# 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. σ+λ/(2πa)
2. σ-λ/(2πa)
3. σ+λ/(2πb)
4. σ-λ/(2πb)
5. None of the above

### Commentary:

(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.