The Large Hadron Collider at CERN

~1200 dipole magnets

My house

The LHC dipole magnets

With an ingenious design intense electric

currents two electric fields are produced in

opposite directions to keep protons in orbit

The larger R, the greater v (and energy)

LHC: R=4.4 Km

Is the LHC Dangerous?

No, it’s

not

We are bombarded every day with energetic

cosmic rays and I am still playing!

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

CERN: the birth place of the internet

Physics 202, Lecture 15

Today’s Topics

!! Faraday’s Law (Ch 31)

!! Change of Magnetic Flux and Emf (!)

!! Lenz’s Law

!! Faraday’s Law of Induction

Electromotive Force (emf, !)

"! 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.

+

-

1.5

V +

-

-------

+++++

!= 1.5V

Demo: Emf and Change of Magnetic Flux 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.

Demo: Eddy Current 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

dt

dB

!"=#

A

B

"#

! •=" AB dB

nominal

direction of !#

Direction of Induced emf

"! !>0, same as nominal direction

!<0, opposite

"! Note that the nominal direction

of ! and the direction of vector A

follows right hand rule

dt

dB

!"=#

A

B

"#

! •=" AB dB

nominal

direction of !#

Methods to Change Electric Flux

"!Change of $B! emf

%! To change $B:

#! Change B ! emf produced by an induced E field

#! Change A ! motional emf

#! Change ! ! motional emf

#! Combination of above

dt

BAd

dt

d

uniform

)cos( !" #

$#= =

B

B