What are magnetic fields?

 Lea el texto y realice las actividades.

What are magnetic fields?

Most of us have some familiarity with everyday magnetic objects and recognize that there can be forces between them. We understand that magnets have two poles and that depending on the orientation of two magnets there can be attraction (opposite poles) or repulsion (similar poles). We recognize that there is some region extending around a magnet where this happens. The magnetic field describes this region.

There are two different ways that a magnetic field is typically illustrated:

1. The magnetic field is described mathematically as a vector field. This vector field can be plotted directly as a set of many vectors drawn on a grid. Each vector points in the direction that a compass would point and has length dependent on the strength of the magnetic force. Arranging many small compasses in a grid pattern and placing the grid in a magnetic field illustrates this technique. The only difference here is that a compass doesn't indicate the strength of a field.

   

   Figure 1: Vector field plot for a bar magnet

   Figure 1: Vector field plot for a bar magnet.
2. An alternative way to represent the information contained within a vector field is with the use of field lines. Here we dispense with the grid pattern and connect the vectors with smooth lines. We can draw as many lines as we want.

   

   Figure 2: Field line plot for a bar magnet

   Figure 2: Field line plot for a bar magnet

   The field-line description has some useful properties:
   - Magnetic field lines never cross.
   - Magnetic field lines naturally bunch together in regions where the magnetic field is the strongest. This means that the density of field lines indicates the strength of the field.
   - Magnetic field lines don't start or stop anywhere, they always make closed loops and will continue inside a magnetic material (though sometimes they are not drawn this way).
   We require a way to indicate the direction of the field. This is usually done by drawing arrowheads along the lines. Sometimes arrowheads are not drawn and the direction must be indicated in some other way. For historical reasons the convention is to label one region 'north' and another 'south' and draw field lines only from these 'poles'. The field is assumed to follow the lines from north to south. 'N' and 'S' labels are usually placed on the ends of a magnetic field source, although strictly this is arbitrary and there is nothing special about these locations. 
   Field lines can be visualized quite easily in the real world. This is commonly done with iron filings dropped on a surface near something magnetic. Each filing behaves like a tiny magnet with a north and south pole. The filings naturally separate from each other because similar poles repel each other. The result is a pattern that resembles field lines. While the general pattern will always be the same, the exact position and density of lines of filings depends on how the filings happened to fall, their size and magnetic properties.

   • 

     Figure 3: Magnetic field lines around a bar magnet visualized using iron filings.

     Figure 3: Magnetic field lines around a bar magnet visualized using iron filings.

How do we measure magnetic fields?

Because a magnetic field is a vector quantity, there are two aspects we need to measure to describe it; the strength and direction.

The direction is easy to measure. We can use a magnetic compass which lines up with the field. Magnetic compasses have been used for navigation (using the Earth's magnetic field) since the 11ᵗʰ century.

Interestingly, measuring the strength is considerably more difficult. Practical magnetometers only came available in the 19ᵗʰ century. Most of these magnetometers work by exploiting the force an electron feels as it moves through a magnetic field.

Very accurate measurement of small magnetic fields has only been practical since the discovery in 1988 of giant magnetoresistance in specially layered materials. This discovery in fundamental physics was quickly applied to the magnetic hard-disk technology used for storing data in computers. This lead to a thousand-fold increase in data storage capacity in just a few years immediately following the implementation of the technology (0.1 to 100 $\mathrm{Gbit/inch^2}$G, b, i, t, slash, i, n, c, h, squared between 1991 and 2003 [2]). In 2007 Albert Fert and Peter Grünberg were awarded the Nobel Prize in Physics for this discovery.

In the SI system, the magnetic field is measured in tesla (symbol $\mathrm{T}$T, named after Nikola Tesla). The Tesla is defined in terms of how much force is applied to a moving charge due to the field. A small refrigerator magnet produces a field of around $0.001~\mathrm{T}$0, point, 001, space, T and the Earth's field is about $5\cdot 10^{-5}~\mathrm{T}$5, dot, 10, start superscript, minus, 5, end superscript, space, T. An alternative measurement is also often used, the Gauss (symbol $\mathrm{G}$G). There is a simple conversion factor, $1~\mathrm{T} = 10^4~\mathrm{G}$1, space, T, equals, 10, start superscript, 4, end superscript, space, G. Gauss is often used because 1 Tesla is a very large field.

In equations the magnitude of the magnetic field is given the symbol $B$B. You may also see a quantity called the magnetic field strength which is given the symbol $H$H. Both $B$B and $H$H have the same units, but $H$H takes into account the effect of magnetic fields being concentrated by magnetic materials. For simple problems taking place in air you won't need to worry about this distinction.

Available at https://www.khanacademy.org. Acceso el 10 de octubre, 2020

ACTIVIDADES
CUESTIONARIO
1. ¿Con qué estamos familiarizados según este artículo?
2. ¿De qué depende la atracción de los imanes?
3. ¿Qué describe la región alrededor de un imán?
4. ¿Cómo se describe el campo magnético matemáticamente?
5. ¿Cómo se traza este campo?
6. ¿A dónde apunta cada vector?
7. ¿Qué ilustra la técnica de disposición de los vectores?
8. ¿Qué se considera cuando se mide una cantidad vectorial?
9 ¿Cómo se mide la velocidad?
10. ¿Qué se descubrió en 1988?
11. ¿En dónde se aplica este descubrimiento? 

COMPLETE LAS SIGUIENTES ORACIONES
1. An alternative way to represent the information ...
2. Magnetic field lines naturally ...
3. Sometimes arrowheads ...
4. The field is assumed to follow the lines ...
5. While the general pattern ...

LEA DESDE "How do we measure magnetic fields?" EN ESPAÑOL.