11.1.2

Mean Drift Velocity

Test yourself

Current Flowing Through a Wire

You need to be able to derive the equation for the current flowing through a wire in terms of its cross-sectional area and the mean drift velocity of electrons flowing through it.

Illustrative background for Electrons in a wireIllustrative background for Electrons in a wire ?? "content

Electrons in a wire

  • If we take a small section of a wire with electrons flowing through it.
  • The wire has a cross-sectional area AA, and length LL.
  • Let nn be equal to the number density of electrons in the wire, which is the number of electrons per unit volume.
  • We can then write the total change in charge as the electrons pass through the section of the wire as:
    • ΔQ=A×L×n×q\Delta Q = A \times L \times n \times q
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Mean drift velocity

  • Substituting this into the equation for current, we have:
    • I=ALnqΔtI = \frac{ALnq}{\Delta t}
  • We can rewrite this as:
    • I=AnqvI = Anqv
  • Where v=LΔtv=\frac{L}{\Delta t} is called the mean drift velocity of the electrons.

Mean Drift Velocity

Mean drift velocity is the average distance travelled by a charge carrier per unit time.

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Movement of charges

  • An electron moving through a wire has a chaotic, zigzag path, but with a migration in a particular direction if a potential difference is applied to the wire.
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Mean drift velocity

  • With a current of a few amps, the average or mean drift velocity of electrons will be of the order of 0.3 mm per second. A high current is possible because there are very many, closely packed charges.

Jump to other topics

1Physical Quantities & Units

2Measurement Techniques

3Kinematics

4Dynamics

5Gravitational Fields

6Deformation of Solids

7Thermal Physics

8Oscillations

9Communication

10Electric Fields

11Current Electricity

12Magnetic Fields

13Modern Physics

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