Charge and Current - A Level Physics

Charge and Current

Definition of Electric Current

  • Electric Current (I)
    • Defined as the rate of flow of charge
    • Formula: I = \frac{dQ}{dt}
    • SI unit: Amperes (A)
    • Measured using an ammeter, placed in series in the circuit.

Charge and Coulombs

  • Charge (Q)

    • A physical quantity, can be positive or negative.
    • Measured in coulombs (C).
    • 1 coulomb is defined as the charge that flows in 1 second with a current of 1 ampere.
    • SI base units: C = As
    • Like charges repel; opposite charges attract.

  • Charge of Subatomic Particles:

    • Protons have a charge of +1, electrons have a charge of -1.
    • Represents multiples of the elementary charge (e), where e = 1.6 \times 10^{-19} C.

  • Net Charge Calculation:

    • Given by: Q = \pm ne
    • Integer n is the number of electrons added or removed.
    • Neutral atoms have equal numbers of protons and electrons.

Charge Carriers

  • Electric current carried in different materials:

    • In metals, current is carried by electrons.
    • Positive metal ions are fixed; electrons move freely.
    • Current results from electrons moving towards a positive terminal when voltage is applied.

  • In liquids:

    • Conducting liquids called electrolytes (e.g., saltwater).
    • Ions in these solutions (e.g., Na^+ and Cl^-) carry charge.

Conventional Current

  • Defined before the discovery of electrons; treats current as flowing from positive to negative terminals.
  • Direction of Electron Flow:
    • Electrons actually flow from negative to positive, opposite of conventional current.

Kirchhoff’s First Law

  • At any point in a circuit, the sum of currents entering equals the sum of currents leaving that point, adhering to conservation of charge.

Mean Drift Velocity

  • Describes the average speed of electrons as they collide while moving through the metal.
  • Definition of Mean Drift Velocity (v): Average velocity of electrons as they travel through a wire.
  • Number Density (n): Number of free electrons per unit volume.
    • Conductors like metals have a high number density (around 10^{28} m^{-3}); insulators like plastics have lower values.

Current Equations

  • Relation of current (I) to charge (Q):
    • I = nAve
    • where A = cross-sectional area of the wire, V = volume, V = AL (length).
  • Rearranged for Mean Drift Velocity:
    • I = nA(\frac{L}{t})
    • Where L is the distance traveled, and t is the time taken.
  • The equation can be used to derive mean drift velocity given current, area, and electron density.