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# Review - PowerPoint PPT Presentation

Review. Physics 122. Final Exam. Saturday, December 17, 8:30 am (sorry!) - Hoygt Final exam is based on the entire course PHY122. Last homework will be based on the entire course to give you more time to prepare for the final. 40% of the final grade. Grading. Workshops: 5%

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### Review

Physics 122

Lecture XXIV

• Saturday, December 17, 8:30 am (sorry!) - Hoygt

• Final exam is based on the entire course PHY122.

• Last homework will be based on the entire course to give you more time to prepare for the final.

• 40% of the final grade

Lecture XXIV

• Workshops: 5%

• Homework: 5%

• Hour Exams: 40%

• Final Exam: 40%

• Laboratory: 10%

• Total: 100%

• 90% or above: A 88-89.9 – A- 85-87.9 – B+

• 80% - 85% : B 78-79.9 – B- 75-77.9 – C+

• 70% - 75%: C 68-69.9 – C- 65-67.9 – D+

• 60% - 65%: D

• Under 60% : F

Lecture XXIV

• The most important concept - electromagnetic field

• First introduced for convenience

• Electric charge changes the properties of space around them = creates electric field

• Other charges feel this change  interaction between electric charges is mediated by the electric field

• Change in electromagnetic field propagates with finite velocity (c ) = electromagnetic wave – proof that EM field is a reality

Lecture XXIV

• Gauss’ law – field lines start on + and end on -

• No magnetic charge – closed field lines

• Changing magnetic flux acts like a source of electric field

• Amper’s law with extra term

Lecture XXIV

• Describes electric field created by electric charges

Lecture XXIV

Gauss Coulomb

• Calculate E of point like (+) charge Q

• Consider sphere radius r centered at the charge

• Spherical symmetry: E is the same everywhere on the sphere, perpendicular to the sphere

Lecture XXIV

• Infinitely long thick charged wire, radius R

• Charge per unit length Q/l=l

• r>R:

• r<R:

Lecture XXIV

• Describes magnetic field created by moving charges = currents

Lecture XXIV

• In a way similar to Gauss’s law for E and Q

• Not flux, integral over closed path

• Only component of B parallel to dl contributes to integral

Lecture XXIV

• Thick wire of radius R carries current I. What is B(r ) in and outside of the wire?

• Current through unit area:

• Current through circle radius r<R

• Path integral over radius r

• Ampere’s law

• For r>R – same as thin wire

Lecture XXIV

• Maxwell’s equations tell us about the EM field

• They tell nothing about the force that this field exerts on charges – Lorentz force

Lecture XXIV

• Electric field changes kinetic energy of charged particle

qDV=mv2/2

• Magnetic field does not change KE, only direction

• Lorentz force:

• Force due to E is parallel to E

• Force due to B is perpendicular to B and to velocity of moving particle

Lecture XXIV

a is always || to net F

Remember v is not always || to a (projectile, circular motion)

Motion in electric fields

- - - -

vx

vy

+ + + +

vx=const

vy=at=tF/m=tEq/m

Lecture XXIV

• Magnetic force F is perpendicular to the velocity v of a particle with charge q

F=qvB sin q

• Magnetic field does not change the magnitude of particle’s velocity, only its direction. Charged particles move in circles in constant magnetic field

Lecture XXIV

• Circular motion

• Magnetic force = Centripetal force

F=qvB

• Centripetal acceleration

a=v2/R

• Newton’s second law

F=ma

qvB=mv2/R

In experiments B is known,

q=e most of the time,

Measure R- measure mv

For a given v measure m – magnetic spectrometer

Lecture XXIV

Frequency does not depend on v!

For a given particle type (q,m=const) it depends only on B

Lecture XXIV

• Crossed E and B fields:

• For one given velocity electric force exactly compensates magnetic force

• Only particles with this value of velocity emerge from particle selector

+ + + +

- - - -

+

Lecture XXIV

• Sources of E

• Electric charges

• Changing magnetic field

• Sources of B

• Currents

• Changing electric field

• In vacuum

• No currents, no charges

Lecture XXIV

• Changing electric field creates magnetic field

• Changing magnetic field creates electric field

• Change propagates with a finite velocity

• Electromagnetic wave – proof of unification

Lecture XXIV

Lecture XXIV

This is the speed of light!!!

Light is an EM wave!!!

Lecture XXIV

• EM waves transport energy

• Energy density:

Lecture XXIV