Classical Mechanics vs Electricity and Magnetism
Classical Mechanics
Electromagnetism
(Mechanics of macroscopic
objects whose velocity is much How many have we covered?
smaller than the speed of light.)
Newton’s three laws
(Only one of the laws is an
equation.)
Four laws
(Every one of them is an
equation.)
Newton’s three laws and relations
derived from them hold for objects
experiencing electromagnetic forces.
Electricity vs Magnetism
Electricity
Magnetism
The one law
No “magnetic charges”
(monopoles) have been found.
Coulomb’s law Ù Gauss’ law
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About the force between two
point charges. (Dependence of the
electric field on the source charge
and distance.)
Alternatively, about the property of
the electric field.
Everything else is derived from this
law, which completely agrees with all
observed electrical phenomena.
(Talked about situations in which this
law alone is insufficient.)
The equation about the force
between two small bar magnets
turns out to be not fundamental.
What are fundamental?
1. Dependence of the magnetic
field on the strength of the
source (a current) and distance.
Chapter 29
2. The force due to a magnetic
field. The force on what?
Chapter 28
Force on a charge due to magnetic field
r
Magnetic Field B
At this point, we do no get into the question of how it is
produced by a current or by a bar magnet, etc. and how its
strength is varied.
Fundamental relation based on a large number of
experimental observations:
r
r r
Fm ∝ qv × B
Magnetism is inseparable from electricity.
Choose the unit (tesla) for magnetic field such that this
relation is written in the simplest form:
r
r r
Fm = qv × B