MAGNETIC FIELDS. - magnets have been used by our species for thousands of years. - for many of these years we had no clue how they worked:

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Allan Cunningham
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1 MAGNETIC FIELDS A SHORT HISTORY OF MAGNETS: - magnets have been used by our species for thousands of years - for many of these years we had no clue how they worked: 200 BC an ancient civilization in Asia Minor in a region known as Magnesia, found rocks that would attract each other - many explanations that described how these rocks worked involved some form of magic 850 AD The Chinese were using pieces of this mineral in a navigational tool which became known as a compass - the needle of the compass was a small piece of this magic rock (which was later named lodestone) 1269 AD an Italian scientist named Petrus Peregrims discovered that metal needles align with the poles of spherical pieces of lodestone 1600 AD a court physician for Queen Elizabeth named William Gilbert explained how compasses work: - he believed that the earth behaves like a giant magnet - this was the first time that a human being provided an explanation that is consistent with what we know today 1750 AD John Mitchell discovers that forces exerted in the polar regions of a magnet are equal in strength - his contribution suggested that magnets possess a predictable nature - this discovery allowed magnets to studied by science 1820 AD Hans Christian Oersted discovers that a current-carrying wire causes a compass needle to move - this discovery led to the creation of a new branch of physics know as electromagnetism - the models we use today to describe why magnets exhibit the properties they do involves the field theory

2 PROPERTIES OF MAGNETS - Magnets have two poles o We ve decided to call them North and South o The north pole will point north if the magnet is allowed to freely rotate about (as in a compass) - either of the two poles cannot and does not exist in isolation o physicists have never seen or detected the presence of a monopole in nature Even if a magnet is cut in half, each half will have both poles Every magnet has a north pole and a south pole o this is a hotly debated topic in physics due to the many similarities between charge and magnets, it seems logical to assume magnetic poles should exist in isolation (just like charges do) As with charge, like poles repel while unlike poles attract more and more research groups are suggesting they are close to discovering the monopole - magnets make fields in the space surrounding them o these fields are called magnetic fields o if a second magnet is placed in this field, the field exerts a force on the magnet this force is labeled F B on a free body diagram the direction of the force depends on the direction of the field and the nature (north or south) of magnetic pole in the field more on this later - magnets can be either permanent or temporary o as the names suggest, permanent magnets always make a magnetic field and the field made by temporary magnets can be turned on or off

3 MAGNETIC FORCES & FIELDS - in much the same way as mass and electric charge, magnets also produce fields o please understand that mass, electric charge and magnets produce different kinds of fields: mass makes a gravitational field electric charge makes an electric field magnets make magnetic fields - magnetic fields are capable of exerting a force (labeled F B ) on any magnet that resides in the field o magnetic fields can exert a force at a distance on other magnets o we can test for the presence of a magnetic field and/or map out the shape of a magnetic field by using something called a test magnet using a test magnet, we can create a magnetic field diagram for any situation the test magnet shows the shape of the field it does NOT show the direction that the field flows CONVENTION: A magnetic field generated by any magnet emerges from the north end and flows into the south end o The closeness (or density) of the field lines in a diagram shows the intensity or strength of the field We measure the intensity of a magnetic field in a unit called a tesla (T) Named in honour of the contributions made to physics by Austrian Hungarian inventor Nikola Tesla ( ) A tesla is equivalent to 1 N/A m More on this later - Knowing this we can create and understand magnetic field diagrams MAGNETIC FIELD DIAGRAMS:

4 GEOMAGNETISM QUESTION: Why do compasses always point north? ANSWER: Due to the fact that the earth behaves like a giant magnet - magnetic fields exert forces on magnets o a compass needle is a tiny magnet o the earth generates a magnetic field this field makes a compass needle align in what we have labeled a north-south orientation - as shown in the diagram above, the magnetic field of the earth is NOT tangential to the earth s surface at most locations o this is described as an angle of dip o it refers to the angle the magnetic field makes with the horizontal - the magnetic poles of the earth DO NOT exactly coincide with the geographic poles o the geographic North Pole is about 1300 km away from the nearby magnetic pole (in the Arctic ocean) o the magnetic poles are constantly moving with respect the geographic poles this is very important to know if you are navigating by a compass! o An angle of magnetic declination is a measure of the angular difference between these two locations this can vary from 0 to 25 in North America depending on your location.

5 MAGNETIC DOMAINS & THE DOMAIN THEORY - All materials are affected by the presence of a nearby magnet o For most materials, however, these effects are far too small to be detected normally - Ferromagnetic materials are materials that readily show large attractive forces to nearby magnets Examples: iron, cobalt, nickel and gadolinium - Paramagnetic materials are those that are very weakly attracted to either pole of a magnet Examples: aluminum, magnesium, tungsten - Diamagnetic materials are those that actually repel either end of a magnet (though extremely weakly) Examples: bismuth, copper, gold and silver - Why are some materials ferromagnetic, others paramagnetic and still others diamagnetic? o Let s focus on ferromagnetic materials for a minute: These materials become magnetic when placed in an external magnetic field In most cases, when the external field is removed, the material loses it s magnetic properties BUT: Some ferromagnetic materials hold these magnetic characteristics even when the external magnetic field is removed - The magnetic properties of any object come from the electrons found in the object o Each electron acts as a tiny magnet This still doesn t explain why some materials behave magnetically and others don t o The working theory used to explain this strange phenomenon was developed by Andre Ampere in the early 19 th century He suggested that ferromagnetic materials consist of large numbers of tiny regions known as domains These domains are at most about 1 mm across Each domain acts like a tiny magnet, with a north and a south pole Normally, the magnetic fields associated with these domains are arranged randomly, which results in a cancellation of the fields However, in the presence of an external magnetic field, they tend to line up so as to produce an overall net magnetic field, in the same direction as the external field These magnetic domains, once lined up, may stay lined up for long periods of time a permanent magnet.

