module m2 electrical engineering -...

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Module M2‐2Electrical Engineering

LECTURE 2 ELECTROSTATICS I

http://youtu.be/P_JtU0dKITE (Electrostatic song)

VIDEO 1:

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Topics3

Review What is electrostatics Electrostatics phenomena Transfer of charges Insulators and conductors Polarization Electrostatic generator Electrostatic discharges Risks and applications of static electricity Risks and applications of static electricity Electrostatic charge- Conclusion Coulomb’s law Fields and forces Superposition of forces

Review : quantized charge4

Charge Mass Diameterelectron - e 1 ~10-18m

q = multiple of an l h

proton +e 1836 ~10-15mneutron 0 1839 ~10-15m

positron +e 1 ~10-18m

elementary charge e

e = 1.6 x 10-19 C

All charges available must be

integral multiples of “e”

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What is Electrostatics?5

Electrostatics is the phenomena and properties of stationary or slow-moving properties of stationary or slow-moving electric charges with no acceleration.

Electrostatic phenomena arise from the forces that electric charges exert on each other other.

Electrostatics phenomena (1)6

Since classical antiquity, it has been known that some materials such as amber attracts lightweight particles after rubbing (e.g. with fur).

The Greek word for amber, ήλεκτρον (elektron), was the origin of the word 'electricity'.

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Transfer of charges

SILK

Glass Rod

Some materials attract electronsmore than others.

Transfer of charges

SILK

Glass Rod

-+

As the glass rod is rubbed against silk, electrons are pulled off the glass onto the silk.

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Transfer of charges

SILK

Glass Rod

--+

+

Usually matter is charge-neutral, because the number of electrons and protons are equal. But here the silk has anexcess of electrons and the rod has a deficit.

Transfer of charges

SILK

Glass Rod

-

+++++

Gl d ilk i l t h t k th t t

----

Glass and silk are insulators: charges stuck on them stay put.

What materials trend to give up/attract electrons when brought in contact with other materials? Why?

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Two materials only need to come into contact and then separate for electrons to be exchanged.

After coming into contact, h i l b d i f da chemical bond is formed

between some parts of the two surfaces, called adhesion, and charges move from one material to the other to equalize their electrochemical potential. This is what creates the net charge imbalance between the objects.

When separated, some of the bonded atoms have a tendency to keep extra electrons, and some a tendency to give them away,

Insulators and Conductors

In insulators, electrons are bound in “orbit” to the nucleus in each atom.

When charge is placed on an insulator it stays in one region

Insulators

When charge is placed on an insulator, it stays in one region and does not distribute.

In conductors electrons can move from atom to atom, thus electricity can “flow”.

Conductors

Wood, plastic, glass, air, and cloth are good insulators.

CHARGED INSULATOR

When a charge is placed on a conductor, it redistributes to the outer surface.

Metals (copper, gold, and aluminum) are good conductors.

CHARGED CONDUCTOR

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Polarization

In a conductor, “free” electrons can move around the surface of the material, leaving one side positive and the other side negative.

Polarization is the separation of charges

In an insulator, the electrons “realign” themselves within the atom (or molecule), leaving one side of the atom positive and the other side of the atom negative.

Polarization is not necessarily a charge imbalance!

Electrostatic generator

Van de Graaff generator

http://www.youtube.com/watch?v=WS9ISUXBsa8(Van de graaff generator)

VIDEO 2:

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Electrostatics discharges (1)15

Grounding: connecting an object to a large body, like Earth, that is capable of removing any electric charge on the object

Electrostatics discharge (2)

electric charge on the object

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Potential risks of electrostatic electricity17

• Aircraft and spacecraft lying in weather will develop static charge from air friction on the airframe. Static gdischarge causes disruption in communication.• Static discharges causes ignitions and explosions in presence of volatile liquids and flammable gasses( petrol stations, fuel tanks of air craft/fuel transport trucks), grain dusts (in grain mills).

