Download 8. Magnetic fields

8. Magnetic fields
Many historians of science believe that the compass, which uses a magnetic
needle, was used in China as early as the 13th century BC, its invention being
of Arabic or Indian origin. The early Greeks knew about magnetism as early
as 800 BC. They discovered that the stone magnetite (Fe3O4) attracts pieces of
iron.
Subsequent experiments showed that every magnet, regardless of its shape, has
two poles, called north (N) and south (S) poles, that exert forces on other
magnetic poles similar to the way electric charges exert forces on one another.
That is, like poles (N–N or S–S) repel each other, and opposite poles (N–S)
attract each other.
Isolated magnetic poles do not exist!
Magnetic poles are always found in pairs. All attempts thus far to detect an
isolated magnetic pole have been unsuccessful. No matter how many times a
permanent magnet is cut in two, each piece always has a north and a south
pole.
Notice that the magnetic field lines outside the magnet
point away from the north pole and toward the south pole.
Cork screw rule
An electron in oscilloscope moves along the y axe with a speed of v = 8106 m/s. A magnetic field B = 0,03 T,
directed at an angle 30° to the x axis and lying in the plane xy, is used to deflect this beam.
1. Draw the magnetic force vector on the figure.
z
2. Find the magnitude of the magnetic force.
v
x
y
B
A charged particle (q) moves with a velocity v in a region in which the magnetic field is B
Determine the magnetic force vector acting on the particle if:
1.
2.
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Note that for electron charge is -1.6X10-19 C and mass is 9.1X10-31 kg
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A velocity selector consists of electric and magnetic fields described by the expressions E=E k and B=BJ
with B =15 mT. Find the value of E such that a 750-eV electron moving in the negative x direction is
undeflected. Note: One electron-volt is equal to 1.6·10-19 joules
z
E = 244 kV/m
x
Consider the mass spectrometer shown this Figure. The magnitude of the electric field
between the plates of the velocity selector is 2.5 103 V/m, and the magnetic field in
both the velocity selector and the deflection chamber has a magnitude of 0.035 T.
Calculate the radius of the path for a singly charged ion having a mass m=2.18 10-26 kg.
r = 0.278 m
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y
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Earth’s Magnetic field
B = 4.32 10-5 T
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