- 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.