Transcript A Brief History of Electricity

A Brief History of Electricity
and Magnetism
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 electrons
more 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 an
excess 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
History
600 BC
1600 AD
1735 AD
1750 AD
1770 AD
1890 AD
Greeks first discover attractive
properties of amber when rubbed.
Electric bodies repel as well as attract
du Fay: Two distinct types of electricity
Franklin: Positive and Negative Charge
Coulomb: “Inverse Square Law”
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
electron
proton
neutron
Charge
-e
+e
0
Mass
1
1836
1839
Diameter
0
~10-15m
~10-15m
positron
+e
1
0
Coulomb’s Law
q1
q2
F12
r12
F 12
kq1q2

2
r12
Force on 2 due to 1
k = (4pe0)-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
M
+e
r12
-e
m
Gravitational force
m = 9.1 10-31 kg
M = 1.7 10-27 kg
r12 = 5.3 10-11 m
Electric Force

Mm
Fg  G 2 r
r12
  1  Qq
 2
Fe  
 4pe0  r12
Fg = 3.6 10-47 N
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
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
Resistance in a Series Circuit
• Each resistance in a
series circuit adds to the
total resistance of the
circuit.
Rtotal = R1 + R2 + R3...
Total resistance
(ohms)
Individual resistances (W)
• Each separate resistance creates a
voltage drop as the current passes
through.
• As current flows along a series
circuit, each type of resistor
transforms some of the electrical
energy into another form of energy
• Ohm’s law is used to calculate the
voltage drop across each resistor.
Resistance in Parallel Circuit
• In parallel circuits the current can take more than one path.
• Because there are multiple branches, the current is not the same
at all points in a parallel circuit.
Short Circuit
• A short circuit is a parallel path in a circuit with
zero or very low resistance.
• Short circuits can be made accidentally by
connecting a wire between two other wires at
different voltages.
• Short circuits are dangerous because they can
draw huge amounts of current.
Circuit Analysis
What is magnetism?
Magnetism is the properties and interactions of
magnets
The earliest magnets were found naturally in the mineral
magnetite which is abundant the rock-type lodestone.
These magnets were used by the ancient peoples as
compasses to guide sailing vessels.
Magnets produce magnetic forces and have magnetic
field lines
Magnets have two ends or poles, called north and
south poles. At the poles of a magnet, the magnetic field
lines are closer together.
Unlike poles of magnets attract
each other and like poles of
magnets repel.
The earth is like a giant magnet!
The nickel iron core of the earth gives the earth a
magnetic field much like a bar magnet.
What are magnetic domains?
Magnetic substances like iron, cobalt, and nickel are
composed of small areas where the groups of atoms are
aligned like the poles of a magnet. These regions are
called domains. All of the domains of a magnetic
substance tend to align themselves in the same direction
when placed in a magnetic field. These domains are
typically composed of billions of atoms.
Electricity and Magnetism – how
are they related?
When an electric current passes through a wire a
magnetic field is formed.
What is an electromagnet?
When an electric current is passed through a coil of wire
wrapped around a metal core, a very strong magnetic
field is produced. This is called an electromagnet.
What are electric motors?
An electric motor is a device which changes electrical
energy into mechanical energy.
How does an electric motor work?
Go to the next slide 
Simple as that!!
We have seen how electricity can produce a magnetic
field, but a magnetic field can also produce electricity!
How?
What is electromagnetic induction?
Moving a loop of wire through a magnetic field produces
an electric current. This is electromagnetic induction.
A generator is used to convert
mechanical energy into electrical energy by
electromagnetic induction.
Carefully study the next diagrams: