<< Chapter < Page Chapter >> Page >

Learning objectives

By the end of this section, you will be able to:

  • Describe the difference between the north and south poles of a magnet.
  • Describe how magnetic poles interact with each other.
Bar magnets, horseshoe magnets, and disc-shaped magnets attract and repel each other. Metal paperclips stick to some of the magnets.
Magnets come in various shapes, sizes, and strengths. All have both a north pole and a south pole. There is never an isolated pole (a monopole).

All magnets attract iron, such as that in a refrigerator door. However, magnets may attract or repel other magnets. Experimentation shows that all magnets have two poles. If freely suspended, one pole will point toward the north. The two poles are thus named the north magnetic pole    and the south magnetic pole    (or more properly, north-seeking and south-seeking poles, for the attractions in those directions).

Universal characteristics of magnets and magnetic poles

It is a universal characteristic of all magnets that like poles repel and unlike poles attract . (Note the similarity with electrostatics: unlike charges attract and like charges repel.)

Further experimentation shows that it is impossible to separate north and south poles in the manner that + and − charges can be separated.

A globe of the Earth with a bar magnet inside it. The south pole of the bar magnet inside the globe is at the north magnetic pole and is near, but not exactly on, the north geographic pole. The north pole of the bar magnet inside the globe is near the south geographic pole. Another bar magnet hangs beside the globe. The north pole of this magnet is pointing toward the north pole of the globe (or the south pole of the magnet inside the globe).
One end of a bar magnet is suspended from a thread that points toward north. The magnet’s two poles are labeled N and S for north-seeking and south-seeking poles, respectively.

Misconception alert: earth’s geographic north pole hides an s

The Earth acts like a very large bar magnet with its south-seeking pole near the geographic North Pole. That is why the north pole of your compass is attracted toward the geographic north pole of the Earth—because the magnetic pole that is near the geographic North Pole is actually a south magnetic pole! Confusion arises because the geographic term “North Pole” has come to be used (incorrectly) for the magnetic pole that is near the North Pole. Thus, “North magnetic pole” is actually a misnomer—it should be called the South magnetic pole.

Two sets of bar magnets. The first set of magnets are oriented with the unlike poles adjacent to each other. Force arrows show that these magnets are pulling on each other. The second set of magnets is oriented with the like poles adjacent to each other. Force arrows show that these magnets are pushing each other away.
Unlike poles attract, whereas like poles repel.
A bar magnet is split in half several times. The original magnet has a south pole and a north pole. Each time the magnet is split, each new half has both a south pole and a north pole.
North and south poles always occur in pairs. Attempts to separate them result in more pairs of poles. If we continue to split the magnet, we will eventually get down to an iron atom with a north pole and a south pole—these, too, cannot be separated.

Real world connections: dipoles and monopoles

[link] shows that no matter how many times you divide a magnet the resulting objects are always magnetic dipoles. Formally, a magnetic dipole is an object (usually very small) with a north and south magnetic pole. Magnetic dipoles have a vector property called magnetic momentum. The magnitude of this vector is equal to the strength of its poles and the distance between the poles, and the direction points from the south pole to the north pole.

A magnetic dipole can also be thought of as a very small closed current loop. There is no way to isolate north and south magnetic poles like you can isolate positive and negative charges. Another way of saying this is that magnetic fields of a magnetic object always make closed loops, starting at a north pole and ending at a south pole.

With a positive charge, you might imagine drawing a spherical surface enclosing that charge, and there would be a net flux of electric field lines flowing outward through that surface. In fact, Gauss’s law states that the electric flux through a surface is proportional to the amount of charge enclosed.

With a magnetic object, every surface you can imagine that encloses all or part of the magnet ultimately has zero net flux of magnetic field lines flowing through the surface. Just as many outward-flowing lines from the north pole of the magnet pass through the surface as inward-flowing lines from the south pole of the magnet.

Some physicists have theorized that magnetic monopoles exist. These would be isolated magnetic “charges” that would only generate field lines that flow outward or inward (not loops). Despite many searches, we have yet to experimentally verify the existence of magnetic monopoles.

