Transcript Magnetism and Electricity

Magnetism and Electricity
Chapter 19
Magnets, Magnetic Poles,
and Magnetic Field Direction
Magnets have two
distinct types of
poles; we refer to
them as north and
south.
Pole Force Law, or
Law of Poles
Like magnetic poles repel, and
unlike poles attract.
Magnets, Magnetic Poles, and
Magnetic Field Direction
Two magnetic poles of opposite kind form a
magnetic dipole. All known magnets are dipoles
(or higher poles); magnetic monopoles could
exist but have never been observed.
A magnet creates a magnetic field:
The direction of a magnetic field (B) at any
location is the direction that the north pole of a
compass would point if placed at that location.
B, Magnetic Field, is a Vector Quantity,
Described by both Magnitude and Direction
North magnetic poles are attracted by south
magnetic poles, so the magnetic field points
from north poles to south poles.
The magnetic field may be represented by
magnetic field lines.
The closer together (that is, the denser) the B
field lines, the stronger the magnetic field. At
any location, the direction of the magnetic field
is tangent to the field line, or equivalently, the
way the north end of a compass points.
Magnetic Force
A magnetic field can
exert a force on a
moving charged
particle.
Calculating Magnetic Field Strength
The magnitude of the force is
proportional to the charge and to the
speed of the charged particle moving
through the magnetic field:
SI unit of magnetic field: the tesla, T
Evaluating Metric Units
• Gauss is commonly used by geologists to
describe Earth’s magnetic field
Calculating Magnetic Force
In general, if the particle is moving at an
angle to the field,
The force is perpendicular to both the
velocity and to the field.
EXAMPLE: 19.2, p. 628
The Right-Hand Rule Gives
the Direction of the Force
F
v
B
RHR is for a + charge
LHR is for a – charge
Acceleration is in the
direction of the force
F
v
F
v
B
B
v
B
F
Sketching Magnetic Fields If the orange "diamond"
particle shown above is negatively charged, in which
direction is the magnetic force acting on it while it is
at the position shown?
A. towards the right side of
the page (+x)
B. towards the left side of
the page (-x)
C. towards the top of the
page (+y)
D. towards the bottom of the
page (-y) none of the above
X represents a magnetic
field pointing into the
plane of the page
• represents a magnetic
field pointing out of the
page
True or False? As the magnetic force acts on the
particles shown in the diagram above, it does
work on each one. This would be evidenced by
a change in the kinetic energy of each particle.
Bellwork 04/27/09
1. What is the magnitude of
force on a particle of charge
+3.0 μC that moves through a
magnetic field of 1.5 T at 7.8
x 104 m/s at 30o from the
parallel? **F=qvBsin**
2. What is the direction of the
force on a positive charge
that travels through a
magnetic field as shown to
the right?
S
N
+
Bellwork Solutions
1. F = (3.0x10-6 C)(7.8x104 m/s)(1.5T)(sin30o)
F= 0.18N
2. The index finger points in the direction of
motion, the other fingers north to south in the
direction of the magnetic field, leaving the
thumb, showing the direction of the force
pointing out of the page, or upwards.