Unit 13: Magnetism, Electromagnetism and Electromagnetic Induction

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Humankind has known about magnetism since before 600 BC. Ancient Greeks discovered lodestone, an ore of iron that is magnetic. When these stones were suspended on a string, they always assumed a North - South orientation. As early as 121 AD, the Chinese knew that a piece of iron could be magnetized by bringing it in close proximity to lodestone. Since lodestone was discovered near the ancient city of Magnesia in Asia Minor, the term given to this phenomena was magnetism.


Magnets and Magnetic Fields

Any magnet, straight, horeshoe or disc shaped always have two poles, referred to as North and South .


The magnetic poles are distinct and cannot be separated into monopoles. Breaking a magnet into two will always result in two, smaller dipoles.


We observe both repulsive and attractive forces with magnets. Just as with electric charges, opposite poles attract and like poles repel.


Magnetic field can be visualized in a manner similar to electric fields. We determine the direction of the field lines by imagining the direction of force acting on a hypothetical North monopole. We can see that the field lines would appear to be coming out of the North pole of the magnet and would be going into the South pole. We used iron filings and a compass in the lab to help us visualize the field lines and direction. Remember, the field lines aren't really there. They simply serve as a mental model to help us understand the action at a distance exhibited by magnets.

When magnets are placed near other magnets, their fields interact in a way similar to what we observed with electric fields. Just as gravitational field intensity (g) and electric field intensity (E), magnetic field intensity (B) is also a vector quantity. Two or more magnets with fields interacting have fields that add together like any two or more vectors to produce a resultant magnetic field.

Two Opposite Poles


Two Like Poles


The Magnetic Field Of The Earth

The earth can be thought of as a gigantic magnet. The magnetic South Pole is in the Northern Hemisphere, in the Northwest Territories in Canada about a thousand miles from the geographic North Pole. Magnetic North is about the same distance from the geographic South Pole in Antarctica.

Magnetic Declination

When you use a compass to orient yourself, remember that the compass points to the magnetic South Pole, not the geographic North. This means that at any point on the earth, there will be an angle formed from two lines drawn from that location, one to geographic North and the other to magnetic South. The angle between these two lines is referred to as the magnetic declination. In New England, the magnetic declination ranges from 15 to 20 degrees west. This means that a compass points west of true North by that angle of declination.


Magnetic Inclination

Magnetic Inclination can be used to get an idea of your latitude. It is measured with a dipping needle. The needle indicates the direction of a cord drawn from the location to magnetic south. The angle measured is the angle between the cord and the horizontal line drawn towards the magnetic South.

Earth's magnetic field is continually changing. The poles precess like a spinning top and move a couple of km per year. In the geological record, there is evidence that the earth's magnetic field has reversed itself on numerous occasions.

Electricity and Magnetism

Magnetic Field Around a Wire Conducting Current

Up until the eighteenth century, many people recognized the similarity between electricity and magnetism, but it wasn't until 1820 that Hans Christian Oersted came upon evidence making a direct connection between electricity and magnetism. When a compass was left near a wire, its needle deflected when the wire had current flowing through it.

The magnetic field produced by a current is a series of concentric circles with the conductor at the center


The direction of these field lines can be determined by the 1st right hand rule
1) Grasp the wire so that your thumb points towards the direction of the conventional current.
2) Your fingers wrap around the conductor in the direction of the magnetic field.

Notice that there are no north or south poles produced, even though there is a magnetic field produced.

Magnetic Field Around a Coil of Wire (Solenoid)

If a conductor is wound into a coil,  not only will a magnetic field be produced by current, but a definite north and south pole will be produced. We can see why this is by examining one loop of wire and applying the 1st right hand rule. If we grasp the wire and slide our hand around the coil, the direction of the field lines inside of the coil all point in one direction. 


If we put several coils together we can see a definite North and South Pole. We can determine the location of the North Pole on a solenoid conducting current by applying the 2nd right hand rule. If we grasp the solenoid so our fingers point in the direction of the conventional current, our thumb will point to the North Pole.


