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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.

Lecture 1 August 5, 2015 2 / 24

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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.


Ch i b i d i

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Charging by induction


Ch i b i d i

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Ch i b i d i

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Ch i b i d ti

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Ch i b i d ti

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Application of charging by induction

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Application of charging by induction


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?


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.


lec 1 physics 72

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