PHYS 012A Lecture Notes - Lecture 18: South Magnetic Pole, Geographical Pole, Electric Field
12 Jun 2018
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Chapter 29: Magnetic Fields
Bar Magnets are dipoles. Magnetic field lines leave from north and end at south.
Are there any magnetic charges (monopoles)?
- No, a single magnetic pole has never been isolated. They are always found in pair. You will
always end up with two magnetic poles even when you break the bar.
Magnetic Dipole Electric Dipole
Magnetic Poles
- Every magnet has a north and south pole that exert force on each others. Like repels and
unlike attract.
- If a bar magnet is suspended so that it can move freely, then it will rotate.
o The earth’s orth geographi pole is a ageti south pole ad vie versa
o The earth is a magnetic dipole.
Earth’s Mageti Field
Magnetic Fields
- An electric field surrounds any electric charge
- The region of space surrounding any moving electric charge also contains a magnetic field
- A magnetic field surrounds a magnetic substance making up a permanent magnet.
- They are a vector quantity
- Magnetic field lines can be used to show how the magnetic field would look
Magnetic Field Lines, Bar Magnet
- Iron fillings show pattern of the electric field lines
- The direction of the field is the direction a north pole would point
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Magnetic Field Lines, Opposite Poles Like Poles
Definition of Magnetic Field
- The field at some point in space can be defined in terms of the magnetic force FB and this
force exerted on a charged particle moves with velocity v.
Force on a Charge Moving in a Magnetic Field
- The properties can be summarized in a vector equation:
Bq=F v B
Right – Hand Rule
- Thumb – direction of the force if q is positive
- The force is the opposite direction of your thumb if q is negative.
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Magnitude of F
-
sinqvBFB=
- Theta is the angle between v and B
- FB is zero when the field and velocity are
parallel or antiparallel. Theta = 0 or 180
degrees
- FB is a max when the field and velocity are
perpendicular. Theta = 90 degrees.
Differences Between Electric and Magnetic Fields
- Direction of force
o The electric force acts along the direction of the electric field.
o The magnetic force acts perpendicular to the magnetic field.
- Motion
o The electric force acts on a charged particle regardless of whether the particle is
moving.
o The magnetic force acts on a charged particle only when the particle is in motion.
- Work
o The electric force does work in displacing a charged particle.
o The magnetic force associated with a magnetic field does no work when a particle is
displaced.
o This is because the force is perpendicular to the displacement of its point of
application.
Work in Field
- When a charged particle is moving with a given velocity through a magnetic field, the field can
alter the direction of the velocity, but not the speed or KE.
Units
- The SI unit for the magnetic field is the tesla (T).
2( / )
Wb N N
Tm C m s A m
= = =
- Wb is a weber
- A non-SI unit is a gauss (G).
o 1 T = 104 G.
Magnetic field lines coming out of the page are indicated by dots, representing the tips of arrows coming
outward.
Magnetic field lines going into the page are represented by crosses, representing the feathers of arrows
going inward.
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Document Summary
Magnetic field lines leave from north and end at south. No, a single magnetic pole has never been isolated. You will always end up with two magnetic poles even when you break the bar. Every magnet has a north and south pole that exert force on each others. If a bar magnet is suspended so that it can move freely, then it will rotate: the earth"s (cid:374)orth geographi(cid:272) pole is a (cid:373)ag(cid:374)eti(cid:272) south pole a(cid:374)d vi(cid:272)e versa, the earth is a magnetic dipole. The region of space surrounding any moving electric charge also contains a magnetic field. An electric field surrounds any electric charge. A magnetic field surrounds a magnetic substance making up a permanent magnet. Magnetic field lines can be used to show how the magnetic field would look. Iron fillings show pattern of the electric field lines. The direction of the field is the direction a north pole would point.
