Electromagnetism

Electromagnetism is one of the fundamental forces in nature, and the the dominant force in a vast range

of natural and technological phenomena

The electromagnetic force is solely responsible for the structure of matter, organic, or inorganic Physics, chemistry, biology, materials science

The operation of most technological devices is based on electromagnetic forces. From lights, motors, and batteries, to communication and broadcasting systems, as well as microelectronic devices.

Electrostatics

Electric Charge

The Transfer of Charge

SILK

Glass Rod

Some materials attract electronsmore than others.

Electric Charge

The Transfer of Charge

SILK

Glass Rod

-+

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

Electric Charge

The Transfer of Charge

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 a deficit.

Electric Charge

The Transfer of Charge

SILK

Glass Rod

-

+++++

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

----

Electric Charge

+ +

Two positively charged rodsrepel each other.

Electric Charge

History

600 BC Greeks first discover attractiveproperties of amber when rubbed.

1600 AD Electric bodies repel as well as attract1735 AD du Fay: Two distinct types of electricity1750 AD Franklin: Positive and Negative Charge1770 AD Coulomb: “Inverse Square Law”1890 AD J.J. Thompson: Quantization of

electric charge - “Electron”

Electric Charge

Summary of things we know:

– There is a property of matter called electric charge. (In the SI system its units are Coulombs.)

– Charges can be negative (like electrons) or positive (like protons).

– In matter, the positive charges are stuck in place in the nuclei. Matter is negatively charged when extra electrons are added, and positively charged when electrons are removed.

– Like charges repel, unlike charges attract.

– Charges travel in conductors, not in insulators

– Force of attraction or repulsion ~ 1 / r2

Charge is Quantized

q = multiple of an elementary charge e:

e = 1.6 x 10-19 Coulombs

Charge Mass Diameterelectron - e 1 0proton +e 1836 ~10-15mneutron 0 1839 ~10-15m

positron +e 1 0

Coulomb’s Law

q1 q2

r12

F12

Force on 2 due to 11 2

12 212

kq qF

r

k = (40)-1 = 9.0 x 109 Nm2/C2

Coulomb’s law describes the interaction between bodies due to their charges

Gravitational and Electric Forces in the Hydrogen Atom

+e -eM

m

r12m = 9.1 10-31 kgM = 1.7 10-27 kgr12 = 5.3 10-11 m

Gravitational force Electric Force

Gravitational and Electric Forcesin the Hydrogen Atom

+e -eM

m

r12

m = 9.1 10-31 kgM = 1.7 10-27 kgr12 = 5.3 10-11 m

Gravitational force Electric Force

Fg = 3.6 10-47 N

212r

MmGFg

Gravitational and Electric Forces in the Hydrogen Atom

+e -eM

m

r12m = 9.1 10-31 kgM = 1.7 10-27 kgr12 = 5.3 10-11 m

Gravitational force Electric Force

Fg = 3.6 10-47 N

F G

Mm

rrg

122

2

1204

1

r

QqFe

Fe = 3.6 10-8N

Conductors and Insulators

• Conductors are materials in which the electric charges move freely

– Copper, aluminum and silver are good conductors (more or less all metals!)

– When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material

Insulators

• Insulators are materials in which electric charges do not move freely

– Glass and rubber are examples of insulators

– When insulators are charged by rubbing, only the rubbed area becomes charged

• There is no tendency for the charge to move into other regions of the material

Semiconductors

• The characteristics of semiconductors are between those of insulators and conductors

• Silicon and germanium are examples of semiconductors

Charging by Conduction

• A charged object (the rod) is placed in contact with another object (the sphere)

• Some electrons on the rod can move to the sphere

• When the rod is removed, the sphere is left with a charge

• The object being charged is always left with a charge having the same sign as the object doing the charging

Metallic sphere

Charging a Metal Object by Induction

• When an object is connected to a conducting wire or pipe buried in the earth, it is said to be grounded

• A negatively charged rubber rod is brought near an uncharged sphere

• The charges in the sphere are redistributed– Some of the electrons in the

sphere are repelled from the electrons in the rod

Charging by Induction

• The wire to ground is removed, the sphere is left with an excess of induced positive charge

• The positive charge on the sphere is evenly distributed due to the repulsion between the positive charges

• Charging by induction requires no contact with the object inducing the charge

Polarization

• In most neutral atoms or molecules, the center of positive charge coincides with the center of negative charge

• In the presence of a charged object, these centers may separate slightly

– This results in more positive charge on one side of the molecule than on the other side

• This realignment of charge on the surface of an insulator is known as polarization

Examples of Polarization

• The charged object (on the left) induces charge on the surface of the insulator

• A charged comb attracts bits of paper due to polarization of the paper