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Physics IClass 22

The Magnetic Field

Rev. 13-Aug-04 GB

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Magnetism in Ancient Times

The ancient Greeks knew that the mineralmagnetite, named for the Magnesia regionof Asia Minor, attracts small iron objectslike nails. Magnetite is also known aslodestone.

Ancient Chinese discovered the compassin the year 271 AD (or CE).

The Chinese compass seen at the left usesa piece of magnetite shaped like a spoon.The handle of the spoon points south.

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Peter Peregrinus

Peter Peregrinus (Pierre de Maricourt) wasa member of the army of the King of Sicilyand probably served as an engineer. In 1269he described everything known aboutlodestones and how to make instrumentsusing these magnets. He discovered theexistence of magnetic poles and was the firstto use the word "poles." (Latin polus.) Hedid experiments referring to how the northand south poles attract or repel.

He also described how to make a perpetual motion machine usingmagnets. People are still trying this (unsuccessfully) today!

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William Gilbert

William Gilbert, 1544-1603

Gilbert published De Magnete (On the Magnet)in 1600. This book became a standard referenceon electricity and magnetism in Europe. Gilbertwas the first to distinguish the electric force(named for the Greek word for amber) from themagnetic force. Gilbert debunked many folkmyths about the curative properties of magnets.

Based on his observations of the preferred directions of thin ironneedles near spherical lodestones, and the similarity of thisphenomenon to the tendency of compass needles to tilt withrespect to the horizontal plane as well as point north-south,Gilbert deduced that the earth itself must be a giant magnet.

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Some Old (and Debunked)Ideas Linger On

“This premium quality magnetic bracelet will help relieve even themost severe or chronic discomfort in the hand, wrist, arm, elbow orshoulder. Constructed of surgical stainless steel with 24K titaniumgold plating.”

Magnetic Material: Samarium Cobalt Manufacturer's Rating: 11,000 Gauss Surface Gauss: 1000 Gauss (each magnet) Penetration:Polarity: North Pole (-) Brand: Serenity2000

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Magnetic Forces Between Poles“Opposites Attract”

NS N S

NS NSattract

repel

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Why Is A Magnet Attracted toSome Materials (Refrigerators)?

attractNS

N

N

N

N

N

N

S

S

S

S

S

S

The permanent magnet causes tiny magneticdomains in the nearby material to line up.

This effect is most noticeable in ferromagneticmetals: iron, cobalt, nickel (& some “rare earth”elements).

In paramagnetic materials (like liquid oxygen), themagnetic domains line up weakly.

In diamagnetic materials (like silver), the domainsline up against the permanent magnet and cause aweak repulsion.

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What Happens If You Try to Separate North and South Poles?

NS

saw cut

NS

NS

Scientists have searched in iron ore moon rocks cosmic rays high-energy colliders and many other placesbut “magnetic monopoles” havenever been found.

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Magnetic Field Lines

From N to S.

In direction ofcompassneedle.

Try this web site:http://www.walter-fendt.de/ph11e/mfbar.htm

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Electromagnetism

Hans Christian Oersted1777-1851

Oersted discovered in 1820 that electric currentcreates a magnetic field like a permanent magnet.This is a Physics 2 topic, but we will use this effectin the Physics 1 activities involving magnetism.

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Units of Magnetic Field

Nikola Tesla1856-1943

The SI unit of magnetic field (actually magnetic flux density, but wedon’t make a distinction in Physics 1) is the tesla, T.

The magnetic field in the gap between the rotor and stator of an electricmotor is about 1 T.

We also sometimes use gauss. 1 T = 10,000 gauss.

The magnetic field in a typical home environment is about 0.1-1.0 gauss.

Inventions: a telephone repeater, rotating magnetic fieldprinciple, polyphase alternating-current system, inductionmotor, alternating-current power transmission, Tesla coiltransformer, wireless communication, radio, fluorescentlights, and more than 700 other patents.

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Class #22Take-Away Concepts

1. The magnetic force was known in ancient times.

2. Magnets have two poles, N and S.

3. Opposite poles attract, like poles repel.

4. N and S poles are always in pairs, never alone.

5. Electric currents also cause magnetic fields.

6. Magnetic field lines start at N and go to S,following the direction of a compass needle.

7. Units of tesla (T) and gauss. 1 T = 10,000 gauss.

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Class #22Problems of the Day

___1. A scientist claims that he has discovered magneticmonopoles, and he sends you a sample cube of materialthat he claims contains only north magnetic poles. Whichtest below would help you verify or dispute his claim?

A) Check if all sides of the cube stick to a refrigerator door.B) Measure the magnetic flux density (magnitude) near the

surface to check if it exceeds the strongest normal magnet.C) Hang the cube on a string and see if it is attracted to or

repelled by a sphere with a negative electrical charge.D) See if the south pole of a compass needle points toward all

sides of the cube when placed near the respective sides.E) No test would help you unless you had other samples of pure

north and south poles to check for attraction and repulsion.

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Class #22Problems of the Day

___2. If magnetic monopoles have never been discovered, whatis/are the source(s) of magnetic fields?

A) Some elementary particles, like electrons, are tiny magneticdipoles.

B) Moving electric charges create magnetic fields.C) Since magnetic fields exist, there must be magnetic

monopoles. We just haven’t discovered them yet.

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Activity #22Introduction to Magnetic Fields

Objective of the Activity:

1. Think about magnetic fields.2. Investigate the magnetic field created by a bar magnet.3. Learn how to measure magnetic fields using a Hall probe.

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Chinese Voyages of ExplorationAdmiral Zheng He (1371-1435)