IB DP Physics: SL复习笔记5.1.2 Drift Speed

Drift Speed

  • In conductors, only negatively charged (delocalised) electrons are allowed to move between atoms
    • In general, an electric current can arise from the flow of either positive or negative particles
  • Charged particles moving through a material or through vacuum are known as charge carriers


Charge carriers drift towards the positive terminal of the conductor. Conventional current flows in the opposite direction

Movement in a Conductor

  • Delocalised electrons are the charge carriers in a conductor
    • These electrons normally move randomly through the conductor



Random path of a delocalised electron through a length of conductor

  • If a potential difference is applied between two points in the conductor, an electric field is created
  • As a consequence:
    • An electric force will act on the charge carriers
    • The charge carriers will drift along the conductor
    • A steady average current will flow through the conductor

Electric Current & Drift Speed

  • The average speed at which the charge carriers move through a conductor is known as the drift speed
  • The value of the drift speed is usually very small
    • For most everyday situations, v ∼ 10–4 m s–1
  • The electric current arising from the movement of a given number of charge carriers through a conductor is calculated as follows:

I = nAvq

  • Where:
    • n = number of charge carriers per unit volume, i.e. charge density (m–3)
    • A = cross-sectional area of the conductor (m2)
    • v = average drift speed of the charge carriers (m s–1)
    • q = charge of the charge carriers (C)

Worked Example

A number N of charge carriers, each with a charge q, moves through a conductor of length L and cross-sectional area A. Show that the electric current flowing through the conductor is given by:

I = nAvq

Where n is the number of charge carriers per unit volume and v is their drift speed.