the large hadron collider at cern - department of physics€¦ · the large hadron collider at cern...
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The Large Hadron Collider at CERN
~1200 dipole magnets
My house
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The LHC dipole magnets With an ingenious design intense electric currents two electric fields are produced in opposite directions to keep protons in orbit
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The larger R, the greater v (and energy) LHC: R=4.4 Km
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Is the LHC Dangerous?
No, it’s not
We are bombarded every day with energetic cosmic rays and I am still playing!
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Energy Spectrum of Cosmic Rays
Earth is bombarded everyday with cosmic particles with huge energy, much larger than the proton collisions at the LHC
LHC
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CERN: the birth place of the internet
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Physics 202, Lecture 15 Today’s Topics
Faraday’s Law (Ch 31)
Change of Magnetic Flux and Emf (ε) Lenz’s Law Faraday’s Law of Induction
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Electromotive Force (emf, ε) Electromotive “force”, emf, is a measure of the voltage
that can be provided by a source. For a given device, if a charge Q passes through that
device, and gains an energy U, the net emf for that device is the energy gained per unit charge, or U/Q.
emf is not a force, it has a unit of volts sources of emf: • chemical process (battery) • change of magnetic flux • semiconductors…..
e.g. battery: notice that emf has a direction emf may exist even if no current.
+
-
1.5V
+ -
-------
+++++
ε= 1.5V
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Demo: Emf and Change of Magnetic Flux
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Lenz’s Law The emf due to change of magnetic flux tends to
create a current which produces a magnetic field to compensate the change of original magnetic flux. Lenz’s law is a convenient way to determine the
direction of the emf due to magnetic flux change.
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Demo: Eddy Current
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Formulation of Faraday’s Law The emf induced in a “circuit” is proportional to the
time rate of change of magnetic flux through the “circuit”.
Notes: “Circuit”: any closed path
does not have to be real conducting circuit
The path/circuit does not have to be circular, or even planar
dtd BΦ−=ε
A
B
θ
∫ •=Φ AB dB
nominal direction of ε
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Direction of Induced emf
ε>0, same as nominal direction
ε<0, opposite
Note that the nominal direction of ε and the direction of vector A follows right hand rule
dtd BΦ−=ε
A
B
θ
∫ •=Φ AB dB
nominal direction of ε
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Methods to Change Electric Flux
Change of ΦB emf To change ΦB:
Change B emf produced by an induced E field Change A motional emf Change ϑ motional emf Combination of above
dtBAd
dtd
uniform
)cos( θε −Φ
−= =B
B