To five significant figures, what are the cyclotron frequencies in a 3.0000 T magnetic field of the ions (a) O₂⁺, (b) N₂⁺, and (c) CO⁺? The atomic masses are shown in the table; the mass of the missing electron is less than 0.001 u and is not relevant at this level of precision. Although N₂⁺ and CO⁺ both have a nominal molecular mass of 28, they are easily distinguished by virtue of their slightly different cyclotron frequencies. Use the following constants: 1 u = 1.6605 x 10⁻²⁷ kg, e = 1.6022 x 10⁻¹⁹ C.
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28. Magnetic Fields and Forces
Force on Moving Charges & Right Hand Rule
Problem 28
Textbook Question
What is the force (magnitude and direction) on the proton in FIGURE P31.28? Give the direction as an angle cw or ccw from vertical.


1
Step 1: Identify the forces acting on the proton. The proton experiences two forces: the electric force due to the electric field \( E \) and the magnetic force due to the magnetic field \( B \). The electric force is given by \( F_E = qE \), and the magnetic force is given by \( F_B = qvB \sin \theta \), where \( \theta \) is the angle between the velocity \( \vec{v} \) and the magnetic field \( \vec{B} \).
Step 2: Calculate the electric force. The charge of the proton is \( q = 1.6 \times 10^{-19} \, \text{C} \), and the electric field \( E \) is \( 1.0 \times 10^6 \, \text{V/m} \). Use the formula \( F_E = qE \) to find the magnitude of the electric force.
Step 3: Calculate the magnetic force. The proton's velocity \( v \) is \( 1.0 \times 10^7 \, \text{m/s} \), the magnetic field \( B \) is \( 0.10 \, \text{T} \), and the angle \( \theta \) between \( \vec{v} \) and \( \vec{B} \) is \( 90^\circ \) (since \( \vec{v} \) is horizontal and \( \vec{B} \) is perpendicular to the plane). Use the formula \( F_B = qvB \sin \theta \) to find the magnitude of the magnetic force.
Step 4: Determine the net force. The electric force \( \vec{F_E} \) acts vertically upward, while the magnetic force \( \vec{F_B} \) acts horizontally to the left. Use vector addition to find the magnitude and direction of the net force. The magnitude is \( F_{\text{net}} = \sqrt{F_E^2 + F_B^2} \), and the direction is given by \( \tan \phi = \frac{F_B}{F_E} \), where \( \phi \) is the angle of the net force relative to the vertical.
Step 5: Express the direction of the net force. Convert the angle \( \phi \) into a clockwise (cw) or counterclockwise (ccw) direction from the vertical, depending on the orientation of the forces. The magnetic force is to the left, and the electric force is upward, so the net force will be at an angle \( \phi \) ccw from the vertical.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electric Field (E)
An electric field is a region around a charged particle where other charged particles experience a force. It is quantified in volts per meter (V/m) and indicates the direction a positive test charge would move. In this scenario, the electric field strength is given as 1.0 × 10^6 V/m, which influences the proton's motion.
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Intro to Electric Fields
Magnetic Field (B)
A magnetic field is a vector field that exerts a force on moving charges and magnetic dipoles. It is measured in teslas (T) and can affect the trajectory of charged particles. In this case, the magnetic field strength is 0.10 T, which will interact with the proton's velocity to determine the net force acting on it.
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Magnetic Fields and Magnetic Dipoles
Lorentz Force
The Lorentz force is the combined force exerted on a charged particle due to electric and magnetic fields. It is calculated using the formula F = q(E + v × B), where q is the charge, E is the electric field, v is the velocity, and B is the magnetic field. This force will dictate both the magnitude and direction of the force acting on the proton in the given scenario.
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Lorentz Transformations of Velocity
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