D l i d i bl CMOS• Destroy electronic devices, most notably CMOSintegrated circuits and MOSFET transistors

Applications of static electricity (1)

Electrostatic Spray Painting: positively charge the object and negatively charge the paint particlesj g y g p p

Opposite chargesattract

-paint particles are attracted to the object and less is lost as over spray

http://youtu.be/LAEGkaqKZhQ(Electrostatic spraying)

VIDEO 3:

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Electrostatic Duster: rubbing the duster across a surface causes the duster to become charged by s c c s s d s b c c g d byfriction.

-neutral dust particles stick to charged duster


Electrostatic Precipitator: a device to remove dust particles from the exhaust gas from industryp c s s g s d s y

http://youtu.be/LYvs8w9CcvM(Electrostatic precipitator)

VIDEO 4:

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Photocopying: selenium is an insulator but becomes a conductor when exposed to lighta conductor when exposed to light

http://youtu.be/3wDaOAp31Bo(Photocopier)

VIDEO 5:


Capacitor:

A capacitor can be charged by connecting it to aA capacitor can be charged by connecting it to a battery or any other source of current.

A capacitor can be discharged by connecting it to any closed circuit that allows current to flow.

http://www.youtube.com/watch?v=enmQy_2qv4o (Charging capacitor)

VIDEO 6:

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Electrostatic charge- Conclusion23

• Objects can lose or gain electric charges.

• The net charge is also sometimes called excess charge because a charged object has an excess of either positive or negative charges.

• A tiny imbalance in either positive or negative charge on anpositive or negative charge on an object is the cause of static electricity.

• The electrostatic forces are described by Coulomb's law.

Coulomb’s law (1)

Charles-Augustin de Coulomb (14 June 1736 – 23 August 1806) was a French physicist.

He was best known for developing Coulomb's law, the definition of the electrostatic force of attraction and repulsion.

The SI unit of electric charge, the coulomb, was named after him.

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Coulomb’s law (2)The magnitude of the electrostatics force of interaction

between two point charges is

:directly proportional to the scalar multiplication of:directly proportional to the scalar multiplication of the magnitudes of charges (q1 and q2) and

:inversely proportional to the square of the distances (r2) between them.

Coulomb's law (3)

The force between two charges gets stronger as the charges move closer together.

The force also gets stronger if the amount of h b lcharge becomes larger.

http://www.youtube.com/watch?v=g_3vRfA1wSU&feature=youtu.be (Coulomb’ s law)VIDEO 7:

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Fields and forces (1)

The concept of a field is used to describe any quantity that has a value for all points in space.

Y hi k f h fi ld h f i d You can think of the field as the way forces are transmitted between objects.

Charge creates an electric field that creates forces on other charges.

VIDEO 8: http://www.youtube.com/watch?v=F1z5UaX96j8 (Electric field line)


Comparing to gravitational force

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Mass creates a gravitational field that exerts forces gon other masses.


Comparing gravitational (attractive) force between two electrons and electric (repulsive) force betweentwo electrons and electric (repulsive) force between them .

Gravitational forces are far weaker than electric forces

( 10 36 times weaker).

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+Q

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+Q+q

Test charge

Q qF


Definition of electric field:

qF

E qQ

q

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VIDEO 9: http://www.youtube.com/watch?v=bG9XSY8i_q8&feature=youtu.be (Electric field of a dipole)

VIDEO 10 : electric field lines(castor oil, semolina seeds)

Superposition of forces from two chargesBlue charges fixed , negative, equal charge (-q)What is force on positive red charge +q ?

y

x

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Superposition of forces from two charges

Consider effect of each charge separately:

Blue charges fixed , negative, equal charge (-q)What is force on positive red charge +q ?

y

x


Take each charge in turn:


y

g

x

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Create vector sum:


y

x


Find resultant:


y

NETFORCE

x

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Interesting Links

http://webphysics.davidson.edu/physlet_resources/bu_semester2/c01_magnitude.html/ _ g

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Interesting Links

http://www.rmutphysics.com/charud/virtualexperiment/Explore/InteractionEC/index.htm/ /

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Interesting Links

http://www.dgp.toronto.edu/~mjmcguff/research/electrostatic/applet1/main.htmlpp /

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Interesting Links

http://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.html

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