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Ch. 27 - Magnetism
Giancoli Douglas - Physics for Scientists and Engineers 5th edition
Giancoli Douglas5th editionPhysics for Scientists and EngineersISBN: 9780137488179Not the one you use?Change textbook
Chapter 26, Problem 57

One form of mass spectrometer accelerates ions by a voltage V before they enter a magnetic field B. The ions are assumed to start from rest. Show that the mass of an ion is m = qB²R²/2V, where R is the radius of the ions’ path in the magnetic field and q is their charge.

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Start by recognizing that the ions are accelerated from rest by a voltage \( V \), which gives them kinetic energy. The kinetic energy gained by the ion is equal to the work done by the electric field: \( KE = qV \), where \( q \) is the charge of the ion.
The kinetic energy of the ion is also expressed as \( KE = \frac{1}{2}mv^2 \), where \( m \) is the mass of the ion and \( v \) is its velocity. Equating the two expressions for kinetic energy, we get \( qV = \frac{1}{2}mv^2 \). Solve for \( v \): \( v = \sqrt{\frac{2qV}{m}} \).
When the ion enters the magnetic field \( B \), it experiences a centripetal force due to the magnetic force. The magnetic force is given by \( F_B = qvB \), and the centripetal force is \( F_c = \frac{mv^2}{R} \), where \( R \) is the radius of the circular path. Equating these forces, \( qvB = \frac{mv^2}{R} \).
Simplify the equation \( qvB = \frac{mv^2}{R} \) to solve for \( v \): \( v = \frac{qBR}{m} \).
Substitute the expression for \( v \) from step 4 into the equation \( v = \sqrt{\frac{2qV}{m}} \) from step 2. This gives \( \frac{qBR}{m} = \sqrt{\frac{2qV}{m}} \). Square both sides to eliminate the square root: \( \frac{q^2B^2R^2}{m^2} = \frac{2qV}{m} \). Rearrange to solve for \( m \): \( m = \frac{qB^2R^2}{2V} \).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Mass Spectrometry

Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions. In this process, ions are generated, accelerated, and then subjected to magnetic and electric fields, allowing for the determination of their mass. Understanding the principles of mass spectrometry is essential for analyzing the behavior of ions in the given scenario.
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Lorentz Force

The Lorentz force is the force experienced by a charged particle moving through electric and magnetic fields. It is given by the equation F = q(E + v × B), where E is the electric field, v is the velocity of the particle, and B is the magnetic field. This concept is crucial for understanding how ions move in the magnetic field after being accelerated by the voltage.
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Kinetic Energy and Potential Energy

In the context of the mass spectrometer, the kinetic energy gained by the ions after being accelerated by a voltage V is equal to the work done on them, which is given by KE = qV. This kinetic energy is then related to the motion of the ions in the magnetic field, where it can be expressed in terms of the radius of their circular path, leading to the derivation of the mass equation.
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Related Practice
Textbook Question

Suppose the electric field between the electric plates in the mass spectrometer of Fig. 27–34 is 2.84 x 10⁴ V/m and the magnetic fields are B = B'= 0.58 T. The source contains carbon isotopes of mass numbers 12, 13, and 14 from a long-dead piece of a tree. (To estimate atomic masses, multiply by 1.67 x 10⁻²⁷ kg.) Does it matter if the ion charge is positive (lost electrons) or negative (gained electrons)? Explain.

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Textbook Question

\(\What\) is the value of q/m for a particle that moves in a circle of radius 8.0 mm in a 0.46-T magnetic field if a crossed 320-V/m electric field will make the path straight?

1311
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Textbook Question

Suppose the electric field between the electric plates in the mass spectrometer of Fig. 27–34 is 2.84 x 10⁴ V/m and the magnetic fields are B = B'= 0.58 T. The source contains carbon isotopes of mass numbers 12, 13, and 14 from a long-dead piece of a tree. (To estimate atomic masses, multiply by 1.67 x 10⁻²⁷ kg .) How far apart are the marks formed by the singly charged ions of each type on a detector or photographic film? What if the ions were doubly charged?

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Textbook Question

A long copper strip is 3.0 cm wide and thick. When it carries a steady 42-A current in a 0.80-T magnetic field it produces a 6.5-μV Hall emf. Determine:

(a) the Hall field in the conductor;

(b) the drift speed of the conduction electrons;

(c) the density of free electrons in the metal.

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Textbook Question

In a mass spectrometer, germanium atoms have radii of curvature equal to 21.0, 21.6, 21.9, 22.2, and 22.8 cm. The largest radius corresponds to an atomic mass of 76 u. What are the atomic masses of the other isotopes?

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Textbook Question

A mass spectrometer is monitoring air pollutants. It is difficult, however, to separate molecules of nearly equal mass such as CO (28.0106 u) and N₂ (28.0134 u). How large a radius of curvature must a spectrometer have (Fig. 27–34) if these two molecules are to be separated on the detector by 0.50 mm?

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