli Sankaran Email: mouli.sankaran@yahoo.com 3
Session 15A: Focus
 What is a Capacitor?
 Nature’s Huge Capacitor
 Construction of Capacitors
◦ Unit of Capacitor (Farad)
 Dielectric Materials

Capacitors

What is a Capacitor?
 Capacitors (sometimes known as condensers) are energy-
storing devices that are widely used in
◦ Television, Radios, electronic equipments
 Tune a radio into a station, take a flash photo with a digital
camera, you're making good use of capacitors
 Capacitors have the ability to both store and deliver finite
amounts of energy
 They differ from ideal sources in this respect, since they
cannot sustain a finite average power flow over an infinite
time interval
6

Nature’s Huge Capacitors

Huge Capacitors in Nature!!!
Discharging of
Nature’s
Huge Capacitors
is Lightning!!
Clouds are the
Floating
Huge Capacitors!!!

Huge Capacitors in Nature Explained
1. The top of a cloud becomes positively
charged when smaller ice particles
swirl upward
2. The bottom of a cloud becomes
negatively charged when the heavier
ice particles gather lower down
◦ The separation of positive and negative
charges in a cloud makes a kind of moving
capacitor
3. The huge negative charge at the bottom
of the cloud repels negative charge
away from it, so the ground effectively
becomes positively charged
9

Huge Capacitors in Nature Explained
• The separation of charge between the
bottom of the cloud and the ground beneath
means that this area of the atmosphere is
also, effectively, a capacitor
• Over time, enormous electrical charges can
build up inside clouds, resulting in an
enormous amount of electrical potential
energy
4. When the voltage reaches a certain level
(millions of volts), the air is transformed
from being an insulator into a conductor,
and electricity will flow through it as
though it were a metal wire, creating a
giant spark better known as a bolt of
lightning
10

Construction of Capacitors

Definition of a Capacitor
 Capacitor is a component which has the
ability or “capacity” to store energy in the
form of an electrical energy producing a
potential difference (Static Voltage) across
its plates, much like a small rechargeable
battery.

Construction of a Capacitor
 Take two electrical
conductors/plates (things that
let electricity flow through
them) and
 Separate them with
an insulator (a dielectric
material that doesn't let
electricity flow very well) and
 You make a capacitor:
13
 Something that can store electrical energy.
 Adding electrical energy to a capacitor is called charging;
 Releasing the energy from a capacitor is known is discharging.

Unit of Capacitor (Farad)
 Capacitance is the electrical property of a
capacitor
 And is the measure of a capacitor’s ability to
store an electrical energy onto its two plates
 The unit of capacitance is
the Farad (abbreviated to F) named after the
British physicist Michael Faraday
 A capacitor has the capacitance of One
Farad when electrical energy equivalent
to One Coulomb of charge is stored in it by a
voltage of One volt
◦ C = Q/V
Inventions:
Electromagnetism
Benzene
Bunsen Burner
Popularized:
Anode, Cathode,
Electrode, Ion, …

Unit of Capacitor .. Continued
 Capacitance, C is always positive and has no
negative units.
 However, the Farad is a very large unit of
measurement to use on its own
◦ One Coulomb of charge = 6.25x1018 electrons
 So, sub-multiples of the Farad are generally used
such as:
◦ 1 micro-farads (μF) = 10-6 F
◦ 1 nano-farads (nF) = 10-9 F
◦ 1 pico-farads (pF) = 10-12 F

Dielectric Materials

Insulator Vs Dielectric Material
 The term insulator is generally
used to indicate electrical
obstruction
 While the term dielectric is used
to indicate the energy storing
capacity of the material
◦ By means of polarization
 A common example of
a dielectric is electrically
insulating material between the
metallic plates of a capacitor.

Different Dielectric Materials Used in
Capacitors
 When they are placed in the electric field between two capacitor
plates, they line up with their charges pointing opposite to the
field, which effectively reducing the E.
 That reduces the potential (V) on the plates and, which
increases their capacitance
◦ Ability to store more electrical energy
 The best dielectric
materials are made of
polar molecules
 Have more positive
electric charge on one side
and more negative electric
charge on the other

Permittivity (ε) Explained
 Different materials make better or worse dielectrics
according to how well they insulate the space between a
capacitor's plates and
◦ Reduce the electric field between them, there by reducing the
voltage across the plates
 A measurement called the relative permittivity (ε) tells
us how good a dielectric material is.
 A vacuum is the worst dielectric and is given a relative
permittivity of one
◦ Other dielectrics are measured relative (by comparing them) to a
vacuum
◦ Paper is about 3 times higher
◦ Water is 80 times better than vacuum (but not used in capacitors)

Session 15A: Summary
 What is a Capacitor?
 Nature’s Huge Capacitor
 Construction of Capacitors
◦ Unit of Capacitor (Farad)
 Dielectric Materials

References
Ref 1 Ref 2

Basic Electric Circuits Session 15A

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