# 22.11 More applications of magnetism

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## Learning objectives

By the end of this section, you will be able to:

• Describe some applications of magnetism.

## Mass spectrometry

The curved paths followed by charged particles in magnetic fields can be put to use. A charged particle moving perpendicular to a magnetic field travels in a circular path having a radius $r$ .

$r=\frac{\text{mv}}{\text{qB}}$

It was noted that this relationship could be used to measure the mass of charged particles such as ions. A mass spectrometer is a device that measures such masses. Most mass spectrometers use magnetic fields for this purpose, although some of them have extremely sophisticated designs. Since there are five variables in the relationship, there are many possibilities. However, if $v$ , $q$ , and $B$ can be fixed, then the radius of the path $r$ is simply proportional to the mass $m$ of the charged particle. Let us examine one such mass spectrometer that has a relatively simple design. (See [link] .) The process begins with an ion source, a device like an electron gun. The ion source gives ions their charge, accelerates them to some velocity $v$ , and directs a beam of them into the next stage of the spectrometer. This next region is a velocity selector that only allows particles with a particular value of $v$ to get through.

The velocity selector has both an electric field and a magnetic field, perpendicular to one another, producing forces in opposite directions on the ions. Only those ions for which the forces balance travel in a straight line into the next region. If the forces balance, then the electric force $F=\text{qE}$ equals the magnetic force $F=\text{qvB}$ , so that $\text{qE}=\text{qvB}$ . Noting that $q$ cancels, we see that

$v=\frac{E}{B}$

is the velocity particles must have to make it through the velocity selector, and further, that $v$ can be selected by varying $E$ and $B$ . In the final region, there is only a uniform magnetic field, and so the charged particles move in circular arcs with radii proportional to particle mass. The paths also depend on charge $q$ , but since $q$ is in multiples of electron charges, it is easy to determine and to discriminate between ions in different charge states.

Mass spectrometry today is used extensively in chemistry and biology laboratories to identify chemical and biological substances according to their mass-to-charge ratios. In medicine, mass spectrometers are used to measure the concentration of isotopes used as tracers. Usually, biological molecules such as proteins are very large, so they are broken down into smaller fragments before analyzing. Recently, large virus particles have been analyzed as a whole on mass spectrometers. Sometimes a gas chromatograph or high-performance liquid chromatograph provides an initial separation of the large molecules, which are then input into the mass spectrometer.

## Cathode ray tubes—crts—and the like

What do non-flat-screen TVs, old computer monitors, x-ray machines, and the 2-mile-long Stanford Linear Accelerator have in common? All of them accelerate electrons, making them different versions of the electron gun. Many of these devices use magnetic fields to steer the accelerated electrons. [link] shows the construction of the type of cathode ray tube (CRT) found in some TVs, oscilloscopes, and old computer monitors. Two pairs of coils are used to steer the electrons, one vertically and the other horizontally, to their desired destination.

#### Questions & Answers

A weather vane is some sort of directional arrow parallel to the ground that may rotate freely in a horizontal plane. A typical weather vane has a large cross-sectional area perpendicular to the direction the arrow is pointing, like a “One Way” street sign. The purpose of the weather vane is to indicate the direction of the wind. As wind blows pa
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If a prism is fully imersed in water then the ray of light will normally dispersed or their is any difference?
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the same behavior thru the prism out or in water bud abbot
Ju
If this will experimented with a hollow(vaccum) prism in water then what will be result ?
Anurag
What was the previous far point of a patient who had laser correction that reduced the power of her eye by 7.00 D, producing a normal distant vision power of 50.0 D for her?
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What is the far point of a person whose eyes have a relaxed power of 50.5 D?
Jaydie
What is the far point of a person whose eyes have a relaxed power of 50.5 D?
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A young woman with normal distant vision has a 10.0% ability to accommodate (that is, increase) the power of her eyes. What is the closest object she can see clearly?
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29/20 ? maybes
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In what ways does physics affect the society both positively or negatively
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how can I read physics...am finding it difficult to understand...pls help
rerry Reply
try to read several books on phy don't just rely one. some authors explain better than other.
Ju
And don't forget to check out YouTube videos on the subject. Videos offer a different visual way to learn easier.
Ju
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I have a exam on 12 february
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what is velocity
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the speed of something in a given direction.
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what is a magnitude in physics
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What is meant by dielectric charge?
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what happens to the size of charge if the dielectric is changed?
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omega= omega not +alpha t derivation
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u have to derivate it respected to time ...and as w is the angular velocity uu will relace it with "thita × time""
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define variable velocity
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displacement in easy way.
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