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Lecture

PHYS 102 Lecture Notes - Electromagnet, Inverse-Square Law


Department
Physics
Course Code
PHYS 102
Professor
Zaven Altounian

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Phys 102 Alanna Houston
March 6, 2008
Simple Right Hand Rule:
-Imagine holding the wire carrying the current with your right hand,
the thumb in the direction of the current. Your fingers will then
give the sense of direction of the magnetic field B
-A circle with a dot coming out of it is a vector pointing out of the
plane
-A circle with an x is a vector pointing into the plane
-The simple right hand rule applies to many geometries including
current in a loop:
-
Determination of Magnetic Field Strengths Produced by Electric
Currents:
-General Case:
oWhat is the magnetic field at point P due to the current I?
o
oLet us take a small segment of the wire, delta L and
calculate the field, delta B it produces at P. It has been
found, experimentally, that delta B ~ I delta L
o
oThe inverse square law applies as well:
o
oIn addition, the component of the displacement r
perpendicular to the current contributes to the field at point
P
oDeltaB ~ sin ϕ (The current does not produce a field on
itself

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Phys 102 Alanna Houston
oPutting it all together,
o (Biot-Savart Law)
oThe proportionality constant
o
o
oFor a medium other than vacuum, we replace mu not by
mu, the permeability of the medium. For permanent
materials, such as Fe, mu can be very large (mu ~ 5000 mu
not)
oTo find the total magnetic field at point P, we must ad
vectorially all contributions delta B from each current
segment Delta L. Integration may be necessary. The
direction of the B-field is determined directly by the right
hand rule
-To summarize:
-
Important Geometries:
-The B-field at the centre of a circular current loop of radius r:
-
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