Chapter Thirty Six Notes:

Magnetism

July 1820: Oersted and electromagnetism

 

Hans Christian Oersted

By the end of the 18th century, scientists had noticed many electrical phenomena and many magnetic phenomena, but most believed that these were distinct forces. Then in July 1820, Danish natural philosopher Hans Christian Oersted published a pamphlet that showed clearly that they were in fact closely related.

During a lecture demonstration, on April 21, 1820, while setting up his apparatus, Oersted noticed that when he turned on an electric current by connecting the wire to both ends of the battery, a compass needle held nearby deflected away from magnetic north, where it normally pointed. The compass needle moved only slightly, so slightly that the audience didn’t even notice. But it was clear to Oersted that something significant was happening.

Even in this day and age, most of the public regards magnetism as a mystery. That has led to magnetic bracelets and similar "health products," to magnets taped to fuel lines for better gas mileage, and to widespread worries about possible reversal of the Earth's field, encouraged by Hollywood movies.

    In the minds of most Americans magnetism is forever associated with specially treated iron, with patterns of iron filings and with the way the compass needle lines up with the north-south direction. Few schools teach much more, because, (1) physics is an elective, and (2) magnetism is covered near the end of the textbook, the school year is short, and teachers are happy if they just make it to Ohm's law.

    Some people may also know that a current-carrying wire coil wrapped around an iron bar turns it into a magnet, and about use of electromagnets in electric machinery. But it's always with iron, or with some magnetic substance. Why sunspots would be magnetic remains completely unclear.

    In ancient times, both Greeks and Chinese knew about natural magnets, rare chunks of iron-rich mineral known as lodestones. The Chinese also knew that if you rubbed a steel needle against a lodestone, in a fixed direction, it also became a magnet. Around the

year 1000, they furthermore found that if a magnet or lodestone was placed on a little "boat" floating in a bowl of water, it always pointed in a fixed direction--and for a magnetized iron bar, that direction was always north-south. You could rotate the bowl, but the magnet would keep pointing in the same

    The reason, we now know, is that the Earth, too, is magnetic. From that came the magnetic compass, quickly copied by Arab navigators and then by Europeans. We may wonder today--if lodestones did not exist, the compass might have stayed undiscovered for a long time, and would Columbus have ventured so far from land without it?

Every magnet has two poles. This is where most of its magnetic strength is most powerful. These poles are called north and south or north-seeking and south seeking poles. The poles are called this as when a magnet is hung or suspended the magnet lines up in a north - south direction. When the north pole of one magnet is placed near the north pole of another magnet, the poles are repelled. When the south poles of two magnets are placed near one another, they also are repelled from one another. When the north and south poles of two magnets are placed near one another, they are attracted to one another.

The attraction and repelling of two magnets towards one another depends on how close they are to each other and how strong the magnetic force is within the magnet. The further apart of the magnets are the less they are attracted or repelled to one another.

The magnetic and electric fields are both similar and different. They are also inter-related.

Similar: Just as the positive (+) and negative (−) electrical charges attract each other, the N and S poles of a magnet attract each other.

In electricity like charges repel, and in magnetism like poles repel. Different : The magnetic field is a dipole field. That means that

every magnet must have two poles. On the other hand, a positive (+) or negative (−) electrical charge

can stand alone. Electrical charges are called monopoles, since they can exist without the opposite charge.◦ • Monopole – a single magnetic pole or electric charge◦ • Dipole – a pair of opposite poles◦ • The so-called magnetic moment is the measure of the strength of the dipole.

The magnetic moments are expressed as multiples of Bohr Magnetons. A Bohr magneton has a value of 9.27 x 10-24 joules/tesla.

When a magnet is broken into little pieces, a north pole will appear at one of the broken faces and a south pole. Each piece, regardless of how big or small, has its own north and south poles.

The area around a magnet can also behave like a magnet. This is called a magnetic field. The larger the magnet and the closer the object to the magnet, the greater the force of the magnetic field.

Magnetic Materials The term magnetism is derived from Magnesia, the name of a

region in Asia Minor where lodestone, a naturally magnetic iron ore, was found in ancient times. Iron is not the only material that is easily magnetized when placed in a magnetic field; others include nickel and cobalt.

• The magnetic field is the central concept used in describing magnetic phenomena.

• A region or a space surrounding a magnetized body or current-carrying circuit in which resulting magnetic force can be detected.

• A magnetic field consists of imaginary lines of flux coming from moving or spinning electrically charged particles. Examples include the spin of a proton and the motion of electrons through a wire in an electric circuit.

Magnetic field or lines of flux of a moving charged particle

A magnetized bar has its power concentrated at two ends, its poles; they are known as its north (N) and south (S) poles, because if the bar is hung by its middle from a string, its N end tends to point northwards and its S end southwards. The N end will repel the N end of another magnet, S will repel S, but N and S attract each other. The region where this is observed is loosely called a magnetic field.

Either pole can also attract iron objects such as pins and paper clips. That is because under the influence of a nearby magnet, each pin or paper clip becomes itself a temporary magnet, with its poles arranged in a way appropriate to magnetic attraction.

