Physics 154 April 13, 2009 Announcements Review Ampere's Law

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```					                                                                 Announcements
• HW deadline changed, due Thursday at
Physics 154                              5:00pm instead of tomorrow
April 13, 2009                          • Today we start chapter 28
Instructor: Katrina Hay                  • Exam next Friday, April 24th

Clarification on right
Review: Ampere’s Law                            hand rule (r.h.r)
• Ampere’s Law does for magnetostatics what
Gauss’s Law does for electrostatics
• In Gauss’s Law you had enclosed charge, in       Use it to
Ampere’s Law you have enclosed current           1. Find cross product directions
• Like Gauss’s Law, it can only be used in cases   2. Find direction of B inside a wire
loop or coil of loops (B goes
of symmetry (where B and dl are parallel and B
straight through the wire)
is constant along an “Amperian Loop”)
3. Find direction of B around a
• Amperian Loop: use like the Gaussian Surface,       straight wire with current
except to contain current, not charge               running through it (B curls
around the wire)

Example uses of Ampere’s Law                            Gauss’s Law for Magnetism
1. Find the magnetic field near an                   • What it means: magnetic monopoles do
infinitely long straight wire. (Notice how          not exist!
much easier this is than using Biot-              • If we discover that they do, this
Savart Law)                                         equation would have to be altered.
2. Find the magnetic field inside a Toriod.

1
Magnetic field application,                               (Begin Chapter 28)
Example 27-7                                          Magnetic Flux
• Similar to Electric Flux in math form
Find the magnetic field at the center of a     • Magnetic flux is the measure of the field
square current loop that has edges of          strength through an area (“number” of
length L = 50 cm and carries a current         filed lines passing through an area)
equal to 1.5 A.
• Units: Webers
• It’s a scalar

Example 28-1                                      Induced Emf
• Faraday observed: If the magnetic flux
• Find the magnetic flux through a               through a wire loop changes, an Emf is
solenoid that is 40cm long, has a radius       induced in the wire
– Which induces an electric field in the wire
of 2.5cm, has 600 turns and carries a
– Which moves charges (induces a current!)
current of 7.5A.
direction of induced Emf
emf is generated to oppose the change in flux!

1. A square loop of sides a lies in the yz
plane with one corner at the origin. A varying
Electromagnetic Induction                  magnetic field B = ky passes through the
loop and points in the +x direction. The
magnetic flux through the loop is
1. Pulled a loop of wire through a magnetic
field.                                      A. ka2
2. Moved the magnet and held the loop still.   B. ka2/2
3. With both the magnet and the loop held
C. ka3/2
still, he changed the strength of the
magnetic field (varied the current in an    D. ka3/3
electromagnet).                             E. None of these

2
3. For which of the diagram(s) will current
2. You can change the magnetic flux
flow through the light bulb? (In 3 and 4
through a given surface by                   assume the magnets move in the plane of the
loop.)
A. changing the magnetic field.
B. changing the surface area over which
the magnetic field is distributed.        A.1
C. changing the angle between the           B.2
magnetic field and surface in question.   C.3
D. any of the above.                        D.4
E. none of these strategies.                E.1 and 2

3

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