Lecture 9 Electricity

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Lecture 9Electricity
Chapter 5.1 ? 5.11
Outline

• Electric Charge
• Coulombs Law
• Conductors and Insulators
• Superconductivity
• Ohms Law

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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.
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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.
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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.
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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
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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.
Coulombs Law
Q1 Q2 F K -------- R2
K 9 109 N m2/C2
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Electricity and Gravity
The law of gravity and Coulombs 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)
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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.
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Superconductivity
At low temperatures (1 K) some substances lose
their electric resistance.
This phenomenon, called superconductivity, was
discovered in 1911.
In 1980s superconductivity at higher
temperatures was discovered (200 K).
Superconductivity saves a lot of energy, which is
otherwise wasted as heat.
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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
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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.
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Ohms Law
The electric current in a wire is proportional to
the potential difference.
This is known as Ohms law.

• The property of a conductor that opposes the flow
  of charge in it is called resistance.

I current V voltage
R resistance
V I --- R
1 ohm 1 volt/ampere 1 ? 1 V/A
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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
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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