Magnetic Fields. Physics 4B

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Luke Grant Wood
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1 Physics 4B The term magnetism comes from the region of Magnesia, a province of Greece where certain stones were found by the Greeks more than 2000 years ago. Slide 1 Slide 2 Slide 3 Slide 4 Slide 5 Slide 6 Slide 7 Slide 8 Slide 9 Slide 10 Slide 11 Slide 12 Slide 13 Slide 14 Slide 15 Slide 16 Slide 17 Slide 18 Slide 19 Slide 20 Slide 21 Slide 22 Slide 23 Slide 24 These stones, called lodestones, had the unusual property of attracting pieces of iron. Magnets were first fashioned into compasses and used for navigation by the Chinese in the twelfth century. The subjects of magnetism and electricity developed almost independently of each other until 1820, when a Danish science professor named Hans Christian Oersted discovered in a classroom demonstration that an electric current affects a magnetic compass. Shortly thereafter, the French physicist Andre-Marie Ampere proposed that that electric currents are the source of all magnetic fields. magnetic field A force field that surrounds the space around a magnet or moving electric charge. There are two ways to create a magnetic field: 1) moving electric charges (ex: electromagnet) 2) the magnetic fields of electrons in some materials add together to give a net magnetic field (ex: permanent magnets) 1

If you break a magnet in half, you do not obtain isolated north and south poles; two new magnets are

Like poles repel Opposite poles attract No isolated N or S poles (magnetic monopoles) have ever been

The magnitude (strength) and direction of a magnetic field are represented by magnetic field lines.

2 If you break a magnet in half, you do not obtain isolated north and south poles; two new magnets are produced, each with a N and S pole. Like poles repel Opposite poles attract No isolated N or S poles (magnetic monopoles) have ever been observed. The magnitude (strength) and direction of a magnetic field are represented by magnetic field lines. Magnetic field lines always form closed loops. These lines point from the north to the south pole. The closer together the magnetic field lines, the stronger the magnetic field. The north pole of a compass points in the direction of a magnetic field at any point. 2

The Earth s Magnetic Field If like magnetic poles repel, why does the north pole of

3 Opposite poles Like poles The Earth s Magnetic Field The Earth s magnetic field is similar to the magnetic field from a giant bar magnet. The Earth s Magnetic Field If like magnetic poles repel, why does the north pole of a magnet point to the Earth s north pole? The Earth s geographic north pole is actually a magnetic south pole. 3

Scientists believe it has something to do with electric currents deep within the Earth s interior. The magnetic field of the Earth is not constant.

4 The Earth s Magnetic Field The Earth s magnetic north pole and geographic north pole are not aligned. magnetic declination angle between the orientation of a compass and true north The Earth s Magnetic Field Scientists do not know for certain what causes the Earth s magnetic field. Scientists believe it has something to do with electric currents deep within the Earth s interior. The magnetic field of the Earth is not constant. It has decreased to zero (zero phase) and even reversed over the years. The Earth s magnetic field has reversed about 20 times during the last 5 million years. Cosmic Rays The universe is full of charged particles called cosmic rays that consist of protons and atomic nuclei. Theses charged particles are traveling at insane speeds and are hazardous to astronauts. Most of these charged particles are deflected away from the Earth by the Earth s magnetic field. Van Allen Radiation Belts 4

Van Allen Radiation Belts Magnetic Bottle

in the Earth s magnetic field and make up the

Charged particles trapped in the Earth s

5 Van Allen Radiation Belts Magnetic Bottle Some of the charged particles become trapped in the Earth s magnetic field and make up the Van Allen radiation belts. Charged particles can become trapped by strong magnetic fields. Charged particles trapped in the Earth s magnetic field bounce back and forth between the magnetic poles. Some of these charged particles can escape the magnetic field and enter the Earth s atmosphere, creating an aurora borealis. 5

6 from Space 6

7 from Space 7

Conceptual Physics 11 th Edition. Magnetism. The Magnetic Force. Magnetic Poles. Chapter 24: MAGNETISM

Conceptual Physics 11 th Edition Magnetic Force Magnetic Poles Magnetic Field Magnetic Domains Electric Currents and Chapter 24: MAGNETISM Electromagnets Magnetic Force on Moving Charged Particles Magnetic