AP Physics C Electricity and Magnetism

Electric Circuits - Movement of Electric Charges

Current

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

  • Equation: I = dq/dt

• Charge Carriers: Current consists of charge carriers traveling through a conductor with an average drift velocity.

  • Equation: I = n q v A

• Electric Potential Difference: Charge moves in response to an electric potential difference (emf, ε).

• Zero Current: In a zero-current section, net motion of charge carriers is zero, but individual carriers still possess velocity.

Current Density

• Definition: Flow of charge per unit area.

  • Equation: J = I/A

• Relation to Charge Carriers: Current density relates to the motion of charge carriers.

  • Equation: J = n * q * v_d

• Vector Quantity: Current density is a vector and involves electric field and resistivity.

  • Equation: E = ρ * J

Total Current

• Integration of Current Density: Total current can be determined by integrating current density over an area.

  • Equation: I = ∫(J·dA)

• Direction of Current: Current is a scalar but has direction, chosen as the movement direction of positive charge.

  • Note: Actual current often results from electron movement (negative charge carriers).

Behavior of Circuits

Learning Objective 11.2.A

• Describe the behavior of a circuit.

Circuit Composition

• Components: Circuits are composed of wires, batteries, resistors, lightbulbs, capacitors, inductors, switches, ammeters, and voltmeters.

• Electrical Loops: A closed electrical loop allows charge flow.

  • Closed Circuit: Charges can flow.

  • Open Circuit: No charge flow.

  • Short Circuit: Charge flows without potential difference change.

Circuit Schematics

• Definition: Representations used to describe and analyze electric circuits.

• Dependence on Arrangement: Circuit properties depend on the arrangement of components.

• Common Symbols: Circuit elements have standardized symbols; variable elements are marked with a diagonal strikethrough.

  • Elements: Battery, Bulb, Switch, Capacitor, Resistor, Ammeter, Voltmeter, Inductor.

Suggested Skills

1. Create diagrams, tables, charts, or schematics for physical situations.

2. Derive expressions from known quantities logically.

3. Apply appropriate laws and definitions to make claims.