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Lecture 9
Electricity
Chapter 5.1  5.11
Outline
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Electric Charge
Coulomb’s Law
Conductors and Insulators
Superconductivity
Ohm’s Law
The Electrical Phenomenon
Discovered in ancient Greece more than 2500 years ago
The phenomenon occurs from rubbing of one material
(e.g., amber  electron in Greek) with another
material (e.g., fur).
It manifests itself by attraction or repellence of
‘charged’ materials.
There are 2 types of charge: positive and negative.
Like charges repel, unlike charges attract.
The Nature of Charge
Charge is not produced by rubbing.
Uncharged objects contain equal amounts of negative
and positive charge. They are electrically neutral.
The nature of charge can be traced to the composition of
matter, to atoms.
Atomic Structure
92 chemical elements have been identified in the Universe.
Nearly 20 more have been created artificially.
Each chemical element is made from a different type of
atom.
Atoms are made from particles called protons, neutrons,
and electrons.
Protons and neutrons form the nucleus in the center of the
atom.
Electrons surround the nucleus.
Atomic Structure
Positively charged protons are hold together by the strong
force, which overcomes electrical repulsion.
Negatively charged electrons are attracted to the nucleus.
Proton has a mass 1.673 10-27 kg and is positively charged
Neutron has a mass 1.675 10-27 kg and is uncharged
Electron has a mass 9.11 10-31 kg and is negatively charged
Proton and neutron are ~2000 times heavier than electron
The Coulomb
The unit of electric charge is the coulomb (C).
The proton has a charge of + 1.6 1019 C.
The electron has a charge of  1.6 1019 C.
The quantity of charge is abbreviated e.
Electric charge occurs only in multiples of e.
Coulomb’s Law:
Q1 Q2
F = K -------R2
K = 9 109 N m2/C2
Electricity and Gravity
The law of gravity and Coulomb’s law have the same form.
However, gravitational force is always attractive.
It is harder to collect a large electric charge of either sign
than a large mass of matter.
Thus, gravity is more significant on a large scale (cosmic
size), while electric forces are more significant on a small
scale (atomic size)
Conductors and Insulators
A conductor is a substance through which electric charge
flows readily. Example, metals.
In an insulator charge flows with great difficulty.
The difference between the 2 types of substances is in
the strength of connection between outer electrons and
nuclei.
Conductivity in fluids and gases involves ions.
Superconductivity
At low temperatures (~1 K) some substances lose their
electric resistance.
This phenomenon, called superconductivity, was discovered
in 1911.
In 1980’s superconductivity at higher temperatures was
discovered (~200 K).
Superconductivity saves a lot of energy, which is
otherwise wasted as heat.
The Ampere
A buttery turns chemical energy into electric energy.
Electrons flow in a wire from negative to positive terminal.
The moving electrons are the electric current.
The unit of electric current is called the ampere.
1 ampere = 1 Coulomb/1 second  1A = 1C/s
Potential Difference
Consider a coulomb of negative charge on the negative
terminal.
It has potential energy (PE).
While flowing from  to +, it produces work due to its PE.
The decrease of PE is called the potential difference
between the 2 terminals.
The potential difference between 2 points is equal to the
corresponding energy difference per coulomb.
It is measured in volts (1 volt = 1J/C) and called voltage.
Ohm’s Law
The electric current in a wire is proportional to the
potential difference.
This is known as Ohm’s law.
The property of a conductor that opposes the flow of charge
in it is called resistance.
I = current
V
I = --R
V = voltage
R = resistance
1 ohm = 1 volt/ampere
1  = 1 V/A
Electric Power
Electric energy is useful, because it is:
• carried by wires
• easily converted into other kinds of energy
The rate at which a current is doing work is called the
power of the current.
Power P = I V (current x voltage) is measured in Watts.
1 Watt = 1 ampere x 1 volt
Summary
• Electrical forces are different from gravity.
They are more influential on small scales.
• Electric properties of matter allow us to transmit and
store energy.
• Most properties of ordinary matter can be traced to
electrical forces