Magnet Field of a Solenoid: Understanding the Derivation Step by Step

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SUMMARY

The forum discussion focuses on the derivation of the magnetic field of a solenoid, specifically addressing the relationship between the differential length element (dl) and the angle (θ) in the context of a right triangle. The user struggles with the equation dl = ρ/sinθ dθ, questioning its validity compared to their own derivation using cosine. The resolution highlights that ρ is not a constant, while h remains constant, leading to a differentiation of cotθ = l/h. This clarification resolves the user's confusion regarding the derivation step.

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  • Understanding of basic trigonometry, particularly sine and cosine functions.
  • Familiarity with the concept of differential calculus.
  • Knowledge of magnetic fields and solenoids in physics.
  • Ability to interpret geometric relationships in right triangles.
NEXT STEPS
  • Review the derivation of magnetic fields in solenoids using Ampère's Law.
  • Study the application of differential calculus in physics problems.
  • Explore the relationship between angles and sides in right triangles.
  • Learn about the implications of variable versus constant parameters in calculus.
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Students of physics, particularly those studying electromagnetism, as well as educators and anyone looking to deepen their understanding of the mathematical derivations involved in magnetic field calculations.

zezima1
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Homework Statement


Attached file shows the problem statement.

The Attempt at a Solution


I can do this problem easily. Problem is that there's just one step in my book's derivation that I don't understand. It sets ρ equal to the hypotenuse in the triangle and then says that:

dl = ρ/sinθ dθ (1)

For some reason I can't get this result. If we look at the cosine of θ then:
cosθ = l/ρ
=>
l = ρcosθ
=>
dl/dθ = -ρsinθ
=>dl = -ρsinθdθ

Incosistent with (1). What am I doing wrong?
 
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hi zezima1! :smile:
zezima1 said:
… It sets ρ equal to the hypotenuse in the triangle and then says that:

dl = ρ/sinθ dθ (1)

For some reason I can't get this result. If we look at the cosine of θ then:
cosθ = l/ρ
=>
l = ρcosθ
=>
dl/dθ = -ρsinθ
=>dl = -ρsinθdθ

ah, but ρ isn't constant, h is …

differentiate cotθ = l/h :wink:
 

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