Velocity of Free Electrons in the Wire

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    11/28/13 schoolphysics ::Welcome::

    www.schoolphysics.co.uk/age16-19/Electricity and magnetism/Current electricity/text/Free_electron_motion/index.html

    Velocity of free electrons in a wire

    The free electrons in a metal have three distinct velocities associated with them:

    (a) a random velocity ( about 105ms-1)

    (b) a velocity with which electrical energy is transferred along the wire (about 108ms-1)

    (c) a drift velocity of the electrons as a whole when a current flows through the wire (this

    depends on the applied voltage but is usually a few mms-1for currents of a few amps in normal

    connecting leads).

    The diagram in Figure 1 shows a simplified and enlargedview of a section of a wire carrying a

    current. The electrons are in random motion but if a potential difference is applied across the wire

    with the right hand end positive the free electrons drift slowly towards that end.

    It is possible to measure the electron drift velocity (v) using the experiment outlined in the

    following Student Investigation.

    Electron drift velocity

    You can work out an equation for the electron drift velocity as follows:

    Consider a wire of cross sectional areaA and carrying a current I amps. Let the number of free

    electrons per unit volume be n and the drift velocity be v. (See Figure 3).

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    11/28/13 schoolphysics ::Welcome::

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    In one second an electron will have moved a distance v down the wire but since there are n

    electrons per unit volumethe total number moving through this distance will be nAv. Therefore

    since the charge on an electron is e the current I (which is the charge moving past any point in

    the wire) is:

    The table below shows some free electron concentrations

    The electron steeplechase

    When a current flows round a series circuit

    the current at any point in the circuit is the

    same. The same number of electrons flow

    past any point in the circuit every second -

    no electrons are lost. Although the number

    of electrons is always the same their energy

    gets less as they move round the circuit.

    This energy appears as heat, light or

    magnetism in say an electrical heater, a

    light bulb or an electromagnet.

    You can compare this energy loss with the

    changein energy of runners in a

    steeplechase. The energy loss of the

    athletes when going over the barriers represents the energy that electrons transfer when they

    pass through a resistor.

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    Keith Gibbs 2008

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