Unit 7. Magnetism Electromagnetism Electromagnetic Induction

Transcription

1 Unit 7 Magnetism Electromagnetism Electromagnetic Induction 1

2 Magnetism Origin of the term magnetism The Ancient Greeks discovered a mysterious mineral which attracted iron, and would point North if it were allowed to rotate freely. This mineral was found near a place called Magnesia, in Greece, hence the term magnetism. This mineral later became known as lodestone because it always pointed towards the leading, or "lode" star. Today this star is called the North Star. 2

3 Definition: North and South Poles The North end of a compass or any magnet is the NORTH SEEKING end, and is called the North Pole. The South end of a compass or any magnet is the SOUTH SEEKING end, and is called the South Pole. 3

4 Properties of Magnets Like Poles REPEL Different Poles ATTRACT 4

5 Magnetic Field Around every magnet there is a magnetic field of force (magnetic field). It acts at a distance - similar to gravitational field or electrical field. The direction of the field lines is given in the way that a compass would point. The lines point away from the North end of the magnet and towards the South end. When drawing magnetic fields, remember that the field lines never cross and are perpendicular to the surface. 5

6 Draw magnetic fields around the following: N S 6

7 N N S S S N 7

8 N S N N S 8

9 Earth's Magnetic Field The earth's magnetic field enables people to navigate using a compass. It is believed that certain animals have a magnetic sense of direction which enables them to navigate. (biomagnetism) It protects us from harmful cosmic radiation by deflecting them into outer space. Provides us with entertainment. It produces the Aurora Borealis and Aurora Australis. Northern Lights and the Southern Lights. 9

10 What causes the earth's magnetic field? It is believed that it results from the continuous motion of magnetic material (mainly molten iron and nickel) in the liquid outer core of the earth. 10

11 Magnetic field reversals Reversals have been documented as far back as 330 million years. During that time more than 400 reversals have taken place, one roughly every 700,000 years on average. However, the time between reversals is not constant, varying from less than 100,000 years, to tens of millions of years. 11

12 Many authors have pointed out that the dipole part of the magnetic field has been weakening during historic times, and that if the present trend continues, the dipole field will go to zero in roughly 1500 years. Some people take this to mean that we are entering a reversal. Although this possibility cannot be discounted, many investigators believe that the trend will not continue and that the field will regain its strength, as it has many times in the past. 12

13 Magnetic Declination: Magnetic North and True Geographic North are not in the same place. Magnetic north is near Baffin Island. The angle between the geographic north pole and magnetic north is called magnetic declination. 13

14 Declination varies from place to place and from year to year. 14

15 To use a map and compass you must add (or subtract) declination to your compass bearing to get true north. The link below shows how to use a compass ag/field/compass_e.php 15

16 Magnetic Deviation: In a vehicle such as a ship or aircraft, a compass is influenced by the magnetism of the iron used in the construction of the vehicle as well as the Earth's magnetic field. This causes the compass needle to point in the wrong direction. This directional error is called "deviation". Many people incorrectly use deviation when they mean declination. 16

17 Magnetic Inclination (or Magnetic Dip): It is the vertical component of the earth's magnetic field. It is the angle between the magnetic field and a horizontal line at any point. It is measured with a dipping needle which is basically a compass that is suspended vertically at its centre of mass. 17

18 Magnetic Inclination What direction will a magnet point if it were suspended vertically at its centre of mass at the following places? A) magnetic north B) equator C) magnetic south D) in our classroom 18

19 Types of magnetic substances: Ferromagnetic substances are strongly attracted by magnets. They can become strong induced magnets when placed in the field of another magnet. Iron, nickel, cobalt, and their alloys are ferromagnetic. Paramagnetic substances are only slightly attracted by very strong magnets. Oxygen and aluminium are paramagnetic substances. Diamagnetic substances are repelled by strong magnets. Copper, silver, and water are diamagnetic. 19

20 Magnets and Magnetic materials A MAGNET is an object composed of magnetic material that is permanently magnetized. made from hard ferromagnetic material A MAGNETIC material is one that is influenced by the force of magnetism. Made from soft ferromagnetic material 20

21 Question: Assume that you are given a bar magnet and an identical looking object O. Describe how you would determine whether or not: A) The object is magnetic If the object is magnetic the two objects will attract regardless of the ends that are put together. B) The object is a magnet If the object is a magnet then two ends will repel when brought together in the correct fashion 21

22 Domain Theory Suppose we take a bar magnet like the one below and break it into 2 equal pieces. N S Each piece will act like a bar magnet N S N S 22

23 If we do this again we get: N S N S N S N S If we continue to an atomic level you would discover that each atom acts as a tiny magnet. We call each magnetic atom a dipole. 23

24 A number of these dipoles can act on each other so that they all have basically the same orientation they all point in the same direction. This area is called a domain. 24

25 A ferromagnetic substance that has never been close to a magnet will have its domains pointing in all directions. A piece of Unmagnetized Iron. 25

26 If such a substance is placed in a magnetic field the domains will start to line up with the external field. Strong Magnet Not Shown When stroked with a magnet the domains of the piece of iron (i) line up (ii) grow Induced S Pole Induced N Pole Piece of Iron Becomes a Magnet 26

27 The stronger the field, the more alignment will take place. In a magnet the domains are lined up If the external field is reversed, then over a period of time the domains of the substance will also reverse direction. The induced magnetic poles of the ferromagnetic substances will switch. 27

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29 Magnetic Induction Magnets can be made by stroking a hard ferromagnetic material, such as a nail or needle, with another magnet. It can also be done with a hammer! How? 29

30 Magnetic Saturation Occurs when as many as possible of the dipoles are aligned. 30

31 Keepers for Bar Magnets Prevents the bar magnets and horseshoe magnets from losing their magnetism. What type of material is used for keepers? soft ferromagnetic 31

32 Reverse Magnetism This occurs when a magnet is exposed to a stronger magnetic field, that is opposite in orientation, over a period of time. Why shouldn't a compass be transported in a gun case? 32