TEMPORARY MAGNETS: - when a temporary magnet is not in a magnetic field, the domains point in random directions o this results in the cancellation of the magnetic fields of each small domain o the

6 TEMPORARY MAGNETS: - when a temporary magnet is not in a magnetic field, the domains point in random directions o this results in the cancellation of the magnetic fields of each small domain o the net effect is the object creates no magnetic field - when placed in an external field, the domains align o the object now creates a magnetic field - this field disappears when the external field is removed, and the domains return to a random arrangement PERMANENT MAGNETS: - the domains remain aligned with or without an magnetic field present o this results in a magnet that is always on - The orderly arrangement of domains in a permanent magnet may be disrupted by vibrations, or by heat o In either case, (heating a magnet, or causing it to vibrate - such as by dropping it), this will cause the domains to lose their order, resulting in the magnet weakening.

7 CURRENT VS ELECTRON FLOW - electricity that passes through any material is actually caused by the movement of electrons through that material - electrons can be pushed through a conducting material by a device called a battery Convention: electrons flow: OUT OF THE NEGATIVE TERMINAL INTO THE POSITIVE TERMINAL HOWEVER: - the ultimate nature of electricity was not known until relatively recently o Benjamin Franklin and other physicists who worked with electricity early on didn t know this and devised their own working theory to explain electricity o They suggested that electricity is actually current passing through a material o The convention they chose was that current flows: OUT OF THE POSITIVE TERMINAL INTO THE NEGATIVE TERMINAL - these two descriptions of electricity are at odds with each other o they are essentially opposites o we know today that Ben Franklin was wrong BUT we still use his description of current both conventions are used in textbooks and laboratories today! - the convention that you choose to use will affect the hand rule that you use: Current = Right Hand Rules Electron flow = Left Hand Rules

8 B FORCE ON A CURRENT CARRYING WIRE - a magnetic field is created when an electric current flows through a conductor o these magnetic fields possess all the same properties as those created by standard magnets o this magnetic field disappears when the current stops flowing a magnet of this kind is called a temporary magnet - French physicist Andre Ampere ( ) knew that a magnetic field will exert a force on a permanent magnet o He reasoned that a magnetic field should therefore exert a force on a current-carrying wire - we can calculate the magnitude of the force a current-carrying wire will experience when placed in an external magnetic field using the following equation: F ILBsinθ B = F B = force exerted on wire by magnetic field (N) I = magnitude of current flowing through wire (A) L = length of wire that resides in magnetic field (m) B = strength of external magnetic field (T) θ = angle between direction of current flow and magnetic field ( ) - we can determine the direction of the force a current-carrying wire will experience when placed in an external magnetic field using Right Hand Rule #3: RIGHT-HAND RULE #3: 1. Fingers of right hand point in the direction of the magnetic field 2. Thumb of right hand points in the direction of current flow 3. The palm of right hand indicates the direction in which the force is acting on the wire - this hand rule highlights the fact that the force on a current is not towards either of the poles, but rather at right angles to the magnetic field Example: A wire carrying a current of 3.0 A is sitting in a uniform magnetic field directed from the east to the west. If 1.25 m of wire is sitting in the field and the field is 7.0 T in strength directed to the north, what is the magnitude and direction of the force acting on the wire?

ELECTROMAGNETISM - A connection exists between electricity and magnetism - Although it might not be obvious, the unveiling of this connection has turned out to be one of the most important

(N or S) o This connection was discovered by Hans Christian Oersted in 1820 o Oersted found that a compass needle is influenced by an electric current in a wire This means that electric current

9 ELECTROMAGNETISM - A connection exists between electricity and magnetism - Although it might not be obvious, the unveiling of this connection has turned out to be one of the most important discoveries our species has made o For example, an electrically charged object does not show an attraction or repulsion to a magnet, regardless of the type of charge (+ or -) or type of magnetic pole (N or S) o This connection was discovered by Hans Christian Oersted in 1820 o Oersted found that a compass needle is influenced by an electric current in a wire This means that electric current (moving charge) produces a magnetic field This magnetic field pushes the compass needle The magnetic field lines form concentric circles perpendicular to, and centered on the current o The direction of the field around these circles may be determined by Right Hand Rule #2: Thumb of the right hand indicates the direction of current The curled fingers point in the direction of the magnetic field about that current.

SOLENOIDS - a solenoid is a long coil of wire shaped like a helix - Solenoids can be used to produce large magnetic fields - a solenoid generates a magnetic field similar to that of a bar magnet o

10 SOLENOIDS - a solenoid is a long coil of wire shaped like a helix - Solenoids can be used to produce large magnetic fields - a solenoid generates a magnetic field similar to that of a bar magnet o the magnetic field inside the coil is very strong o the direction of the field can be determined using Right Hand Rule #2 o the magnitude of the magnetic field can be changed by: 1. altering the current through the wire more current = stronger magnetic field 2. changing the number of turns/coils of wire more coils = stronger magnetic field 3. inserting an iron core into the coil intensifies the field greatly - solenoids are often referred to as electromagnets

Magnets & Magnetic Fields

Magnets & Magnetic Fields Magnets Magnets have 2 poles, North and South if broken in half, each half will have both poles at the ends. Like poles repel, unlike poles attract. Hard Magnets- materials that