Trying to solve for B in that equation

Question

I am trying to solve for the variable B in that long expression. It is giving me an error that it cannot convert to float. I am not really sure what that exactly means and what I need to do in order to fix that and get a value for B. I am very new to Python so any help would be appreciated.

Code:

import matplotlib.pyplot as plt
import numpy as np
import math
from sympy.solvers import solve
from sympy import Symbol 

W = 1*(10**(-4)) 
mu = 300  
Cox = 6.906*(10**-6)
Leff = 1*(10**(-4))
Vg1 = 0.5
Vg2 = 1.0
Vg3 = 1.5
Vg4 = 2.0
Vth = 0.0259
tsi = 5 * (10 ** -7)
Vg = [0.5, 1, 1.5, 2]
Esi = 11.7
tox = 1.5 * (10 ** -7)
tsi = 5 * (10 ** -7)
ni = 1.5 * (10 ** 10)
q = 1.602 * (10 ** -19)

B = Symbol('B')

solve((Vg1/2*Vth) - math.log((2/tsi)*(np.sqrt((2*Esi*Vth)/(ni * q)))) - 
math.log(B) + math.log(np.cos(B)) - 2*11.7*(tox)/(3.9 * tsi), B)

Error:

TypeError                                 Traceback (most recent call last)
<ipython-input-3-003cda8dedbd> in <module>()
 18 B = Symbol('B')
 19 
 ---> 20 solve((Vg1/2*Vth) - math.log((2/tsi)*(math.sqrt((2*Esi*Vth)/(ni * 
  q)))) - math.log(B) + math.log(math.cos(B)) - 2*11.7*(tox)/(3.9 * tsi), B)

~\Anaconda3\lib\site-packages\sympy\core\expr.py in __float__(self)
237         if result.is_number and result.as_real_imag()[1]:
238             raise TypeError("can't convert complex to float")
 --> 239         raise TypeError("can't convert expression to float")
240 
241     def __complex__(self):

TypeError: can't convert expression to float

Check out this post https://stackoverflow.com/questions/22555056/typeerror-cant-convert-expression-to-float — Stephen Strosko, May 24 '18 at 17:00
You are trying to use symbolic values with regular mathematical functions. Add `import sympy` at the top of the file and replace `math.log(B)` with `sympy.log(B)` and `math.log(np.cos(B))` with `sympy.log(sympy.cos(B))`. — jdehesa, May 24 '18 at 17:02
I tried using sympy.log(B) but it just kept loading and it will not compute any output. — Aashiv Patel, May 24 '18 at 17:09

Facorazza · Answer 1 · 2018-05-24T17:57:44.493

0

As suggested by @jdehesa use sympy expressions:

from sympy.solvers import nsolve
from sympy import Symbol, log, sqrt, cos

W = 1*(10**(-4)) 
mu = 300  
Cox = 6.906*(10**-6)
Leff = 1*(10**(-4))
Vg1 = 0.5
Vg2 = 1.0
Vg3 = 1.5
Vg4 = 2.0
Vth = 0.0259
tsi = 5 * (10 ** -7)
Vg = [0.5, 1, 1.5, 2]
Esi = 11.7
tox = 1.5 * (10 ** -7)
tsi = 5 * (10 ** -7)
ni = 1.5 * (10 ** 10)
q = 1.602 * (10 ** -19)

B = Symbol('B')

nsolve((Vg1/2*Vth) - sympy.log((2/tsi)*(sympy.sqrt((2*Esi*Vth)/(ni * q)))) - sympy.log(B) + sympy.log(sympy.cos(B)) - 2*11.7*(tox)/(3.9 * tsi), 1)

The expression you are trying to solve is nonlinear so you should try to solve it numerically with nsolve.

Note: The second parameter is a guess of what the result should be close to.

With 1 I get a complex result of:

-17910.2176436326 - 37.1544916945789*I

edited May 24 '18 at 17:57

answered May 24 '18 at 17:21

Facorazza

317
1
15

I understand but I left it on for like 5 minutes and still nothing. – Aashiv Patel May 24 '18 at 17:25
I've updated the answer – Facorazza May 24 '18 at 18:01
Okay, but now how do I use that value to compute another answers? Because I had assigned B for that. – Aashiv Patel May 24 '18 at 18:22
I don't think I understand what you're trying to accomplish then – Facorazza May 24 '18 at 19:18
Okay, so I need to solve for B in that expression as a normal value, it should not be imaginary. Because I need to use that value of B to calculate some other quantities. – Aashiv Patel May 24 '18 at 19:30
The value of B should be between 0 and pi/2 or atleast a real number if that equation was solved correctly – Aashiv Patel May 24 '18 at 19:33

Trying to solve for B in that equation

1 Answers1