One Chapter 6

Magnetic Fields and Electromagnetism

• In this chapter and the next, conventional current is used.

Natural Magnetism

• 600 BC the Greeks had discovered that lodestone (ore containing iron oxide) was able to attract small iron objects.
• Also they discovered that lodestone, when floated, always oriented in the same direction with respect to north and south.
• These effects became known as magnetism.

Magnets

• Now, lodestone is rarely used.
• Iron, nickel, cobalt and gadolinium are used instead.
• Pole - the ends of a magnet where the magnetic forces seem to be greatest.
  • Named:
    • N-Pole - north seeking pole
    • S-Pole - south seeking pole
• Law of Magnetic Poles - Opposite magnetic poles attract. Similar magnetic poles repel.

Magnetic Fields

• Magnetic force
  • acts at a distance
  • is transmitted by a field
• Magnetic Field - a region in space where magnetic effects can be detected
• Magnetic fields can be detected by its effect on a small test compass.
• We depict magnetic fields by drawing lines and arrows on them pointing in the direction of the N-Pole on a test compass. In the diagrams below, the N-pole of the needle on the test compass will point in the directions indicated by the arrows.
• Iron filings can be used to illustrate the shape of a field.

• Each filing behaves like a small magnet in a magnetic field, therefore, they align with the field lines.
• Relative strength of the field is indicated by the relative spacing between field lines
  • Close means strong field
  • far means weak field
• The field strength (|B|) is related to the torque on a test compass not aligned with the field

t = F X r

• t = Torque, F = force, and r = distance from the turning point of the needle in the compass.

Magnetic Field of Earth

• A test compass placed in the Earth's magnetic field has the N-pole pointing to Earth's geographic north pole.
• Since like poles repel and unlike attract, then the Earth's magnetic pole in the northern hemisphere is a south pole.

• The magnetic lines point away from the north pole (in the southern geographic hemisphere) and towards the south pole (in the northern geographic hemisphere).
• Magnetic declination - the angle between the magnetic pole and the geographic pole.
  • Varies from place to place.
• Earth's magnetic field is three dimensional.
• A compass only indicates the horizontal components of the magnetic field
• A magnetic dipping needle is needed to measure the component not parallel to the earth's surface

• The plane of the protractor is aligned with north - south on a compass. The needle dips to show the angle the magnetic lines make with the Earth's surface.
• The Earth's magnetic field is slowly changing.
• One hypothesis as to why the Earth's field is changing, is that the field is rotating about the earth's axis once every 1000 years or so.

Induced Magnetism

• Some materials (iron, nickel cobalt and gadolinium) may, under the right circumstances, become magnetized.
• Ferromagnetic materials - materials that under certain circumstances become magnetized
  • Examples of these materials are iron, nickel, cobalt and gadolinium.
• Domain Theory of Magnetism - Ferromagnetic substances are composed of a large number of tiny magnetic domains (regions of dimensions less than one micro meter) each of which behaves as a tiny magnet. The behaviour of the domains is a consequence of the electrical properties of the atoms the make them up.
• Normally the magnetic domains are randomly oriented and so cancel each other out.
  • When a ferro magnetic material is placed in a strong magnetic field, many of the domains will turn to align with the external field.
  • Some aligned domains will grow in size to eliminate domains that are not aligned.
  • How long the arrangement lasts once the external field disappears, depends upon the material.
• The domain model provides explanations for:
  • A needle is magnetized by stroking in one direction
  • When a magnet is broken, two smaller magnets are produced.
  • Magnets made of soft iron demagnetize quickly but magnets made of hard steel remain magnetized for a long time.
  • Heating or dropping a magnet can cause it to lose its magnetization.
  • Strong magnetic fields can reverse the magnetism in bar magnets
  • Metal objects used in the construction of buildings and other items often become magnetized by the combined effect of the Earth's magnetic field and the vibrations during construction.

