In this equation u is the velocity of the fluid, B is the magnetic field, and eta is the magnetic diffusivity. The first term on the right hand side of the induction equation is a diffusion term. The motion of the molten outer iron core is sustained by convection, or motion driven by buoyancy. The temperature increases toward the center of Earth, and the higher temperature of the fluid lower down makes it buoyant.
The Coriolis effect, caused by the overall planetary rotation, tends to organize the flow into rolls aligned along the north-south polar axis. Electric currents induced in the ionosphere generate magnetic fields ionospheric dynamo region.
Such a field is always generated near where the atmosphere is closest to the sun, causing daily alterations that can deflect surface magnetic fields by as much as one degree.
Typical daily variations of field strength are about 25 nanoteslas nT , with variations over a few seconds of typically around 1 nT. The geomagnetic field changes on time scales from milliseconds to millions of years. Shorter time scales mostly arise from currents in the ionosphere ionospheric dynamo region and magnetosphere, and some changes can be traced to geomagnetic storms or daily variations in currents. At present, the overall geomagnetic field is becoming weaker; the present strong deterioration corresponds to a 10 to 15 percent decline over the last years and has accelerated in the past several years.
Geomagnetic intensity has declined almost continuously from a maximum 35 percent above the modern value achieved approximately 2, years ago. These events are called geomagnetic reversals. Evidence for these events can be found worldwide in basalts, sediment cores taken from the ocean floors, and seafloor magnetic anomalies. Reversals occur at apparently random intervals ranging from less than 0. The most recent such event, called the Brunhes-Matuyama reversal, occurred about , years ago.
Privacy Policy. Skip to main content. Search for:. Magnetism and Magnetic Fields. Electric Currents and Magnetic Fields An electric current will produce a magnetic field, which can be visualized as a series of circular field lines around a wire segment.
Learning Objectives Describe shape of a magnetic field produced by an electric current flowing through a wire. Key Takeaways Key Points A wire carrying electric current will produce a magnetic field with closed field lines surrounding the wire.
Another version of the right hand rules can be used to determine the magnetic field direction from a current—point the thumb in the direction of the current, and the fingers curl in the direction of the magnetic field loops created by it.
The Biot-Savart Law can be used to determine the magnetic field strength from a current segment. A current-carrying wire feels a force in the presence of an external magnetic field. Key Terms Biot-Savart Law : An equation that describes the magnetic field generated by an electric current.
It relates the magnetic field to the magnitude, direction, length, and proximity of the electric current. Permanent Magnets Permanent magnets are objects made from ferromagnetic material that produce a persistent magnetic field. Learning Objectives Give examples and counterexamples of permanent magnets. Key Takeaways Key Points Permanent magnets are objects made from magnetized material and produce continual magnetic fields.
Everyday examples include refrigerator magnets used to hold notes on a refrigerator door. Examples of these materials include iron, nickel, and cobalt. Magnets always have a north pole and a south pole, so if one were to split a permanent magnet in half, two smaller magnets would be created, each with a north pole and south pole.
Permanent magnets are made from ferromagnetic materials that are exposed to a strong external magnetic field and heated to align their internal microcrystalline structure, making them very hard to demagnetize. Key Terms permanent magnet : A material, or piece of such material, which retains its magnetism even when not subjected to any external magnetic fields. Magnetic Field Lines Magnetic field lines are useful for visually representing the strength and direction of the magnetic field.
Learning Objectives Relate the strength of the magnetic field with the density of the magnetic field lines. Key Takeaways Key Points The magnetic field direction is the same direction a compass needle points, which is tangent to the magnetic field line at any given point.
The strength of the B-field is inversely proportional to the distance between field lines. It is exactly proportional to the number of lines per unit area perpendicular to the lines. A magnetic field line can never cross another field line. The magnetic field is unique at every point in space. Magnetic field lines are continuous and unbroken, forming closed loops. Magnetic field lines are defined to begin on the north pole of a magnet and terminate on the south pole.
Key Terms B-field : A synonym for the magnetic field. Key Takeaways Key Points Earth is largely protected from the solar wind, a stream of energetic charged particles emanating from the sun, by its magnetic field. The geomagnetic field varies with time. Currents in the ionosphere and magnetosphere cause changes over short time scales, while dramatic geomagnetic reversald where the North and South poles switch locations occur at apparently random intervals ranging from 0. Heaviside understood them, saw the redundancy, and simplified them into the form we are familiar with today.
I have run across a couple references saying that his original statement had 20 equations. Four vector-valued equations would expand to 12 scalar equations, so vector notation alone does not explain why we have four equations and he had There must have been some redundancy, as you said.
And the Wikipedia article on Heaviside does give the credit for this simplification to Heaviside. The choice of those three letters appears to be an alphabetical accident of history.
The electric field also has D and E which describe it, similar to H and B. Reitz, Milford and Christy is the undergraduate book which most students from my era used. I recommend it. You can get a copy from abe. And yes it Was Heaviside who made the simplification but Maxwell also did shared the idea, this was a good discussion to prove Maxwell, as helpful as johnson law group.
This relationship holds for constant current in a straight wire, in which magnetic field at a point due to all current elements comprising the straight wire is the same. Parallel wires carrying current produce significant magnetic fields, which in turn produce significant forces on currents. The force felt between the wires is used to define the the standard unit of current, know as an amphere. In, the field B 1 that I 1 creates can be calculated as a function of current and wire separation r :.
Magnetic fields and force exerted by parallel current-carrying wires. The field B 1 exerts a force on the wire containing I 2. In the figure, this force is denoted as F 2. Rearranging the previous equation and using the definition of B 1 gives:.
If the currents are in the same direction, the force attracts the wires. If the currents are in opposite directions, the force repels the wires. The force between current-carrying wires is used as part of the operational definition of the ampere. For parallel wires placed one meter away from one another, each carrying one ampere, the force per meter is:. Incidentally, this value is the basis of the operational definition of the ampere. Privacy Policy. Skip to main content. Search for:.
Magnetic Fields, Magnetic Forces, and Conductors. The Hall Effect When current runs through a wire exposed to a magnetic field a potential is produced across the conductor that is transverse to the current. Learning Objectives Express Hall voltage for a a metal containing only one type of charge carriers.
Thus, those charges accumulate on one face of the material. On the other face, there is left an excess of opposite charge. Thus, an electric potential is created.
It is a factor of current I , magnetic field B , thickness of the conductor plate t , and charge carrier density n of the carrier electrons. Key Terms elementary charge : The electric charge on a single proton.
Magnetic Force on a Current-Carrying Conductor When an electrical wire is exposed to a magnet, the current in that wire will experience a force—the result of a magnet field. Learning Objectives Express equation used to calculate the magnetic force for an electrical wire exposed to a magnetic field.
Key Takeaways Key Points Magnetic force on current can be found by summing the magnetic force on each of the individual charges that make this current. The direction of the magnetic force can be determined using the right hand rule , as in fig [[]].
Key Terms drift velocity : The average velocity of the free charges in a conductor. Torque on a Current Loop: Rectangular and General A current-carrying loop exposed to a magnetic field experiences a torque, which can be used to power a motor. Learning Objectives Identify the general quation for the torque on a loop of any shape. Although the forces acting upon the loop are equal and opposite, they both act to rotate the loop in the same direction.
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