AP Physics 2- 11.1 Electric Current- Study Notes- New Syllabus

Electric Current:

Current is the rate at which charge passes through a cross-sectional area of a wire.

\( \text{I} = \dfrac{\Delta \text{Q}}{\Delta t} \)

• Unit: Ampere (A) = Coulomb/second
• If the current is zero in a section of wire, the net motion of charge carriers is also zero, although individual charge carriers still move randomly.

Driving Force (Potential Difference / EMF):

Electric charge moves in a circuit in response to an electric potential difference, also referred to as electromotive force (emf).

Direction of Current:

• Although current is not a vector quantity, it does have a direction.
• The direction of current is defined as the direction in which positive charges would move (conventional current).
• In reality, in metallic conductors, current results from the flow of electrons, which move in the opposite direction to conventional current.

Movement in Conductors:

In metals, charges are carried by free electrons. Electrons drift slowly under the electric field while colliding with atoms.

Drift velocity:

\( \text{v}_d = \dfrac{\text{I}}{n q A} \)

• \( n \) = number density of charge carriers
• \( q \) = charge of each carrier
• \( A \) = cross-sectional area of conductor

Movement in Insulators:

Charges are tightly bound and cannot flow freely. Only polarization (shift of bound charges) occurs.

Movement in Semiconductors:

Both electrons and holes contribute to current. Conductivity depends on temperature and doping.

Movement in Electrolytes (Liquids):

Charges move as positive and negative ions. Example: In NaCl solution, \( \text{Na}^+ \) moves to cathode, \( \text{Cl}^- \) to anode.

Movement in Gases/Plasma:

Ionization of atoms creates electrons and ions that accelerate under the field. Plasma behaves as a conducting medium.