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# Magnetic field (B-field) of a bar magnet

Magnetic field (B-field) of a bar magnet. 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.

## Magnetic field (B-field) of a bar magnet

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1. Magnetic field (B-field) of a bar magnet

2. 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.

3. Source of magnetic field Magnetic fields (B-fields) are created by electrical currents B = (2  10-7)I/R

4. 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.

5. 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.

6. 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!

7. Coils and currents - direction The location of the North and South poles for the coil are determined by another right-hand rule.

8. 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?

9. The Earth’s magnetic field is caused by currents within molten rock. Detailed models of what is going on are still being developed.

10. 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)

11. 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

12. 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 = BIL

13. Magnetic force on current F = BIL Wire can be levitated by magnetic force when a current passes through it

14. Aurora Caused by interaction of charged particles from Sun with Earth’s B-field and atmosphere

15. 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

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