Tag Archives: Induced Charge

A2L Item 206

Goal: Reason regarding induced charges and fields

Source: 283-235 Induced charge in conductors

A positive charge per unit area σ is placed on a cylindrical
conductor of inner radius, r = a, and outer radius, r = b. A positive
charge per unit length, λ, is placed along the axis of the
cylinder. What is the charge density (charge per area) on the outer
surface?

  1. +λ
  2. +σ
  3. -λ
  4. -σ
  5. σ+λ/(2πa)
  6. σ-λ/(2πa)
  7. σ+λ/(2πb)
  8. σ-λ/(2πb)
  9. None of the above

Commentary:

Answer

(7) The biggest problem for students is finding the induced surface
charge density due to the line charge density.

A2L Item 204

Goal: Hone the concept of a conductor

Source: 283-225, Charge induced in a neutral conductor

A positive charge +Q is placed outside a neutral conductor. Inside the
conductor is a cavity containing no charge. What is the net charge on
the surface bounding the cavity?

  1. A positive charge +Q
  2. A positive charge +q < +Q
  3. Zero charge
  4. A negative charge |-q| < |+Q|
  5. A negative charge -Q
  6. Cannot determine the charge.

Commentary:

Answer

(3) Students have difficulty envisioning induced charge distributions,
especially when the conductor has an irregular shape.