But this property of iron is a very special type of magnetism, almost an accident of nature! Out in space there is no magnetic iron, yet magnetism is widespread. For instance, sunspots consist of glowing hot gas, yet they are all intensely magnetic. The Earth's own magnetic powers arise deep in its interior, and temperatures there are too high for iron magnets, which lose all their power when heated to a red glow. What goes on in those magnetized regions? It is all related to electricity.

MAGNETIC FORCE The magnetic field of an object can create a magnetic force on

other objects with magnetic fields. That force is what we call magnetism.

When a magnetic field is applied to a moving electric charge, such as a moving proton or the electrical current in a wire, the force on the charge is called a Lorentz force.

Attraction When two magnets or magnetic objects are close to each other,

there is a force that attracts the poles together.

Force attracts N to S Magnets also strongly attract ferromagnetic materials such as

iron, nickel and cobalt.

 Repulsion When two magnetic objects have like poles facing each other, the

magnetic force pushes them apart.

Force pushes magnetic objects apart Magnetic and electric fields

The magnetic and electric fields are both similar and different. They are also inter-related.

Each atom that makes up a substance is a time magnet. When atoms are arranged not in random directions but all in the same direction, the substance is a permanent magnet. Magnetism and electricity are very closely related, so that the flow of electricity through a conductive wire generates a magnetic field, and conversely a change in a magnetic field produces a flow of current in a conductor.

How is Magnetism Produced? The electrons in an atom spin as they rotate about the nucleus.

This spinning motion creates a magnetic effect in each electron, which together forms a magnetic field around the atom.

 

Normally, the atoms in any substance are oriented in random fashion throughout the substance. This means that their individual magnetic fields cancel each other, and the substance as a whole does not appear magnetically charged.

In a permanent magnet, all of the electrons are oriented in the same direction. This means that the substance as a whole acts as a magnet. The lines that show the direction of the magnetic field are called magnetic lines.

Even when the atoms are oriented randomly, exposure to a nearby magnet may cause them to line up with the magnetic field. Substances that do this easily, such as iron or nickel, can be 'magnetized.‘

Other substances, in which the atoms remain randomly oriented even when exposed to a magnet, such as copper, wood or plastic, cannot be magnetized.

When a coil is wrapped around an iron bar and electric current is passed through the coil, the iron bar picks up a magnetic field, and becomes an 'electromagnet.' The strength of the magnetic field is proportional to the size of the current flow. The relation is similar to the way wind passing through a windmill (current flow) flows at right angles to the plane of rotation of the windmill blades (creation of the magnetic field).

If the wind reverses direction, the windmill will also rotate in the opposite direction.

If a permanent magnet is inserted and withdrawn through the center of the coil, it causes an electric current to flow in the coil. The direction of the current flow is in opposition to the change in the magnetic field, so that the current is reversed each time the permanent magnet is inserted or withdrawn. This is the principle of the electric generator. The relation is similar to the way a windmill revolves (current flows) in response to the wind passing through it (movement of the magnet).

The connection between electric current and magnetic field was first observed when the presence of a current in a wire near a magnetic compass affected the direction of the compass needle. We now know that current gives rise to magnetic fields, just as electric charge gave rise to electric fields.

With positive current, point your thumb in the direction of the current and your fingers wrap around the wire in the direction of the B field.

[B-field = Magnetic field]Compass near a current-carrying wire

Form a loop with current carrying wire, and the concentration of the magnetic field within the loop is much stronger. Double the number of loops, and the magnetic field is twice as strong. The magnetic field intensity increases with the number of loops. A current carrying coil of wire with many loops is an electromagnet.

Sometimes a piece of iron is placed inside the coil of an electromagnet. The magnetic domains of the iron are induced into alignment, increasing the magnetic field intensity. Beyond a certain limit, the magnetic field in the iron “saturates,” so iron is not used in the cores of the strongest electromagnets, which are made of superconducting material (section 34.4)

A charged particle moving in a plane perpendicular to a magnetic field will move in a circular orbit with the magnetic force playing the role of centripetal force.

The direction of the force is given by the right-hand rule.

Equating the centripetal force with the magnetic force and solving for R the radius of the circular path we get

mv2 / R = q v B and  R = m v / q B

Orbit of charged particlein a magnetic field

Right Hand Rule:

Since a charged particle moving through a magnetic field experiences a deflecting force, a current of charged particles moving through a magnetic field also experiences a deflecting force. The direction of that deflection is dependent upon the direction of the current.

The basic galvanometer, devised by William Sturgeon in 1825, allows all of the various combinations of current and magnetic needle direction to be tried out. By making suitable connections to the screw terminals, current can flow to the right or to the left, both above and below the needle. Current can be made to travel in a loop to double the effect, and, with the aid of two identical external galvanic circuits, the currents in the two wires can be made parallel and in the same direction. Note that the wires are insulated from each other where they cross.

Electric Motor A current-carrying loop in a B field is the basis of an electric

motor. Using the right-hand rule one can see that the forces acting on the

wire will cause the loop to rotate. Changing the current direction at the right time will cause the loop to continue rotating on the motor shaft.

