Transcript Hans Christian Oersted

Hans Christian Oersted
• Accidently discovered electric current
would cause a compass needle go
haywire.
• Discovered that moving Charges
(Electric Currents) create Magnetic
Fields
Magnetic Field (B-field)
• The invisible aura around a magnet
Remember:
• If a charged particle moves through a magnetic field,
it will feel a force from that field
• A charged particle will be deflected by a magnetic
field.
A current carrying wire
• A straight current-carrying wire creates
circular magnetic field lines:
B
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Right Hand Rule
Used to draw and show direction of a magnetic
field around a current carrying wire.
Right Hand Rule:
1. Thumb of right hand – points in direction of
current flow
2. Wrap fingers like you are grabbing the wire
3. Fingers show direction of the magnetic field
Symbols
• Magnetic Field (B-Field)
• “Bullets” – out of page
• “Feathers” – into page
Example:
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Wire in a Loop
• If wire is bent into a loop, the B-field lines
become bunched up inside the loop
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Explained
• There is a greater concentration of feathers
inside the loop than bullets on the outside of
the loop, therefore the B-field is stronger
inside the loop.
• Bend wire into another loop, the
concentration of the B-field doubles, and so
on.
• The direction of the B-field depends on the
direction of current flow in the wire.
Solenoid
• A coil of wire
• You can magnetize a
solenoid by running
electric current through
it.
• A Solenoid can be used
to move a piece of
ferromagnetic material
and do physical work like
opening car doors!
B
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Electromagnet
• A Solenoid can become an Electromagnet by
placing a ferromagnetic core in the coil
Examples of Electromagnets
• MRI - magnetic resonance imaging
• Maglev – fast moving transportation,
magnetically levitated
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TV’s
• Old TVs are big and boxy. The box part holds a cathode
ray tube that basically shoots electrons at the screen lined
with a phosphorescent substance on the TV.
• Without an electromagnet (steering coil), those electrons
would hit the screen in only one location.
– These copper coils create a magnetic fields inside the tube, and
the electron beam responds to the fields.
– One set of coils creates a magnetic field that moves the electron
beam vertically, while another set moves the beam horizontally.
– By controlling the voltages in the coils, you can position the
electron beam at any point on the screen
•Human body –
contains billions
of atoms.
•For the MRI scan only concerned
with the hydrogen
atom (very
abundant since
the body is mostly
made up of water
and fat)
•Hydrogen atoms
have a strong
magnetic
moment, which
means that in a
magnetic field,
they line up in the
direction of the
field.
Force of the magnetic field
• Two magnets will apply a force on each other.
• Therefore if you have a moving electric charge (which has a
magnetic field around it) it can be forced to move by
another magnetic field
F=qvB
Fm = Force (Newtons)
q = Charge (Coulombs)
v = Velocity (m / s)
B = Magnetic Field (Tesla)
Many moving charges: called electic current
F=BIl
Fm = Force (Newtons)
B = Magnetic Field (Tesla)
I = Current (Amps)
l = length (meters)
A charge will feel the greatest force when it is moving
PERPENDICULAR to the B-field. It will feel no force if it is moving
PARALLEL to the B-field.
Physics Gang Signs
To determine the direction of the above
variables you will need to know the “Physics
gang signs”
Thumb represents force
Pointer finger represents direction of velocity
All other fingers represent the B-field, where
your knuckles are at the North Pole.
Example
If an electron in an electron beam experiences a downward
force of 2 E –14 N while traveling in a magnetic field of
8.3 E –3 T directed to the west. Find the DIRECTION and
MAGNITUDE of the velocity of the electron.