Transcript Lecture29

Lecture Demos:
E-40 Magnetic Fields of Permanent Magnets (6A-1)
E-41 Oersted’s Experiment (6B-1)
E-42 Force on a Moving Charge (6B-2)
6B-3 Magnetic Field around a wire
6B-10 Forces between parallel conductors
READING QUIZ
Magnetic fields are created by wires
1. carrying any kind of electric current
2. only if there is a steady current
3. only when the current is first turned on
Chapter 14: Magnets and Electromagnetism
1. Permanent magnets have opposite poles: like poles repel,
opposites attract (North, South).
2. Lines of force exist around magnets (field lines).
3. A current in a wire causes a magnetic field outside of the
wire.
4. A current in one wire can cause a force on another currentcarrying wire.
5. Magnetic fields exert a force on a moving charge.
Unlike poles attract one another,
and like poles repel one another.
http://www.geolab.nrcan.gc.ca/geomag/compass_e.shtml
http://www.princeton.edu/~oa/manual/mapcompass2.shtml
Magnetic field lines produced by a magnetic dipole
form a pattern similar to the electric field lines
produced by an electric dipole. However, the
magnetic field lines form continuous loops.
A magnetic dipole lines up with an externally
produced magnetic field just as an electric
dipole lines up with an electric field.
The magnetic field of the earth can be pictured
by imagining a bar magnet inside the earth,
oriented as shown here.
With the wire oriented along a north-south line,
the compass needle deflects away from this
line when there is current flowing in the wire.
The right-hand rule gives the direction of the magnetic
field lines that encircle a current-carrying wire.
The thumb points in the direction of the current and
the fingers curl in the direction of the field lines.
Two parallel current-carrying wires exert
an attractive force on each other when the
two currents are in the same direction.
The magnetic force exerted on the moving charges
of an electric current is perpendicular to both the
velocity of the charges and to the magnetic field.
If the index finger of the right hand points in
the direction of the velocity of the charge, and
the middle finger in the direction of the magnetic
field, the thumb indicates the direction of the
magnetic force acting on a positive charge.
Force per unit length on parallel wires:
F 2k'I1I 2

l
r
K = 1x10-7 N/A2
Force on a moving charge: F = qvB,
where F is perpendicular to both v and B.
Force on a wire with current I in a perpendicular B field:
F = I LB
When a current-carrying wire is bent into a
circular loop, the magnetic fields produced
by different segments of the wire add to produce
a strong field near the center of the loop.
The two most important facts about magnets:
1. Moving a coil of wire near a magnet can cause a current
to flow in the wire.
2. Moving a magnet near a coil of wire can cause a current
to flow in the wire.