Electricity

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Electricity Magnetism

• Seb Oliver
• Lecture 11
• Microscopic Description of Currents
• Magnets Magnetic Fields

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Summary Lecture 10
Ohms Law

• Resistance
• Resistance
• Resistivity
• Resistance in series parallel
• Variation of resistance with temperature
• Power dissipation

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Summary Lecture 10

• Microscopic description of currents
• Drift velocity
• Drift velocity due to electric field

Ohms Law
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Todays Topics

• Microscopic description of currents
• Drift velocity
• Drift velocity due to electric field

Ohms Law
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Introduction to Second Half of Course

• First Half
• Charges
• Force between charges Electric Field
• Moving charges, current
• Second Half
• Magnetic Field
• Force on moving charges in magnetic field
• Generation of magnetic field by moving charges
• Generation of current by moving magnetic field

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Magnets Magnetism
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History

• 13th Century B.C. Compasses being used in China
• lt800 B.C. Greeks know about magnetism due to
  shepherd Magnes
• 1269 Frenchman Pierre de Maricourt maps out field
  on a sphere and discovers poles
• 1600 William Gilbert suggested Earth is a large
  permanent magnet
• 1819 Hans Christian Oersted discovers that
  current influences compass needle

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History

• 1820s Jean-Baptiste Biot Félix Savart Magnetic
  force from currents
• 1820s André Ampère Magnetic force from currents
• 1820s Michael Faraday Current from changing
  magnetic field
• 1820s Henry Current from changing magnetic field
• Maxwell combination

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Examples of Magnets

• Compass
• Earth
• Hi-Fi speakers
• Fridge magnets
• Electric motors
• Scrap yards
• Cupboard doors

• video/audio tapes

Bar magnets Filings
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Properties of magnets
As with charges we find that there are attractive
and repulsive forces.
We find that magnets stick to certain
non-magnetised materials
We find that magnets can both attract and repel
each other
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Magnetic Induction
Magnetism can be induced in materials by rubbing
with another magnetised material
Paramagnetic materials such as steel can become
magnetised but this will only last for a short
time
Ferromagnetic materials such as iron, cobalt,
gadolinium and dysprosium can become permanently
magnetic
A permanently magnetised ferromagnet can thus be
attracted to paramagnetic material by inducing
magnetism in that material
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Magnetic Field
Magnetic Field like the Electric Field is another
example of a vector field
It is defined everywhere
It has a magnitude
It has a direction, the direction that a compass
needle would point
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Magnetic Field Lines
If we move a compass around and record the
direction it points everywhere we can map out the
direction of the magnetic field lines
Direction of Field
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Magnetic Field Lines
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Magnetic Field Lines
The experiments of Pierre de Maricourt mapped out
the field lines on naturally magnetic sphere
Demonstrated that they all pointed to two
diametrically opposed points or poles.
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Magnetic Poles
In electrostatics there are two types of
charges positive and negative
Similarly there are two types of poles North
and South
Like poles repel
Dislike poles attract
By convention
The North pole of a compass needle points to the
geographical north pole.
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Quick Quiz
North
The Geographical North pole is defined as the as
where the axis of rotation of the earth goes
through the arctic
Is this

• Exactly the north magnetic pole
• Nearly the north magnetic pole
• Exactly the south magnetic pole
• Nearly the south magnetic pole

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Magnetic Field Examples
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Monopoles
Unlike with electric charge no isolated magnetic
pole or monopole has ever been discovered
A north pole is always found with a corresponding
south pole
Monopoles
Theories predict that monopoles should exist. It
may even be surprising that we do not see them
and many researchers are actively searching for
magnetic monopoles.
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Todays Topics

• Microscopic description of currents
• Drift velocity
• Drift velocity due to electric field

Ohms Law
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Summary Lecture 11

• Magnetic Field is vector field
• Units
• Direction in which compass needle points (N)
• Poles north and south

Tesla (T)