Download Electricity and Magnetism

Electricity and Magnetism
AP Physics
History
2000 years ago Greeks
Chinese Use for Navigation
1296 Pierre Maricourt – needle orientation
1600 William Gilbert – Earth is a magnet
1820 Hans Christian Oersted – Connection between
electricity & magnetism
Today – Maglev Trains (No Friction!)
Magnets
Have the property of magnetism.
Ferromagnetic materials (ie iron): Spin of electrons
line up in small regions called domains.
Magnetic domains can align in a given direction to
allow a magnet to induce magnetism.
Lines of magnetic flux: the field lines of a magnet
(similar to electric field lines)
Two types of Magnets
1.Temporary or Soft – iron can easily lose its
magnetic property. Iron is magnetic while
placed inside a current carrying coil. This is
the basis for electromagnets.
2.Permanent or Hard - steel
Magnetic Poles
Hang a bar magnet by a thread.
It will line-up in a north-south direction.
The end pointing north is the “north pole”
The other end is the “south pole”
Magnetic Poles
Similar to electric charges
Always occur in pairs
Opposite (unlike) poles attract
Like poles of a magnet repel
Earth as a Magnet
Earth is one big magnet.
If the north pole of a bar magnet is pointing to
the north on Earth, what pole of the Earth is
the bar magnet pointing to?
A bar magnet’s north pole is attracted to the
Earth’s south pole.
Magnetic Fields
Used to describe magnetism.
Describe the condition in space – moving
charges experience a force perpendicular to
their velocity.
a.k.a. Defined by measuring the force the field
exerts on a moving charged particle, such as
an electron.
Magnetic Field is a Vector
 A magnet produces a vector field, the
magnetic field, at all points in the space
around it.
Magnetic Field Lines
Run out of North and into south.
Used to picture the magnetic field
Similar to an electric field but…
Not the same. Electric charges
can be isolated, magnetic poles
cannot be isolated.
Magnetic Field
 Strength of magnetic field measured in
Teslas
 SI unit T
 Symbol is a B
Magnetic Field
 B = (µo /2π) (I/r)
 r = distance from wire (m)
 µo = permeability of free space = 4πx10-7 T m/A
 So µo /2π = 2x10-7T m/A
 I = current
Magnetic Force
One of the four fundamental forces
Part of Electromagnetic Force
Acts within a magnetic field.
Strongest at the poles of a magnet
Force of Magnetic Field on Current
Carrying Wire
 F = ILB
 a.k.a. F = ILB Sin Ө
 I is current,
 L is the length of the wire,
 B is the strength of the magnetic Field,
 Ө is the angle the wire makes with the
magnetic field (angle the direction of the
magnetic field is going to the direction the
current is going in the wire.)
Force on a current carrying wire
 Force is greatest when magnetic field is ┴ to
the magnetic field (Ө = 90º or 270º)
 Into the page symbolized by an x or a
circled x
 No force where Ө = 0º or 180º
Force of a Magnetic Field on a Moving
Charge
 Hans Christian Oersted figured out that
there is a relationship between
electricity and magnetism.
 He aligned a compass and a wire along
the Earth’s magnetic field. When put
current through the wire the compass
swung East-West.
Force of a Magnetic Field on a Moving
Charge
 The force (F) is equal to the charge (q) times
the speed of the particle (v) times the
magnitude of the field (B), or
 F = q*v x B, where the direction of F is at
right angles to both v and B as a result of the
cross product. This defines the magnetic
field's strength and direction at any point.
 F = qvBsinӨ
Right Hand Rules - Purpose
Relate directions when dealing with
magnetism
Right Hand Rule for Magnetic Field of a
Current Carrying Wire
a)Use right hand to grip wire
b)Point thumb in the direction of I
c)Fingers curl in the direction of B
RHR to tell the Pole of a Magnet
a)Wrap fingers of right hand around the
electromagnet in the direction of I.
b)Thumb points to North
RHR for Direction of Force produced by a
Current Carrying Wire in a Magnetic Field
a)Point fingers of the right hand in the
direction of B.
b)Point thumb in the direction of I
c)Palm points in the direction of F.
Sources
http://www.windows.ucar.edu/tour/link=/earth/Magnetosphere/earth_magnetic_poles.
html
http://www.coolmagnetman.com/magfield.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html#c1
http://library.thinkquest.org/16600/advanced/magneticfields.shtml