Math Problem Statement

An infinite conducting cylindrical shell has radius 0.15 m and surface charge density 1.1 μC/m2. What is the magnitude of the electric field, in newtons per coulomb, 1.1 m from the axis of the cylinder? Keep in mind: Make use of cylindrical symmetry. Can you determine the total charge per unit length of the cylinder? The surface area of a cylinder (excluding the ends) is 2πrL.

Solution

Let's break the problem down step by step.

Step 1: Understanding the setup

  • We have an infinite conducting cylindrical shell.
  • The shell has a radius of 0.15 m.
  • The surface charge density is given as σ=1.1μC/m2\sigma = 1.1 \, \mu C/m^2.
  • The electric field is to be calculated at a distance of 1.1 m from the axis of the cylinder.

Step 2: Total charge per unit length on the cylindrical shell

The total charge per unit length, λ\lambda, on the cylindrical shell can be determined by multiplying the surface charge density σ\sigma by the surface area of the cylindrical shell per unit length (assuming length LL = 1 meter). The surface area per unit length of a cylindrical shell is:

A=2πrL=2π×0.15m×1m=0.942m2A = 2 \pi r L = 2 \pi \times 0.15 \, \text{m} \times 1 \, \text{m} = 0.942 \, \text{m}^2

Now, the charge per unit length λ\lambda is:

λ=σ×A=1.1μC/m2×0.942m2=1.0362μC/m\lambda = \sigma \times A = 1.1 \, \mu C/m^2 \times 0.942 \, \text{m}^2 = 1.0362 \, \mu C/m

So, the charge per unit length is:

λ=1.0362μC/m=1.0362×106C/m\lambda = 1.0362 \, \mu C/m = 1.0362 \times 10^{-6} \, C/m

Step 3: Electric field at a distance of 1.1 m from the axis

Using Gauss's Law, we can find the electric field at a distance r=1.1mr = 1.1 \, \text{m} from the axis of the cylindrical shell. Because of cylindrical symmetry, the electric field will be radial and depend only on the distance from the axis.

Gauss's law for a cylinder states that:

ΦE=EdA=Qencϵ0\Phi_E = \oint \vec{E} \cdot d\vec{A} = \frac{Q_{\text{enc}}}{\epsilon_0}

Where:

  • ΦE\Phi_E is the electric flux.
  • QencQ_{\text{enc}} is the charge enclosed by the Gaussian surface.
  • ϵ0\epsilon_0 is the permittivity of free space (8.85×1012C2/Nm28.85 \times 10^{-12} \, C^2/N \cdot m^2).

For a cylindrical Gaussian surface with radius r=1.1mr = 1.1 \, \text{m} and length L=1mL = 1 \, \text{m}, the electric flux is:

ΦE=EA=E(2πrL)\Phi_E = E \cdot A = E \cdot (2 \pi r L)

The enclosed charge is the charge per unit length times the length of the Gaussian surface:

Qenc=λLQ_{\text{enc}} = \lambda L

Thus, Gauss's law becomes:

E(2πrL)=λLϵ0E \cdot (2 \pi r L) = \frac{\lambda L}{\epsilon_0}

Simplifying for EE:

E=λ2πϵ0rE = \frac{\lambda}{2 \pi \epsilon_0 r}

Substitute the known values:

  • λ=1.0362×106C/m\lambda = 1.0362 \times 10^{-6} \, C/m
  • r=1.1mr = 1.1 \, \text{m}
  • ϵ0=8.85×1012C2/Nm2\epsilon_0 = 8.85 \times 10^{-12} \, C^2/N \cdot m^2

E=1.0362×1062π(8.85×1012)(1.1)E = \frac{1.0362 \times 10^{-6}}{2 \pi (8.85 \times 10^{-12}) (1.1)}

Now let's calculate the electric field:

E1.0362×1066.85×101115,126N/CE \approx \frac{1.0362 \times 10^{-6}}{6.85 \times 10^{-11}} \approx 15,126 \, \text{N/C}

Final Answer:

The magnitude of the electric field at a distance of 1.1 m from the axis of the conducting cylindrical shell is approximately:

E15,126N/CE \approx 15,126 \, \text{N/C}

Do you want any additional details or have other questions about this? Here are some related topics to explore:

  1. How does the electric field change if the distance from the cylinder is less than the radius of the shell?
  2. How do the electric fields inside and outside a conducting cylindrical shell compare?
  3. What would happen to the electric field if the shell was not conducting?
  4. What would happen if the shell had a different surface charge density?
  5. Can you apply Gauss’s Law to other symmetrical charge distributions like spherical or planar symmetry?

Tip: When using Gauss’s Law for cylindrical symmetry, always choose a Gaussian surface that matches the symmetry of the problem, such as a co-axial cylinder.

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Math Problem Analysis

Mathematical Concepts

Electrostatics
Gauss's Law
Cylindrical Symmetry

Formulas

Surface Area of Cylinder: A = 2πrL
Charge per Unit Length: λ = σ * A
Electric Field using Gauss's Law: E = λ / (2π * ε₀ * r)

Theorems

Gauss's Law

Suitable Grade Level

College/University Physics