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A view of a section of the accelerator at Fermilab. Down each side of a long corridor are tubes surrounded by orange magnets. Lots of tubes and wires and other electronics are visible.
The Fermilab facility in Illinois has a large particle accelerator (the most powerful in the world until 2008) that employs magnetic fields (magnets seen here in orange) to contain and direct its beam. This and other accelerators have been in use for several decades and have allowed us to discover some of the laws underlying all matter. (credit: ammcrim, Flickr)

Thermonuclear fusion (like that occurring in the Sun) is a hope for a future clean energy source. One of the most promising devices is the tokamak , which uses magnetic fields to contain (or trap) and direct the reactive charged particles. (See [link] .) Less exotic, but more immediately practical, amplifiers in microwave ovens use a magnetic field to contain oscillating electrons. These oscillating electrons generate the microwaves sent into the oven.

Figure a shows a tokamak in a lab. Figure b is a diagram of a tokamak. A current-carrying wire wraps around a donut-shaped vacuum chamber. Inside the chamber is plasma. The magnetic field has a toroidal and poloidal shape inside the chamber.
Tokamaks such as the one shown in the figure are being studied with the goal of economical production of energy by nuclear fusion. Magnetic fields in the doughnut-shaped device contain and direct the reactive charged particles. (credit: David Mellis, Flickr)

Mass spectrometers have a variety of designs, and many use magnetic fields to measure mass. The curvature of a charged particle’s path in the field is related to its mass and is measured to obtain mass information. (See More Applications of Magnetism .) Historically, such techniques were employed in the first direct observations of electron charge and mass. Today, mass spectrometers (sometimes coupled with gas chromatographs) are used to determine the make-up and sequencing of large biological molecules.

Section summary

  • Magnetic force can supply centripetal force and cause a charged particle to move in a circular path of radius
    r = mv qB , size 12{r= { { ital "mv"} over { ital "qB"} } ,} {}
    where v size 12{v} {} is the component of the velocity perpendicular to B size 12{B} {} for a charged particle with mass m size 12{m} {} and charge q size 12{q} {} .

Conceptual questions

How can the motion of a charged particle be used to distinguish between a magnetic and an electric field?

High-velocity charged particles can damage biological cells and are a component of radiation exposure in a variety of locations ranging from research facilities to natural background. Describe how you could use a magnetic field to shield yourself.

If a cosmic ray proton approaches the Earth from outer space along a line toward the center of the Earth that lies in the plane of the equator, in what direction will it be deflected by the Earth’s magnetic field? What about an electron? A neutron?

What are the signs of the charges on the particles in [link] ?

Diagram showing magnetic field lines into the page. Charges are moving from the bottom to the top of the diagram and thus perpendicular to the field lines. Charge a curves to the left. Charge b moves in a straight line from bottom to top. Charge c curves to the right.

Which of the particles in [link] has the greatest velocity, assuming they have identical charges and masses?

Diagram showing magnetic field lines out of the page. Charge a curves clockwise with a large radius as it moves from the bottom to the top of the diagram. Charge b curves clockwise with a much smaller radius as it moves from lower middle to upper middle of the diagram.

Which of the particles in [link] has the greatest mass, assuming all have identical charges and velocities?

While operating, a high-precision TV monitor is placed on its side during maintenance. The image on the monitor changes color and blurs slightly. Discuss the possible relation of these effects to the Earth’s magnetic field.

Problems&Exercises

If you need additional support for these problems, see More Applications of Magnetism .

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Source:  OpenStax, Basic physics for medical imaging. OpenStax CNX. Feb 17, 2014 Download for free at http://legacy.cnx.org/content/col11630/1.1
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