Engineering Calculate the bonding energy of two ions

AI Thread Summary
The discussion focuses on calculating the bonding energy of two ions by differentiating the energy equation and setting it to zero. The user initially attempts to solve for the distance r but is unsure about handling the exponent correctly. They arrive at the equation r^{n-3} = Bn/A and question if r can be expressed as the nth root of that fraction. A response indicates an error in the derivative and suggests rewriting the energy equation for clarity, leading to a corrected differentiation approach. The conversation emphasizes the importance of accurate differentiation in solving the bonding energy problem.
Jaccobtw
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Homework Statement
$$E_N = \frac{-A}{r} + \frac{B}{r^{n}}$$

Calculate the bonding energy ##E_0## in terms of the parameters A, B, and n using
the following procedure:
1. Differentiate ##E_N## with respect to r, and then set the resulting expression equal
to zero, because the curve of ##E_N## versus r is a minimum at ##E_0##.
2. Solve for r in terms of A, B, and n, which yields ##r_0##, the equilibrium interionic
spacing.
3. Determine the expression for ##E_0## by substituting ##r_0## for r
Relevant Equations
$$E_N = \frac{-A}{r} + \frac{B}{r^{n}}$$
1.) So first I differentiate and set it equal to 0 and get:
$$\frac{A}{r^2} -\frac{Bn}{r^{n-1}} = 0$$

2.) When solving for r, I'm not quite sure how to take away the exponent so I get up to the second to last step:

$$r^{n-3} = \frac{Bn}{A}$$

Would it be:

$$r = \sqrt[n-3]{\frac{Bn}{A}}$$

?

Am I doing this problem correctly?

Thank you
 
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Correct. You can note the result as ##r_0##, i.e.,
$$
r_0 = \left( \frac{Bn}{A} \right)^{1/(n-3)}
$$
 
Jaccobtw said:
$$E_N = \frac{-A}{r} + \frac{B}{r^{n}}$$
1.) So first I differentiate and set it equal to 0 and get:
$$\frac{A}{r^2} -\frac{Bn}{r^{n-1}} = 0$$
2.) When solving for r, I'm not quite sure how to take away the exponent so I get up to the second to last step:
$$r^{n-3} = \frac{Bn}{A}$$
Would it be:
$$r = \sqrt[n-3]{\frac{Bn}{A}}\ \ \ ?$$
Am I doing this problem correctly?

Thank you
You made an error in taking the derivative.

Writing ##E_N## as ##\displaystyle \quad \quad E_N= -A\,r^{-1} + B\,r^{-n} \quad## may help.

Then ##\displaystyle \quad \quad \dfrac{E_N}{dr}= A\,r^{-2} - B\,r^{-n-1} = A\,r^{-2} - B\,r^{-(n+1)}##

##\displaystyle \quad \quad \quad \quad \quad \quad = \dfrac{A}{r^2} - \dfrac{nB}{r^{(n+1)}} \quad##
 
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