11.1.A—Describe the movement of electric charges through a medium

Describe the movement of electric charges through a medium.

• Current is the rate at which charge passes through a cross-sectional area of a wire. Relevant equation: =Idq dt
  • i. Current within a conductor consists of charge carriers traveling through the conductor with an average drift velocity. Relevant equation: =In qv Ad
  • ii. Electric charge moves in a circuit in response to an electric potential difference, sometimes referred to as electromotive force, or emf ( ).
  • iii. If the current is zero in a section of wire, the net motion of charge carriers in the wire is also zero, although individual charge carriers will not have zero speed.
• Current density is the flow of charge per unit area. Relevant equation:
  • i. Current density is related to the motion of the charge carriers within a conductor. Relevant equation: =Jn qvd
  • ii. Current density is a vector quantity.
  • iii. A potential difference across a conductor creates an electric field within the conductor that is proportional to the resistivity of the conductor and the current density. Relevant equation:
• If a function of current density is given, the total current can be determined by integrating the current density over the area. Derived equation:
• Although current is a scalar quantity, it does have a direction. Because its direction is relative to the current carrier and not space, current does not obey the laws of vector addition and has no vector components.
  • i. The direction of conventional current is chosen to be the direction in which positive charge would move.
  • ii. In common circuits, the current is actually due to the movement of electrons (negative charge carriers).