Magnetic field of Earth Notice that the “North” pole of the Earth is a magnetic South pole! Earth’s field is similar to that of a bar magnet.
Source of magnetic field Magnetic fields (B-fields) are created by electrical currents B = (2 10-7)I/R
Direction of magnetic field around wire The field circles or loops around an electrical current. The direction of the field is determined by a right-hand rule.
Bar magnet Inside a solid material, currentswithin the atom cause each atom to act like a little magnet. In some materials, like aluminum, the atoms are not aligned, so no bar magnet. In iron, many regions (domains) of atoms are lined up magnetically and thus create a bar magnet.
Coils and currents A current in a wire creates a magnetic field (B-field) around the wire. If you make a coil out of the wire, the B-fields from each loop add up. You get a field around the coil that looks like that from a bar magnet!
Coils and currents - direction The location of the North and South poles for the coil are determined by another right-hand rule.
Coil acts like bar magnet When the current is on, the coil acts like a bar magnet (which end is North pole?) and attracts the real bar magnet. Where are the N and S poles for the bar?
The Earth’s magnetic field is caused by currents within molten rock. Detailed models of what is going on are still being developed.
Magnetic vs. Electric • Like charges repel • Unlike charges attract • Electric fields are produced by charges • E-fields exert forces against charged particles • Like poles repel • Unlike poles attract • Magnetic fields are produced by currents • B-fields exert forces against moving charged particles (currents)
Magnetic forces on moving charges • When a charge moves through a B-field, it feels a force. • It has to be moving • It has to be moving across the B-field Direction of force is determined by another right-hand rule
Magnetic forces on moving charges FB = qvB If a current I in a wire of length L passes through the field, the force is: FB = BIL
Magnetic force on current F = BIL Wire can be levitated by magnetic force when a current passes through it
Aurora Caused by interaction of charged particles from Sun with Earth’s B-field and atmosphere
Planets like Jupiter and Saturn have strong B-fields that interact with charged particles from the Sun. This is a picture of an aurora on Saturn as seen in UV light