lec 1 Physics 72
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Transcript of lec 1 Physics 72
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Lecture 1Electric charge
August 5, 2015
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Electric charge
an intrinsic property of matter, just like mass
SI unit: coulomb (C)
can be positive or negative opposite charges attract, like charges repel
Remark
Theres nothing negative about negative charges.
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Structure of an atom
If, in some cataclysm, all of scientific knowledge were to be destroyed,and only one sentence passed on to the next generations of creatures,what statement would contain the most information in the fewest words?
I believe it is. . .that all things are made of atoms. . .
Richard Feynman
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Structure of an atom
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Charge is quantized
Definition (Elementary charge (e))
The magnitude of electron and proton charge. It is equal to 1.
6
1019C
.
Charges come in multiples of the elementary charge
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Conductors and insulators
conductors electrons move freely
insulators electrons are bound to atoms
conductor insulatormetal plasticwater air
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Charging by rubbing
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Charging by rubbing
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Charging by rubbing
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#LaundryDay
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#LaundryDay
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#LaundryDay
Why do clothes stick together in the dryer?
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Charge is conserved
The total charge of a closed system is a constant.
My hair and my comb are initially neutral.If I comb my hair and the comb ends up with a charge Q,
what is the final charge of my hair? +Q!
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Charge is conserved
Consider two identical metalballs.
Total charge initially:
q+ 3q= +2q.If the objects are identical, thistotal charge will be distributedequally to each of them. So eachwould end up with a charge of+2q
2 = +q .
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Charge is conserved
Would the answer be thesame? No! Since the larger
metal ball could take in/spitout more electrons, it wouldend up with the larger chunkof the systems total charge.
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Grounding
Remark
Earth is a very, very large conductor. If we place a charged conductor in
contact with the Earth, essentially all of its charge is sucked in by the
Earth, thereby making the conductor electrically neutral. We say that the
object is grounded.
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Ch i b i d i
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Charging by induction
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Ch i b i d i
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Charging by induction
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Ch i b i d i
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Charging by induction
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Ch i b i d ti
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Charging by induction
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Ch i b i d ti
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Charging by induction
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Application of charging by induction
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Application of charging by induction
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Quiz!
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Quiz!
Consider three identical conducting cubes, A, B, and C, withcorresponding charges as shown in the figure. Cube A and Cube B weremade to touch each other. Cube A was then made to touch Cube C.What are the final charges on each cube?
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Quiz!
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Quiz!
Solution:
When Cube A and Cube Bare made to touch each other, their totalcharge, 2q+ 0 = +2q, were distributed equally to each of them. So eachwould end up with a charge of +2q2 = +q.
When Cube A and Cube Bare then made to touch each other, their total
charge, +q+ (4q) = 3q, were distributed equally to each of them. Soeach would end up with a charge of 3q2 =
32q.
qA = 3
2q, qB= +q, qC =
3
2q.
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