Ch. 08 - Conservation of Energy

Giancoli Douglas - Physics for Scientists and Engineers 5th edition

Giancoli Douglas5th editionPhysics for Scientists and EngineersISBN: 9780137488179Not the one you use?Change textbook

Giancoli Douglas 5th edition

Ch. 08 - Conservation of Energy

Problem 74a

Chapter 8, Problem 74a

The potential energy of the two atoms in a diatomic (two-atom) molecule can be approximated as (Lennard-Jones potential) U(r) = -(a/r⁶) + (b/r¹²), where r is the distance between the two atoms and a and b are positive constants. At what values of r is U(r) a minimum? A maximum?

Verified step by step guidance

Start by understanding the problem: The Lennard-Jones potential is given as U(r) = -(a/r⁶) + (b/r¹²), where r is the distance between two atoms, and a and b are constants. To find the values of r where U(r) is a minimum or maximum, we need to analyze the derivative of U(r) with respect to r.

Take the first derivative of U(r) with respect to r. Using the power rule for differentiation, the derivative is: dU/dr = 6a/r⁷ - 12b/r¹³.

Set the first derivative equal to zero to find the critical points: 6a/r⁷ - 12b/r¹³ = 0. Simplify this equation to isolate r: 6a/r⁷ = 12b/r¹³. Divide through by 6 and simplify further: a/r⁷ = 2b/r¹³.

Solve for r by multiplying through by r¹³: a * r⁶ = 2b. Then isolate r by dividing both sides by 2b and taking the sixth root: r = (2b/a)^(1/6). This gives the value of r where U(r) is a minimum.

To determine if there is a maximum, take the second derivative of U(r) with respect to r and analyze its sign at the critical points. If the second derivative is positive, the critical point is a minimum; if negative, it is a maximum. For this potential, U(r) has a minimum at r = (2b/a)^(1/6) and no maximum within the physical range of r.

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Key Concepts

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

Lennard-Jones Potential

The Lennard-Jones potential is a mathematical model that describes the interaction between a pair of neutral atoms or molecules. It accounts for both attractive forces, which dominate at longer distances, and repulsive forces, which become significant at very short distances. The potential is expressed as U(r) = -(a/r⁶) + (b/r¹²), where 'a' and 'b' are constants that characterize the strength of these interactions.

Potential Energy Minimum and Maximum

In the context of potential energy, a minimum occurs at a point where the potential energy is lower than at surrounding points, indicating a stable configuration. Conversely, a maximum occurs where the potential energy is higher than at neighboring points, indicating an unstable configuration. To find these points, one typically takes the derivative of the potential energy function and sets it to zero to solve for critical points.

Critical Points and Stability

Critical points in a function occur where the first derivative is zero or undefined, indicating potential minima or maxima. To determine the nature of these critical points, the second derivative test is used: if the second derivative is positive, the point is a minimum (stable), and if negative, it is a maximum (unstable). This analysis is crucial for understanding the stability of molecular configurations in the Lennard-Jones potential.

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Textbook Question

The graph of Fig. 8–43 shows the potential energy curve of a particle moving along the 𝓍 axis under the influence of a conservative force. Note that the total energy E > U(𝓍), so that the particle’s speed is never zero. In which interval(s) of 𝓍 is the force on the particle to the right?

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Textbook Question

The graph of Fig. 8–43 shows the potential energy curve of a particle moving along the 𝓍 axis under the influence of a conservative force. Note that the total energy E > U(𝓍), so that the particle’s speed is never zero. At what value(s) of 𝓍 is the magnitude of the force a minimum?

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