Two small charged spheres are 5.0 cm apart. One is charged to +25 nC, the other to −15 nC. A proton is released from rest halfway between the spheres. What is the proton's speed after it has moved 1.0 cm?
Ch 25: The Electric Potential
Knight Calc5th EditionPhysics for Scientists and EngineersISBN: 9780137344796Not the one you use?Change textbook
Chapter 25, Problem 29b
In a semiclassical model of the hydrogen atom, the electron orbits the proton at a distance of 0.053 nm. What is the electron's potential energy?
Verified step by step guidance1
Step 1: Recall the formula for the potential energy of a system of two charges. The potential energy (U) is given by the equation: , where is Coulomb's constant, and are the charges of the proton and electron, and is the distance between them.
Step 2: Identify the values of the constants and variables. The charge of the electron and proton is C and C, respectively. Coulomb's constant is N·m²/C². The distance is given as , which needs to be converted to meters: m.
Step 3: Substitute the known values into the formula. Replace , , , and into the equation: .
Step 4: Simplify the numerator. Multiply the constants and charges in the numerator: . This will yield a negative value because the charges are opposite in sign.
Step 5: Divide the result of the numerator by the denominator. Use the value of in meters to complete the calculation. The final result will give the potential energy in joules (J).

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Coulomb's Law
Coulomb's Law describes the electrostatic force between two charged particles. In the case of the hydrogen atom, it explains the attractive force between the negatively charged electron and the positively charged proton. The potential energy associated with this force can be calculated using the formula U = -k * (e^2 / r), where k is Coulomb's constant, e is the charge of the electron, and r is the distance between the charges.
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Potential Energy in Electric Fields
In the context of electric fields, potential energy represents the work done to move a charge from a reference point to a specific point in the field. For the hydrogen atom, the potential energy of the electron is negative, indicating that work must be done against the electric force to separate the electron from the proton. This negative value reflects the stability of the electron's orbit around the proton.
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Quantum Mechanics and Energy Levels
Quantum mechanics introduces the concept of quantized energy levels for electrons in atoms. While the semiclassical model approximates the electron's orbit, it is important to recognize that in reality, electrons exist in probabilistic clouds rather than fixed orbits. The potential energy calculated in this model is a simplification, but it provides insight into the energy interactions within the atom, which are foundational for understanding atomic structure.
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