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 29a
In a semiclassical model of the hydrogen atom, the electron orbits the proton at a distance of 0.053 nm. What is the electric potential of the proton at the position of the electron?
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Step 1: Recall the formula for electric potential due to a point charge, which is given by \( V = \frac{k \cdot q}{r} \), where \( k \) is Coulomb's constant (\( 8.99 \times 10^9 \, \text{N·m}^2/\text{C}^2 \)), \( q \) is the charge of the proton (\( +1.6 \times 10^{-19} \, \text{C} \)), and \( r \) is the distance from the charge (\( 0.053 \, \text{nm} \) or \( 0.053 \times 10^{-9} \, \text{m} \)).
Step 2: Convert the distance \( r \) from nanometers to meters. Since \( 1 \, \text{nm} = 10^{-9} \, \text{m} \), multiply \( 0.053 \, \text{nm} \) by \( 10^{-9} \) to express \( r \) in meters.
Step 3: Substitute the values for \( k \), \( q \), and \( r \) into the formula \( V = \frac{k \cdot q}{r} \). Ensure all units are consistent (Coulombs for charge, meters for distance).
Step 4: Simplify the expression by performing the division \( \frac{k \cdot q}{r} \). This will yield the electric potential \( V \) in volts (\( \text{V} \)).
Step 5: Interpret the result. The electric potential represents the work done per unit charge to bring a positive test charge from infinity to the position of the electron in the hydrogen atom.

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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Electric Potential
Electric potential, often referred to as voltage, is the amount of electric potential energy per unit charge at a specific point in an electric field. It indicates how much work would be done to move a positive test charge from a reference point to the point in question. In the context of the hydrogen atom, the electric potential due to the proton at the location of the electron can be calculated using the formula V = k * (q / r), where k is Coulomb's constant, q is the charge of the proton, and r is the distance from the proton to the electron.
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Coulomb's Law
Coulomb's Law describes the force between two charged objects. It states that the force is directly proportional to the product of the magnitudes of the charges and inversely proportional to the square of the distance between them. This law is fundamental in electrostatics and is essential for calculating the electric potential in systems like the hydrogen atom, where the proton and electron interact through their electric charges.
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Hydrogen Atom Model
The semiclassical model of the hydrogen atom combines classical mechanics and quantum mechanics to describe the behavior of the electron in relation to the proton. In this model, the electron is treated as a particle moving in a circular orbit around the proton, influenced by the electrostatic force. Understanding this model is crucial for calculating properties like electric potential, as it provides the framework for analyzing the interactions between the charged particles.
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