Transcript February 8 Magnetism

Do Now: 12/16/2013
(on last week’s paper)
 What makes a magnet a magnet?
 Why are some magnets stronger than
others?
 What else do you know about magnets?
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Objectives
Discuss characteristics of magnets.
 Describe magnetic field lines
 Quantify magnetic fields.
 Calculate force on wires and charges.
 Calculate force on wires and charges
 Describe origins of induced emf.
 Apply Faraday’s Law of Induction.
 Apply Lenz’s Law.

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Magnets (some basics)
2 poles…always
 Can’t isolate a single magnetic charge.
 Opposites attract.
 Ferromagnetic materials include iron,
cobalt, nickel, and gadolinium.
 Paramagnetic materials – everything else.

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Magnetic Fields
Described with field lines.
 Direction of field is tangent to line at
any point.
 Number of lines per unit area
proportional to the magnitude of the
field.
 Lines go from N to S.

N
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S
Quantifying Magnetic Field

Direction based on compass needle

Magnitude of B defined as torque exerted
on compass needle

Magnetic Field is a vector with symbol B.

N Pole is really the south magnetic pole.
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Units for B
Tesla (use this for calculations!)
 1T = 1 N/A-m

Gauss
 1G = 1 x 10-4 T

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Electric Currents & Magnetism
1820—Oersted found deflection of
compass needle near electric wire.
 An electric current produces a magnetic
field.
 Direction of field around current carrying
wire described with the “Right Hand Rule”
(See page 591).
 Use conventional current flow.

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Force on I in B
Current exerts force in B, B exerts force
on I.
 Force is perpendicular to current in wire
and magnetic field (B )
 Right hand rule applies
 F = IlBsin()

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Force on I in B
F = IlBsin()
 Assume uniform magnetic field
 Current parallel to field B, Force 0.
 Current perpendicular to field B, Force
max.

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Example:

A proton moves at 8x106 m/s along the xaxis. It enters a region in which there is a
magnetic field 2.5 T, directed at an angle
of 60 with the x-axis and lying along the xy plane. Calculate the initial force and
acceleration of the proton.
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Electric Charge in B
Force on Moving charge in B – Lorentz
Force.
 From F=IlBsin()

We have N particles of charge q passing a
reference point in time t, so I=Nq/t
 Since t is time to travel distance l and
v=d/t, then we can let l=vt and so
F=(Nq/t)(vt)Bsin so…

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Force on a single charge
F = qvBsin()
v is a vector, B is a vector so we have
to take the cross product and use
right hand rule.
 pointer
finger points in direction of v
 middle finger points in direction of B
 Thumb points in direction of F
 Rule for positive charge only!
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Examples

(on handout)
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Long Straight Wires
B = (0/2)(I/r)
 Field strength is proportional to current
and inversely proportional to distance
from wire.
 Constant of proportionality is
0/2
Where mu is permeability of free space =
4x10-7 T-m/A

Review example 20-7.
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2 Long Wires
2 current carrying wires will exert forces on
each other.
 Right hand rules to determine field
direction and force direction on wire.
 Currents same direction—attractive.
 Currents opposite directions—repulsive.

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2 Long Wires
F = I2lB1 and B1 = (0/2)(I1/r)
So
 F = I2l (0/2)(I1/r)
So
 F/l = (0/2)(I1I2/r)


Review example 20-8 and 20-9.
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Practice:

1-4 MC in Chapter 19
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Do Now (12/18/13):
*Pass in your HW, please!
1. A wire carries a current of 22 A from east
to west. Assume that at this location the
magnetic field is 0.5 G and points from
North to South. Find the magnetic force
on a 36 m length of wire.
2. How does this change if the current runs
west to east?
3. If the current is directed north to south,
what is the magnetic force on the wire?
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Torque on a Current Loop
  BIA sin 

Torque
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Example:

A circular loop of radius 50 cm is oriented
at an angle of 30 to a magnetic field of 0.5
T. The current in the loop is 2 A. Find the
magnitude of torque at this instant.
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Galvanometer

Galvanometer is the historical name given to a
moving coil electric current detector. When a
current is passed through a coil in a magnetic
field, the coil experiences a torque proportional to
the current. If the coil's movement is opposed by
a coil spring, then the amount of deflection of a
needle attached to the coil may be proportional to
the current passing through the coil. Such "meter
movements" were at the heart of the moving coil
meters such as voltmeters and ammetersuntil
they were largely replaced with solid state meters
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galvanometer

galvanometer
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Galvanometer

A galvanometer is the basis of an
ammeter and a voltmeter
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Motion of a Charged Particle in
a B-field

motion
mv
F  qvB 
r
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2
Example

A proton is moving in a in a circular orbit of
radius 14 cm in a uniform magnetic field of
magnitude 0.35 T, directed perpendicularly
to the velocity of the proton. Find the
orbital speed of the proton.
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Mass spectrometer

The mass spectrometer is an instrument
which can measure the masses and
relative concentrations of atoms and
molecules. It makes use of the basic
magnetic force on a moving charged
particle.
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Mass Spectrometer

How it works
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Example: Mass Spectrometer
Two singly ionized atoms move out of a slit at a
point S (shown on board) and into a magnetic
field of 0.1 T. Each has a speed of 1 x106 m/s.
The nucleus of the first atom contains one
proton and the second contains a proton and
a neutron. Atoms with the same chemical
properties but different masses are called
isotopes. The two isotopes here are hydrogen
and deuterium. Find their distance of
separation when they strike a photographic
plate at P.
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Practice:
Complete MC 1-4 (5-6 are bonus) in
Chapter 19
 Problem 30
 Will be collected

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