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Section 12.1 ­ Magnetic Fields

A magnetic field is the three­dimensional region of space surrounding a magnet that will exert a force on magnetic objects. The magnetic field exists around the magnet whether the magnetic field is causing a force or not.

Earth has a magnetic field that exists both inside and surrounding it.Other types of fields also exist in nature. A gravitational field exists around Earth because of Earth’s mass, and it causes objects with mass to be attracted toward Earth’s centre. Electric fields surround charged particles such as protons and electrons. The electric fields cause charged particles to experience forces of attraction or repulsion.

What is a FIELD?

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Maglev Train ­ Shanghai, China

Th e type of Maglev train used in Shanghai is based on a German design that usesmagnetic fi elds that cause the train to levitate based on attraction. Th e bottom ofthe train is attracted upward toward the bottom of the track ﴾Figure 6﴿. Th e forwardmotion is caused by both attraction and repulsion forces between the track and thetrain. Attraction forces between the front of the train and the track immediately infront of it pull the train forward. At the same time, repulsion forces at the back of thetrain push it forward.

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In the northern parts of the northern hemisphere, magnetic fields cause beautiful displays of light in the night sky called northern lights or aurora borealis. Charged particles from the Sun travel through space and interact with Earth’s magnetic field, which is strongest at the poles. The shape of the field directs these charged particles toward the ground. These downward­travelling particles collide with oxygen or other gases in the atmosphere, transforming kinetic energy into light.

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Section 12.2 ­ Oersted's Discovery

Is there a link between electricity and magnetism?

Charged particles behave in a similar way to magnetic poles. Like charges (positive and positive or negative and negative) repel each other and unlike charges (positive and negative) attract each other.

In 1819, Danish physicist Hans Christian Oersted was the first scientist to successfully connect electricity and magnetism. We have since found out that the electromagnetic force is one of the four fundamental forces of nature.

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Oersted discovered that a wire with current running through it creates a magnetic field around the wire. The field can be "seen" by using a magnetic compass. The arrow on the compass will seek out magnetic North when there is no current present, but with current present the arrow follows the magnetic field created by the electric current in the wire.

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Oersted’s principle whenever a charge moves through a straight conductor, a circular magnetic field is created around the conductor.

He also noted, that when the current direction changed, so did the direction of the compass arrow.

In Oersted's time, the conventional current model, positive to negative flow, was the norm; hence his principle has two versions (conventional and electron flow).

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Right Hand Rule ­ Magnetic Fields ­ Conventional Flow Model

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Left Hand Rule ­ Magnetic Fields ­ Electron Flow Model

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Representing Currents and Magnetic Fields in a Wire Visual Aids for the Conventional Current Model

Note the red tip of the arrow ­ gives direction of the field.

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