As we observed with our experiment, the strength of the magnetic field depends upon the current, the radius, the number of coils and the presence or absence of a ferromagnetic core material.

The Nature of Magnetism

What makes a substance magnetic? Magnetism is determined at the subatomic level by the presence or absence of unpaired electrons
Presence of Unpaired Electrons Behavior in Magnetic Field Examples
Ferromagnetic several unpaired electrons strongly attracted to a magnet Fe, Co, Ni, Gd
Paramagnetic a few unpaired electrons weakly attracted to a magnet O2, Mn
Diamagnetic no unpaired electrons weakly repelled by a magnet N2

Ferromagnetic substances like iron can either be magnetized or not. What determines whether or not a piece of iron is magnetized? In a ferromagnetic substance, there are groups of atoms whose magnetic poles all line up in the same direction. These groups are referred to as domains.

An unmagnetized piece of iron has domains that are randomly oriented. If you add up the magnetic field of each domain, they "cancel out" because of their random orientation.


A magnetized piece of iron has many domains pointing in the same direction. The magnetic field strength of the magnet will increase as the number of domains pointing in the same direction increase.


This can help us explain a lot about what we observe with magnets. For example:

1) We can make an unmagnetized paper clip magnetized, simply by placing it next to a magnet. The magnetic field of the magnet causes the domains in the paper clip to line up temporarily, allowing it to be magnetized. When the magnet is removed, the domains randomize, and the paper clip loses its magnetism.

2) We can stroke a nonmagnetized steel nail with a magnet and make the nail magnetized. The magnetic field the magnet causes the domains in the nail to line up so that the nail becomes magnetized.

3) An older Canadian Nickel is attracted to a magnet. When it is heated to a very hot temperature, it loses its attraction to the magnet. The thermal energy causes the domains to vibrate and assume a random orientation.

4) A ferromagnetic material used as a core in a solenoid enhances the magnetic field of the solenoid because the domains of the material align with the solenoid's magnetic field. The ability to enhance a solenoids magnetic field is referred to as the magnetic permeability, m. We define the magnetic permeability as the ratio of the magnetic field of the solenoid with the core to the magnetic field of the solenoid with no core.

m = Bwith/ Bwithout

Ferromagnetic substance have a m >>1, paramagnetic substances have a m of only slightly more than 1 whereas diamagnetic substances have a m of slightly less than 1

Interaction of Magnetic Fields Produced By Conductors

In class we observed that magnetic fields produced by conductors interact. 

When 2 conductors conduct current in the same direction, we observe an attraction. Drawing the magnetic field lines around each conductor illustrates that the field lines from each conductor travel in opposite directions where we observe the most interaction.


When 2 conductors conduct current in the opposite direction, we observe a repulsion. Drawing the magnetic field lines around each conductor illustrates that the field lines from each conductor travel in the same direction where we observe the most interaction.


Direction of the Force on a Moving Charge Due to an External Magnetic Field


Picture a wire placed between two magnets. If the wire conducts current, an induced magnetic field resembling concentric circles surrounds the conductor. We can determine the direction of the induced magnetic field by the first right hand rule. We can see that above the conductor the field lines go in the same direction and repel whereas below the conductor, the field lines go in opposite directions and attract. The net force will be downward. 


We can develop a third right hand rule that helps us determine the direction of the force.

1) Our fingers point in the direction of the external magnetic field.

2) Our thumb points in the direction of the current (positive charge)

3) Our palm points in the direction of the force.

The force acting on a conductor conducting current in a magnetic field can be calculated using the equation 

F = BI L 

where F = force in Newtons
B = the external magnetic field
I = current flowing through the wire
L = length of the conductor

The above equation can be generalized for any charge moving through a magnetic field. The force acting on a charge moving through a magnetic field is

F = q v B

where q = charge in Coulombs
v = velocity of charge in m/s

The Motor

Because a charge moving in a magnetic field can experience a force we can now explain how motors work. Electrical current flows through a coil placed between two magnets. The charges moving through a magnetic field experience a force that turns the motor.
We can see how this force can turn the coil in a motor from the following diagram.

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