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.