A parallel-plate, air-filled capacitor is being charged as in Fig. 29.23. The circular plates have radius 4.00 cm, and at a particular instant the conduction current in the wires is 0.520 A. (a) What is the displacement current density jD in the air space between the plates?
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32. Electromagnetic Waves
Intro to Electromagnetic (EM) Waves
Problem 20
Textbook Question
The diameter of the nucleus is about 10 fm. What is the kinetic energy, in MeV, of a proton with a de Broglie wavelength of 10 fm?

1
Step 1: Understand the relationship between the de Broglie wavelength and the momentum of a particle. The de Broglie wavelength (λ) is given by the formula: , where is Planck's constant and is the momentum of the particle.
Step 2: Rearrange the formula to solve for the momentum : . Substitute the values for Planck's constant () and the de Broglie wavelength (, where ).
Step 3: Convert the momentum into kinetic energy using the relativistic energy-momentum relation. For non-relativistic speeds, the kinetic energy can be approximated as: , where is the mass of the proton ().
Step 4: Substitute the calculated momentum and the mass of the proton into the kinetic energy formula. Ensure all units are consistent (e.g., converting to MeV/c if necessary).
Step 5: Simplify the expression to find the kinetic energy in MeV. This will give the final result for the proton's kinetic energy based on its de Broglie wavelength.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
De Broglie Wavelength
The de Broglie wavelength is a fundamental concept in quantum mechanics that relates the wavelength of a particle to its momentum. It is given by the formula λ = h/p, where λ is the wavelength, h is Planck's constant, and p is the momentum of the particle. This concept illustrates the wave-particle duality of matter, indicating that particles like protons exhibit wave-like properties at small scales.
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Kinetic Energy of a Particle
Kinetic energy is the energy that an object possesses due to its motion, calculated using the formula KE = (1/2)mv², where m is the mass and v is the velocity of the object. In quantum mechanics, the kinetic energy of a particle can also be expressed in terms of its momentum, using the relation KE = p²/(2m). Understanding this relationship is crucial for calculating the kinetic energy of particles at the atomic scale.
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Nuclear Scale and Units
The nuclear scale refers to dimensions on the order of femtometers (fm), which are used to measure atomic nuclei. One femtometer is 10^-15 meters, a scale where quantum effects dominate. In nuclear physics, energy is often expressed in mega-electronvolts (MeV), a unit that reflects the energy of particles at this scale. Recognizing these units is essential for converting and interpreting results in nuclear physics problems.
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