28.23 - Gotchas
Compass needles point north because Earth’s magnetic north pole is located near the geographic North Pole. No, the north end of the needle
on a compass is attracted to the magnetic south pole of the Earth. Compass needles point approximately to the geographic North Pole
because the magnetic south pole of the Earth is near its geographic North Pole.
Magnetic force vectors on charged particles point in the same direction as the magnetic field. No, the magnetic force vectors which act on
moving, electrically charged particles are perpendicular to the magnetic field, and to the particles’ velocities, as well.Magnetic fields exert a force on all moving electrically charged particles. Almost true. The charges have to be moving for there to be a force,
but if they are moving parallel to or opposite to the field, there will be no force. There must be at least some component of the velocity
perpendicular to the field for a force to exist.
When I use the right-hand rule for any charged particle moving in a magnetic field, my thumb points in the direction of the magnetic force. No,
the right-hand rule gives the direction of the force on a positively charged particle. For negative particles, the thumb points opposite to the
direction of the magnetic force.28.24 - Summary
A magnet is an object that creates magnetic fields and can exert a magnetic force
on other magnets or on moving charged particles. Magnets always have two poles,
called north and south poles. As with electrical charges, opposite poles attract each
other and like poles repel.
The exterior field lines of the magnetic field generated by a magnet are directed
from its north pole to its south pole. The symbol for a magnetic field is B, a vector.
Magnetic fields are measured in teslas (T). 1 T = 1 N·s / C·m. Magnetic fields í
especially weaker ones í are also measured in smaller units called gauss (G).
1 G = 10í^4 T.
The Earth has its own magnetic field, which is why compasses work on its surface.
The Earth’s magnetic south pole is near the geographic North Pole. Because the
two do not coincide, when using a compass you need to know the declination, the
angle between the magnetic and geographic poles at your location. Compasses can
also help you estimate your latitude if they are allowed to orient in three dimensions.Materials that are able to become permanent magnets, like iron, are said to exhibit
ferromagnetism. Ferromagnetic materials contain small regions with their own magnetic fields called domains. Domains are in turn made up of
electrons whose spins have a net alignment.
A magnetic field exerts a force on a moving charge. The force is perpendicular to both the velocity of the charge and the magnetic field. If you
wrap the fingers of your right hand from the velocity vector to the magnetic field vector, your thumb points in the direction of the force on a
positive charge. Your thumb is pointing in the direction opposite to the force on a negative charge.
A charged particle that is moving perpendicularly to a uniform magnetic field will move in a circular path. If it has a velocity component parallel
to the field, that component will cause helical motion: The particle will move in a circular path (in two dimensions) while moving at a constant
velocity in the third dimension.
A device called a mass spectrometer takes advantage of the circular motion caused by a magnetic field to separate moving particles by their
mass-to-charge ratios.
Since an electric current consists of moving charges, a current-carrying wire can have a force exerted on it by a magnetic field. The strength of
the force is proportional to the magnitude of the current, the length of the wire, the magnetic field strength, and the sine of the angle between
the wire and the field.
An electric motor takes advantage of the torque exerted by a magnetic field on a loop of current to spin a rotor, which turns a shaft to perform
useful work.
Magnetic fields exert a force on moving charges, but moving charges also create magnetic fields of their own.
A current-carrying wire creates a magnetic field that circles around the wire. The strength of the field decreases as you move farther from the
wire, and increases as the current increases. The direction of the field lines is given by the right-hand rule: Your thumb points in the direction of
current and your fingers wrap around the wire in the direction of the field lines.
When two current-carrying wires are placed parallel to each other, the magnetic field of each one affects the other. When the currents in the
wires are in the same direction, the wires attract. When the currents are running in opposite directions, the wires repel each other.Force on charge moving in B fieldMotion of a charge in a B fieldMagnetic force on a wireF = ILB sin ș
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