# Current

## Prerequisites

• Flux (for measuring total current)
• Charge Density (for comparison with current density, and/or as an quantity that contributes to current density)

## In-class Content

• Acting Out Current Density (Kinesthetic Activity - 10 min) Note: Curl is needed for 3 different ideas in this class: Conservative Work, Going from $\vec A$ to $\vec B$, and getting from $\vec B$ to $\vec J$.
• Current Density (lecture) 10 min

## Homework for Static Fields

1. (TotalCurrentGEM55) Calculate surface current density $K$ and volume current density $J$ for a wire carrying total current $I$, from Griffiths E&M book.

A current $I$ flows down a wire of radius $a$.

1. If it is uniformly distributed over the surface, give a formula for the surface current density $\Vec K$.

2. If it is distributed in such a way that the volume current density, $|\Vec J|$, is inversely proportional to the distance from the axis, give a formula for $\Vec J$.

3. For the surface current you found in Problem 1.a, find the magnetic vector potential at a distance $r$ from the center of the wire with length of $2L$.

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