DC motor

    In ancient times, both Greeks and Chinese knew about natural magnets, rare chunks of iron-rich mineral known as lodestones. The Chinese also knew that if you rubbed a steel needle against a lodestone, in a fixed direction, it also became a magnet. Around the year 1000, they furthermore found that if a magnet or lodestone was placed on a little "boat" floating in a bowl of water, it always pointed in a fixed direction--and for a magnetized iron bar, that direction was always north-south. You could rotate the bowl, but the magnet would keep pointing in the same direction.

    The reason, we now know, is that the Earth, too, is magnetic. From that came the magnetic compass, quickly copied by Arab navigators and then by Europeans. We may wonder today--if lodestones did not exist, the compass might have stayed undiscovered for a long time, and would Columbus have ventured so far from land without it?

Robert Norman and an early scientific experiment

Figure 4     This is mainly about explaining a very fundamental concept in

science--the experiment. A scientific experiment is a way of testing nature, to learn how it behaves.

    By 1580, the use and manufacture of compass needles was a well known art. The maker would take a flat steel needle, find its middle by balancing it, install a pivot there, and then magnetize it by stroking it against a magnet or a lodestone. But that was not enough. The north-pointing end always seemed heavier, and a tip had to be snipped off, to make the needle balance again.

 

  The story goes that a compass maker named Robert Norman once snipped off too much and ruined a needle, so he devised an experiment, to find what was happening. Before magnetizing the needle, he balanced it not on a vertical pivot but on a horizontal one, lined up in the east-west direction. (Figure 4, above). Before the needle was magnetized, it stayed horizontal. Afterwards, its north end slanted down. (For some reason, the needle in Figure 4 points straight down, as it would at the magnetic pole.) Aha! The north-pointing magnetic force on the needle was not horizontal, but pointed into the Earth.

Slide 5    It was a classical scientific experiment, one of the first, and was published in 1581. Norman's contemporary was William Gilbert, distinguished physician and later physician to Queen Elizabeth I. Gilbert devoted much of his energy and money to study magnetism, and in 1600 published his research in a book "De Magnete" (Latin for "On the Magnet"). The preceding visualization of the downward "dip angle" of the magnetic force (Slide 5) is from this book.

    Gilbert devised an experiment which suggested a reason for the properties of the compass: the Earth itself was a giant magnet. Using as model for the Earth a lodestone fashioned into a sphere (he named it "terrella" or "little Earth"), Gilbert reproduced not only the north pointing properties of the horizontal needle, but also the downward slanting of the needle which Robert Norman made.     You will find two reviews of Gilbert's book and a lot more, including most of what we are telling you here today, in a web course on Earth magnetism, "The Great Magnet, the Earth" With home page by Dr. David P. Stern :

http://www.phy6.org/earthmag/

demagint.htm .

I found the following story, and thought it was extremely interesting! If you would like to read the entire story go to the following website:http://science.nasa.gov/headlines/y2003/29dec_magneticfield.htm

December 29, 2003: Every few years, scientist Larry Newitt of the Geological Survey of Canada goes hunting.

His quarry is Earth's north magnetic pole. At the moment it's located in northern Canada, about 600 km from

the nearest town: Resolute Bay, population 300, where a popular T-shirt reads "Resolute Bay isn't the end of the world, but you can see it from here.“

Scientists have long known that the magnetic pole moves. Sometimes the field completely flips. The north and the south poles

swap places. Such reversals, recorded in the magnetism of ancient rocks, are unpredictable.

At the heart of our planet lies a solid iron ball, about as hot as the surface of the sun. Researchers call it "the inner core." It's really a world within a world. The inner core is 70% as wide as the moon. It spins at its own rate, as much as 0.2° of longitude per year faster than the Earth above it, and it has its own ocean: a very deep layer of liquid iron known as "the outer core."

At the heart of our planet lies a solid iron ball, about as hot as the surface of the sun. Researchers call it "the inner core." It's really a world within a world. The inner core is 70% as wide as the moon. It spins at its own rate, as much as 0.2° of longitude per year faster than the Earth above it, and it has its own ocean: a very deep layer of liquid iron known as "the outer core."

Earth's magnetic field comes from this ocean of iron, which is an electrically conducting fluid in constant motion. Sitting atop the hot inner core, the liquid outer core seethes and roils like water in a pan on a hot stove. The outer core also has "hurricanes"--whirlpools powered by the Coriolis forces of Earth's rotation. These complex motions generate our planet's magnetism through a process called the dynamo effect.

The next page has some of the figures pertaining to the information given above. If you would like more, go to the website!

The movement of Earth's north magnetic pole across the Canadian arctic, 1831--2001.

Right: a schematic diagram of Earth's interior. The outer core is the source of the geomagnetic field.

Supercomputer models of Earth's magnetic field. On the left is a normal dipolar magnetic field, typical of the long years between polarity reversals. On the right is the sort of complicated magnetic field Earth has during the upheaval of a reversal.