Electro-magnetism

1
Electro-magnetism

• NCEA AS2.6
• Text Chapters

2
Magnetism

• Permanent or ferro-magnetism
• Like poles repel, unlike poles attract
• Magnetic field strength B measured in Tesla T

3
Magnetic Fields

• Magnetic fields can be represented by field lines
• Closer togetherstronger field
• Point from N to S

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Magnetism

• Electromagnetism.
• Caused by moving electric charges.
• The earth has a magnetic field around it because
  of iron atoms spinning around the core.

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Electromagnetism

• Fields are formed around current carrying wires

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Right Hand Curl Rule

• Thumb points in direction of current flow
• Fingers curl in direction of field lines

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Magnetic Force

• Two parallel current carrying wires
• FkI1I2L/d
• Current flowing same direction in both wires
  are attracted
• Current flowing opposite directions wires repel
  each other

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Solenoids

• Fields are formed in solenoids or coils.

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Right Hand Curl Rule

• Fingers curl in direction of current flow around
  the coil
• Thumb points towards the north pole

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Electromagnets

• Electromagnets have a range of uses
• Attached to cranes in scrap metal yards
• Electric bells
• Relay switches
• More useful than permanent magnets as they are
  stronger and can be switched off

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Magnetic force

• Force on moving charges - a charged particle
  moving in a magnetic field will experience a
  force
• FBqv (sin?)
• Bmag field strength
• q charge
• vvelocity
• (This formula applies to a small charge moving at
  right angles to magnetic field lines if not
  right angles multiply by sin?)

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Right Hand Slap Rule

• For a positive charge moving in a magnetic field
• Thumb direction of movement
• Fingers direction of magnetic field
• Palm direction of force acting on charge

• Right hand rule gives direction for positive
  charge negative
• must be reversed

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Force on Moving Charge

• This principle can be used to deflect the
  electron beam in a TV set to make it scan across
  the screen

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Force on Moving Charge

• It can also be used to determine the mass of
  unknown substances in a device called a mass
  spectrometer

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Force on Current-carrying Wire

• A wire carrying a current will experience a force
  when placed into a magnetic field
• FBIL (sinq)
• Applies to current flowing in a wire running at
  right angles to magnetic field lines. (if not
  90, then multiply by sinq)
• Direction (for conventional current) given by
  right hand rule.

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Right Hand Slap Rule

• For a wire carrying current in a magnetic field
• Thumb direction of current
• Fingers direction of field lines
• Palm direction of force on wire

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Magnetic Force

• This principle can be used in magneto-hydro
  propulsion units

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Magnetic Force

• And in loud-speakers...

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Electric Motors

• If magnetic force acts on opposite sides of a
  current carrying coil which is mounted on an
  axis, a torque is produced which makes the coil
  spin

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Electric Motors

• This idea is the basis for all devices that run
  by an electric motor.

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Induction

• If a wire is moved through a magnetic field then
  a voltage can be induced across the ends the
  wire.
• If the wire is connected to a circuit then
  current will flow. The direction of induced
  current is determined by a right hand rule.

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Right Hand Slap Rule

• vdirection of wire movement
• Bdirection of magnetic field lines
• F force on a positive charge (ie direction of
  current flow)

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Induction

• The size of this induced voltage is given by
• VBvL
• (Bmag field strength, vvelocity of movement,
  Llength of wire in field)
• This is known as Faradays Law

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Induction

• The direction of the induced current is such that
  it creates an opposing force on the motion that
  is causing it.
• This is known as Lenzs Law
• Induced voltage/current can be made larger if
• The mag field is stronger
• The wire is longer
• The movement is faster
• (Solenoid has an iron core)

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Induction

• Induction is the principle behind the microphone

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Induction

• And the dynamo..

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Generators

• If a coil is spun around inside a magnetic field
  then current can be induced that can be used to
  operate devices that run on electricity

• This is called generation
• Both DC and AC can be generated depending on
  whether slip rings or split rings are used

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Transformers

• These consist of 2 coils wound close to each
  other.
• Changing the current in one coil makes the field
  around it change. This changing field induces
  current in a nearby coil.

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Transformers

• The ratio of the windings determines how much
  voltage/current is induced
• The voltage can be calculated using
• Nnumber of turns
• VVoltage
• Pprimary coil
• Ssecondary coil

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Transformers

• No transformer is 100 efficient, but assuming it
  was
• Power in Power out

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Transformers

• 3 Types
• Step up Vs gt Vp
• Step down Vs lt Vp
• Isolating (a safety device) Vs Vp