Download Units of Magnetic Field

CH22: Magnetism
The magnificent spectacle of the Aurora Borealis, or northern lights,
glows in the northern sky above Bear Lake near Eielson Air Force
Base, Alaska. Shaped by the Earth’s magnetic field, this light is
produced by radiation spewed from solar storms.
(credit: Senior Airman Joshua Strang, via Flickr)
Outline
•Magnets, Magnetic fields, and Magnetic field of the Earth
•Force on a moving charge in a magnetic field
•Motion of a charge particle in a magnetic field
•The mass spectrometer
•Force on a current in a magnetic field
•Magnetic fields produced by currents
•Magnetic Resonance Imaging (MRI)
•Cathode Ray Tube (CRT)
•Magnetic storage
Magnetic Compass
The needle of a compass is a permanent
magnet that has a north magnetic pole (N) at
one end and a south magnetic pole (S) at the
other.
Navigation in animals:
Vertebrates (ex: migratory birds) and
invertebrates (ex: Spiny lobsters) use the
earth's magnetic field to navigate and
determine their geographic position.
Cow’s magnet and feeding.
Magnetic Poles
Like poles repel each other, and unlike poles
attract each other.
North and south poles always occur in pairs
North and south poles always
occur in pairs. Attempts to
separate them result in more pairs
of poles. If we continue to split
the magnet, we will eventually
get down to an iron atom with a
north pole and a south pole—
these, too, cannot be separated.
https://phys.org/news/2016-08mysterious-magnetic-monopole.html
Magnetic Field of Magnets
Magnetic Field of the Earth
The earth behaves magnetically almost as if a bar magnet were
located near its center. The axis of this fictitious bar magnet does not
coincide with the earth's rotational axis; the two axes are currently
about 11.5° apart.
Angle of Declination
The south magnetic pole does not coincide with the north
geographic pole but, its position is not fixed but moves over
the years.
The angular difference between the magnetic
north and the geographical north is called the
angle of declination.
Angle of Dip
Earth's magnetic field lines are not parallel to the surface at all
points.
For instance, near the north magnetic pole the field lines are
almost perpendicular to the surface of the earth.
The angle that the magnetic field makes with
respect to the surface at any point is known as
the angle of dip.
The Force That a Magnetic Field
Exerts on a Moving Charge
The following two conditions must be met for a charge to
experience a magnetic force when placed in a magnetic
field:
1.The charge must be moving. No magnetic force acts on a
stationary charge.
2.The velocity of the moving charge must have a
component that is perpendicular to the direction of the
magnetic field.
Magnetic Force
Consider a charge q0, moving with velocity v, in a magnetic
field. Magnetic field will be denoted by the symbol B.
The magnetic force is given by,
F  q0vB(Sin ).
where θ is the angle between the
magnetic field and velocity.
10. An electron moving at 4.00×103 m/s in a 1.25-T magnetic field
experiences a magnetic force of 1.40×10−16 N . What angle does the
velocity of the electron make with the magnetic field?
Right-hand Rule No. 1
When the right hand is oriented so the fingers point along the
magnetic field B and the thumb points along the velocity v of a
positively charged particle, the palm faces in the direction of the
magnetic force F applied to the particle.
Application of RHR
1. What is the direction of the magnetic force on a positive charge
that moves as shown in each of the six cases shown below?
Definition of the Magnetic
Field
F  qvB( Sin ).
F
B
.
qv( Sin )
Magnetic field is the force on a unit charge moving with a unit
velocity perpendicular to the magnetic field.
The SI unit of the magnetic field is the tesla, T named after the
Croatian-born American engineer Nikola Tesla (1856-1943).
N
N
N
T


.
C.(m / s) (C / s).m A.m
Units of Magnetic Field
The cgs unit for the magnetic field is the gauss, G.
1 T = 104 G.
The Earth’s magnetic field near its surface is about
0.5 G or 5  10-5 T.
Laboratory electromagnets can produce magnetic fields as
high as 3 T.
Superconducting magnets can produce magnetic fields up to
30 T.
https://www.youtube.com/watch?v=jjcHZuTGWXk
The Motion of a Charged
Particle in an
Electric or Magnetic Field
Velocity Selector
17. A velocity selector in a mass spectrometer uses a 0.100-T
magnetic field. (a) What electric field strength is needed to select a
speed of 4.00×106 m/s ? (b) What is the voltage between the plates if
they are separated by 1.00 cm?
The Circular Motion
Mass Spectrometer
Physicists use mass spectrometers for determining the relative masses and abundances
of isotopes.
Chemists use these instruments to help identify unknown molecules produced in
chemical reactions.
Mass spectrometers are also used during surgery, where they give the anesthesiologist
information on the gases, including the anesthetic, in the patient's lungs.
Problem 20
20. A mass spectrometer is being used to separate common oxygen-16
from the much rarer oxygen-18, taken from a sample of old glacial ice.
(The relative abundance of these oxygen isotopes is related to climatic
temperature at the time the ice was deposited.) The ratio of the masses
of these two ions is 16 to 18, the mass of oxygen-16 is 2.66×10−26 kg,
and they are singly charged and travel at 5.00×106 m/s in a 1.20-T
magnetic field. What is the separation between their paths when they
hit a target after traversing a semicircle?
The Force on a Current in a
Magnetic Field
https://www.youtube.com/watch?v
=tUCtCYty-ns
Loudspeaker
The Torque on a CurrentCarrying Coil
Electric Motor