Electromagnetism

• Prior to Oersted, electricity and magnetism were considered to be separte.
• Oersted noticed that a compass needle was deflected by a current flowing through a wire.
• Basic Principle of Electromagnetism - Moving electric charges produce a magnetic field

Magnetic Field Around a Straight Conductor

• Magnetic lines around a current carrying conductor are circular
• A small test compass indicates the direction of the field lines (North end of the compass needle points in the direction of the field).
• Right Hand Rule Number 1 - for determinging the direction of the magnetic field produced by a current carrying wire.
  • With your right hand, grasp the wire with your thumb pointing in the direction of conventional current. Your fingers will curl around the wire in the direction of the magnetic field.
• A note on symbols:
  • A circle with a dot in it is understood to be an arrow pointing out of the screen or page.
  • A circle with an X in it is understood to be an arrow pointing into the screen or page.

• Note that the bottom half of the above diagrams uses electron flow current, and the top half uses conventional current.

Magnetic Field of a Loop

Magnetic Field of a Coil or Solenoid

• Solenoid - a long conductor wound into a coil

• The magnetic field of a solenoid is the vector sum of the fields produced by each loop.
• The field inside can be very strong and nearly straight
• The field is similar to that of a bar magnet.
• Right Hand Rule Number 2 - For determining the direction of the magnetic field produced by a solenoid.
  • Grasp the solenoid with your right hand. Your finger tips must point and curl around in the direction of conventional current. Your thumb points in the direction of the magnetic field lines inside the solenoid. (i.e. towards the solenoid's N-pole)

Using Electromagnets and Solenoids

• When a ferromagnetic material is placed inside of a solenoid, the magnetic field becomes much stronger
• The reason is that the domains of the material align with the solenoid's field.
• The total field is then the vector sum of the solenoid's and ferromagnetic material's fields.
• The field can be thousands of times stronger
• Permeability of the core - the ratio of the magnetic field strength with a particular core present, to the magnetic field strength without the core.
• Electro magnet - A device that causes a magnetic field by the flow of current.
  • A strong temporary magnet made by coiling wire around a ferromagnetic core, and passing an electric current through the wire.

Applications

• Many devices use a current carrying coil to creat a magnetic field
• The field is commonly used to cause another component to move by magnetic attraction

Lifting Electromagnet

• Uses a soft ferromagnetic core of high permeability
• A conductor is wrapped around the core
• By turning on a current through the conductor, a field is produced which is used to lift heavy ferromagnetic objects.

Electromagnetic Relay

• Relay - a switch that turns an electric circuit on or off, and that is operated magnetically by a current flowing in a separate circuit.
• When the switch at the top of the diagram is closed, current flows through the coils and produces a strong magnetic field. The field pulls the soft iron armature to the right which causes the conductor attached to the armature to touch the contact point. This closes the external circuit which results in the light bulb turning on.
• When the switch at the top of the diagram is opened, then current stops flowing through the coils, the magnetic field collapses, and the spring attached to the armature pulls the armature to the left which breaks the external circuit at the contact point. The bulb goes off.
• Armature - a pivoted bar of soft iron

Electric Bell

• When the switch at the top is closed, the coils produce a magnetic field which pulls the armature to the right and the bell is struck. As the armature is pulled, the contact is broken, and the current through the coils stops. Because the magnetic field produced by the coils now collapses, the spring is able to pull the armature back to the left which closes the contact again. Again the coils produce a magnetic field and the cycle repeats itself over and over.

Loud Speaker

• A current which varies with time pass through the flexible leads which bring the varying current to the voice coil. The varying current in the voice coil causes the coil to produce a magnetic field that varies with time. This magnetic field interacts with the magnetic field produced by the tubular permanent magnet. Because the voice coil field changes with time, the pull of the tubular permanent magnet on the voice coil will vary with time. This causes the paper sound cone attached to the voice coil to be moved in and out. This movement produces the compressions and rarefactions in the air that move to your ears where you preceive them as sound.

Galvanometer

• The coil around the soft iron cylinder is connected to some external circuit to be tested. The current in the external circuit flows through the coil to produce a magnetic field in the coil and the soft iron cylinder. Because the magnetic field produced by the coil is not parallel to the magnetic field produced by the permanent magnet, the coil and cylinder experience a torque which causes it to turn. As the coil and cylinder turn, a force is applied to the control spring. When the force of the control spring trying to restore the coil to its start position, and the magnetic force trying to turn the coil and cylinder away from its start position are equal, then the needle on the galvanometer will point to a specific number, which gives us information about the external circuit.

January 6, 2014