The fact that magnetic poles always occur in pairs of north and south is true from the very large scale—for example, sunspots always occur in pairs that are north and south magnetic poles—all the way down to the very small scale. Magnetic atoms have both a north pole and a south pole, as do many types of subatomic particles, such as electrons, protons, and neutrons.

Making connections: take-home experiment—refrigerator magnets

We know that like magnetic poles repel and unlike poles attract. See if you can show this for two refrigerator magnets. Will the magnets stick if you turn them over? Why do they stick to the door anyway? What can you say about the magnetic properties of the door next to the magnet? Do refrigerator magnets stick to metal or plastic spoons? Do they stick to all types of metal?

Section summary

  • Magnetism is a subject that includes the properties of magnets, the effect of the magnetic force on moving charges and currents, and the creation of magnetic fields by currents.
  • There are two types of magnetic poles, called the north magnetic pole and south magnetic pole.
  • North magnetic poles are those that are attracted toward the Earth’s geographic north pole.
  • Like poles repel and unlike poles attract.
  • Magnetic poles always occur in pairs of north and south—it is not possible to isolate north and south poles.

Conceptual questions

Volcanic and other such activity at the mid-Atlantic ridge extrudes material to fill the gap between separating tectonic plates associated with continental drift. The magnetization of rocks is found to reverse in a coordinated manner with distance from the ridge. What does this imply about the Earth’s magnetic field and how could the knowledge of the spreading rate be used to give its historical record?

Got questions? Get instant answers now!

Questions & Answers

what is mutation
Janga Reply
what is a cell
Sifune Reply
how is urine form
Sifune
what is antagonism?
mahase Reply
classification of plants, gymnosperm features.
Linsy Reply
what is the features of gymnosperm
Linsy
how many types of solid did we have
Samuel Reply
what is an ionic bond
Samuel
What is Atoms
Daprince Reply
what is fallopian tube
Merolyn
what is bladder
Merolyn
what's bulbourethral gland
Eduek Reply
urine is formed in the nephron of the renal medulla in the kidney. It starts from filtration, then selective reabsorption and finally secretion
onuoha Reply
State the evolution relation and relevance between endoplasmic reticulum and cytoskeleton as it relates to cell.
Jeremiah
what is heart
Konadu Reply
how is urine formed in human
Konadu
how is urine formed in human
Rahma
what is the diference between a cavity and a canal
Pelagie Reply
what is the causative agent of malaria
Diamond
malaria is caused by an insect called mosquito.
Naomi
Malaria is cause by female anopheles mosquito
Isaac
Malaria is caused by plasmodium Female anopheles mosquitoe is d carrier
Olalekan
a canal is more needed in a root but a cavity is a bad effect
Commander
what are pathogens
Don Reply
In biology, a pathogen (Greek: πάθος pathos "suffering", "passion" and -γενής -genēs "producer of") in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term pathogen came into use in the 1880s.[1][2
Zainab
A virus
Commander
Definition of respiration
Muhsin Reply
respiration is the process in which we breath in oxygen and breath out carbon dioxide
Achor
how are lungs work
Commander
where does digestion begins
Achiri Reply
in the mouth
EZEKIEL
what are the functions of follicle stimulating harmones?
Rashima Reply
stimulates the follicle to release the mature ovum into the oviduct
Davonte
what are the functions of Endocrine and pituitary gland
Chinaza
endocrine secrete hormone and regulate body process
Achor
while pituitary gland is an example of endocrine system and it's found in the Brain
Achor
what's biology?
Egbodo Reply
Biology is the study of living organisms, divided into many specialized field that cover their morphology, physiology,anatomy, behaviour,origin and distribution.
Lisah
biology is the study of life.
Alfreda
Biology is the study of how living organisms live and survive in a specific environment
Sifune
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply
Practice Key Terms 2

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'College physics for ap® courses' conversation and receive update notifications?

Ask