Section3_Mapping_Magnetic_Fields_soln.notebook April 19, 2012

Section 3: Mapping Magnetic Fields

In this lesson you will

• state the Law(s) of magnetic forces • use iron filings to map the field around various configurations of bar magnets and around a horse shoe magnet • use a test compass (or several test compasses) to map the field around a bar magnet • use several test compasses to map the field around a straight conductor • extrapolate from the observed field around the straight conductor and draw a diagram of the field around (and through) a coil • state left hand rules to describe the magnetic field around a straight conductor (Oersted's Principle) and around a coil.

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Law of Magnetic Forces

Like magnetic poles (north and north or south and south) repel one another with a force, even at a distance apart.

Unlike poles (north and south or south and north) attract one another with a force, even at a distance apart.

A magnetic field is the region around a magnetic material in which the material exerts an influence. The fields always include two poles because a single pole cannot exist alone.

To map a magnetic field we use the north pole of a compass (North Pole test). The north pole of the compass is repelled by the north end of any magnet creating the field. So the north pole of a compass will point away from the North Pole of the magnet and toward the South Pole of the magnet.

Magnetic field lines are drawn tangent to the compass needle at any point in space. The number of lines per unit area is directly proportional to the magnitude of the magnetic field. The direction of the magnetic field is defined as the direction in which the north pole of a test compass would point when placed at that location.

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Characteristics of magnetic field lines • Spacing of lines indicated relative force • Outside a magnet, the lines are concentrated at the poles. • The lines are orientated S to N inside the magnet and N to S outside the magnet. • The lines do not cross

Magnetic fields:

1 Single bar magnet

2 North Pole vs North Pole

3 South Pole vs South Pole

4 North Pole vs South Pole

5 Horseshoe Magnet

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The Earth’s Magnetic Field

­ similar to that of a bar magnet ­ the Earth’s magnetic poles are continually wandering ­ a recent theory suggests the wandering is due the flowing motion of hot liquid metals under the Earth’s crust ­ studies of volcanic rock from different time eras have shown that the Earth’s magnetic poles have actually switched several times over the ages. ­ The Earth’s geographic poles are actually the opposite of the magnetic poles.

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Section 4: Electricity and Magnetism

Similarities between electricity and magnetism:

1. Two types of electric charge ( + and ­). Two types of magnetic poles (N and S). The positive charge and the north pole behave similarly as do a negative charge and the south pole.

2. Like charges and poles repel; unlike charges and poles attract.

3. Charged objects set up electric fields of force. Magnetic objects set up magnetic fields of force.

4. Certain substances may be electrified by rubbing; certain substances may be magnetized by rubbing.

Section 5: Oersted’s Discovery Hans Christian Oersted (1777 ­ 1851) placed a magnetic compass needle directly beneath a long horizontal conducting wire. The wire was placed along the earth’s magnetic north­south line, so that the magnetic needle was naturally aligned parallel to the wire. When the wire was connected to the terminals of the battery, the compass needle swung toward an east­west orientation, nearly perpendicular to the wire.

A charge at rest does not affect a magnet, while a charge in motion does exert a force on a magnet.

Magnetic Field about a current carrying conductor ­ Oersted’s Discovery

http://www.youtube.com/watchv=p_bU2CInQDE&feature=player_detailpage

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Oersted’s principle: The Magnetic Field around a Straight Conductor (The Basic principle of electromagnetism)

Charge moving through a straight conductor produces a circular magnetic field around the conductor. The field is represented by concentric rings around the conductor.

As the distance from the conductor increases the field gets weaker and the lines become more widely spaced. There are no poles.

Reversing the direction of the current through the conductor would cause the field lines to point in the opposite direction.

Left Hand Rule (# 1) for a Straight Conductor:

Grasp the conductor in the left hand and point the thumb in the direction of the electron flow. Wrap your fingers around the conductor. The direction that your fingers curl is the direction of the magnetic field lines.

Cautionary note:

You are going to meet several left­hand rules. It is extremely important to remember that left­hand rules do not work for moving positive charges (only for moving negative charges). Equally important, if you do physics in college or university, you will likely be using conventional current (flowing from + to ­). Left hand rules do not work for conventional current. You will have to use right­hand rules for conventional current.

Symbols

Do questions 1, 2 a, d, c, f on page 638 and questions 6 ­ 8 page 663.

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