Electric Force One of the four fundamental forces Responsible for much of our technology Governs...
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![Page 1: Electric Force One of the four fundamental forces Responsible for much of our technology Governs chemistry which deals with interactions of the outer electrons.](https://reader035.fdocuments.in/reader035/viewer/2022062719/56649eda5503460f94be8f4d/html5/thumbnails/1.jpg)
Electric Force
• One of the four fundamental forces
• Responsible for much of our technology
• Governs chemistry which deals with interactions of the outer electrons between atoms or groups of atoms
• Interesting topic because it also involves the concept of magnetism
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Electric Force
• We are all familiar with static electricity
• When the humidity is low, we can walk across a carpet wearing leather-soled shoes and give someone a big awakening
• What is happening here?
• Transferring some entity into our fingertips
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Electric Charge
• We now know that elementary particles like the electron and proton have a quality known as charge.
• Objects that accumulate charge have either gained or lost electrons
• These objects experience large forces as a result of being “charged”
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Electric Charge
• There are two kinds of charges which we have called positive and negative
• We have defined the charge of the electron to be negative and the charge of the proton to be positive
• What it is interesting is that “Like charges repel and unlike charges attract!”
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Electric Charge
• Earlier we studied some conservation laws that turned out to be very important
• The conservation of energy, linear momentum and angular momentum have major consequences for the way the world behaves
• Electric charge is also a conserved quantity and for every positive charge there seems to be a corresponding negative charge
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Conservation of Charge
• In any process, the net amount of electric charge produced is zero!
• This means that if we add up all the positive charges and all the negative charges moving around in a process, the total charge will sum to zero
• So we can neither create nor destroy charge
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Electric Charge
• Charged objects are composed of atoms, a few of which have either gained or lost an electron
• Those special atoms are called ions
• On a macroscopic level, ions eventually gain or lose an electron from contact with something else and return to neutral status
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Insulators and Conductors
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Insulators and Conductors
• Materials such as metals conduct electricity• This means that charges can move freely
through the material (electrons do the moving)
• Other materials, such as wood or glass do not conduct electricity
• Electrons cannot move freely in these materials
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Insulators and Conductors
• A few materials fall somewhere between these two extremes and are called semiconductors
• Examples are silicon and germanium
• The key is whether or not the electrons in the material are tightly bound to individual atoms
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Insulators and Conductors
• In metals, not all the electrons are required for bonding the atoms into a lattice
• At least one electron per atom is free to move around at will in the material
• It is almost like the molecules of air can move freely anywhere in a room
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Insulators and Conductors
• Since the metals have free electrons, they can easily allow charges to move
• In insulators, the electrons are tightly bound to individual atoms and are not free to move
• The result is they do not conduct electricity• In semiconductors, only a small fraction of
the electrons are able to move. We’ll learn more next semester
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Induced Charge
Touch a neutral metal rod with a charged metal rod and you will transfer some charges to the neutral rod
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Induced Charge
Bring a charged metal rod close to a neutral metal rod and you can separate charges on the neutral rod. Note charge conservation is at work.We have induced charges at the
ends of the neutral rod.
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Induced Charge
The earth is large and can conduct. So, when you separate the charges and allow the electrons to flow to the earth, you can leave a charged object behind.
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Coulomb’s Law
By sharing charges between two objects, Coulomb was able to make charges that were 1/2, 1/4, etc. of the original charge on a rod.
He then could measure the forces between two charged objects.
€
F =kQ1Q2
r2
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Coulomb’s Law
• The standard unit of charge is the Coulomb
• We’ll define it later in terms of magnetic fields, but for now all we need is the constant k in SI units.
• We want force in Newtons, distance in meters and charge in Coulombs
• K = 8.988 x 109 N m2/C2
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Coulomb’s Law
• The Coulomb is a HUGE charge
• The electron has a charge 1.6 x 10-19 C and is designated as e.
• The net charge on any object is some multiple of this fundamental charge
• We say charge is quantized because there is a smallest value, namely e
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Coulomb’s Law
• Strictly speaking, Coulomb’s Law applies only to point charges (no finite extent in space)
• However, for spheres, can consider all the charge concentrated at a point in the center
• Just like gravity applies to a sphere (like the earth)
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Coulomb’s Law
• Compare electrical to gravitational force in a hydrogen atom
€
Felectrical=9×1091.6×10−19( )
2
0.53×10−10( )
2=8.2×10−8
Fgravity=6.67×10−111.67×10−27×9.11×10−31
0.53×10−10( )
2=3.61×10−47
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Coulomb’s Law
• The electric force is a vector!
• Superposition of forces Fnet = F1 + F2 + …
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Coulomb’s Law
• The electric force is a vector!
• Superposition of forces Fnet = F1 + F2 + …