Download - Electromagnetism
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Electromagnetism
Forces in nature:
How does matter interact?
Four Fundamental Forces:
Long range: Gravity
Mid Range: Electromagnetic
Short Range: Strong and
Weak
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Electric Charge (~Mass)Two types of electric charge: positive and negative
Unit of charge is the coulomb [C]
Charge of electron (negative) or proton (positive)is
,e C10602.1 19e
Charge is quantized
NeQ Charge is conserved
, epn ee
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Electric Force (~Gravity)The electric force between charges q1 and q2 is
(a) repulsive if charges have same signs
(b) attractive if charges have opposite signs
Like charges repel and opposites attract !!
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Coulomb's LawCoulomb’s law:Force by q1 on q2
Direction: along the line joining the two chargesRepulsive or attractive
221
r
qqkF e
Coulomb’s constant in vacuum:
229
0
C/Nm1094
1
ek
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The superposition principleIf many charges are present:Net force is on any charge is the
vector sum of the forces from all the individual charges
In general:
23133 FFF
N
iijNjjjj FFFFF
121 ...
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The electric fieldThe electric field at a point is the force acting on a test
charge q at that point divided by the charge q
For a test charge q
q
FE
EqF
has the direction and sense of the force on a positive charge.
E
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Electric field created by a point charge Q
2
1
r
Qqk
FE e
2r
QkE e
is outwards a positive charge and inwards a negative charce
E
E
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The superposition principle for fields
The electric field due to a collection of N point charges is the vector sum of the individual electric fields due to each charge:
N
iiN EEEEE
121 ...
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Example of vector field: gravitation
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Comparison with gravity
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Electric field lines
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(http://ocw.mit.edu/ans7870/8/8.02T/f04/visualizations/vectorfields/02-particleSource/02-ParticleSource_320.html)
Flows With Sources
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(http://ocw.mit.edu/ans7870/8/8.02T/f04/visualizations/vectorfields/01-particleSink/01-ParticleSink_320.html)
Flows with sinks
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(http://ocw.mit.edu/ans7870/8/8.02T/f04/visualizations/vectorfields/03-particleCirculate/03-PartCircMotion_320.html)
Circulating flows
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