Electric Current & Charge Flow — A/L Physics

What must I remember about Electric Current & Charge Flow?

• Electric Current (I): Rate of flow of electric charge across a cross-section: I = dQ/dt = ΔQ/Δt (scalar quantity, SI base unit Ampere = C/s).
• Microscopic Drift Velocity (v_d): Average net velocity acquired by conduction electrons under an electric field: I = n A v_d e, where n is free electron number density (m⁻³), A is cross-sectional area, and e is electron charge.
• Current Density (J): Current per unit cross-sectional area perpendicular to flow: J = I / A = n v_d e (vector quantity directed along electric field E).
• Thermal velocity vs Drift velocity: Thermal random motion of electrons is very high (~10⁵-10⁶ m/s) with zero net displacement; Drift velocity under typical laboratory currents is remarkably slow (~10⁻⁴ m/s = fractions of mm/s); Electrical signals propagate near speed of light c as electromagnetic wavefronts.

Test yourself (1 minute)

1. Conventional Current Direction

   By historical convention, direction of electric current is defined as the direction in which positive charges would flow; conduction electrons (negative) physically drift in the OPPOSITE direction to conventional current.

2. Charge Carriers across Media

   In metallic conductors: Conduction electrons; In electrolytes: Positive and negative ions (cations and anions); In semiconductor crystal lattices: Electrons and holes; In ionized gases/plasmas: Positive ions and free electrons.

3. Vector nature of current

   Although current has an associated direction, it is a SCALAR quantity because it obeys ordinary scalar addition laws (Kirchhoff's Current Law), not vector addition rules.

Practise Electric Current & Charge Flow questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 7: Current Electricity

What comes next?

Next in the syllabus

Where does Electric Current & Charge Flow fit in the syllabus?

Unit 7.1 · Year 2 · Term 1