Electricity and drift velocity.

Discussion in 'Physics & Math' started by ajanta, Apr 16, 2018.

  1. ajanta Registered Senior Member

    In the case of a 12 gauge copper wire carrying 10 amperes of current (typical of home wiring), the individual electrons only move about 0.02 cm per sec or 1.2 inches per minute (in science this is called the drift velocity of the electrons.) And the electric bulb lights up immediately when we turn the switch on......it sets the electrons along the way into motion all along the wire. So it appears as if the electrons are moving very fast, when in fact they don't.

    From wiki
    (u = mσΔV/ρefℓ

    u is again the drift velocity of the electrons, in m⋅s−1.
    m is the molecular mass of the metal, in kg.
    σ is the electric conductivity of the medium at the temperature considered, in S/m.
    ΔV is the voltage applied across the conductor, in V.
    ρ is the density (mass per unit volume) of the conductor, in kg⋅m^−3.
    e is the elementary charge, in C.
    f is the number of free electrons per atom.
    ℓ is the length of the conductor, in m.)

    Now about a step up transformer.....
    Suppose, Secondary winded coil that the coil wire is 3×10^8 m long. Primary coil wire is perfect for 50 Hz AC frequency.
    My questions....

    1.When the output AC frequency is same as input AC frequency?
    2. If input AC frequency is 50 Hz then what is the output frequency, 50 Hz or O Hz?
    Last edited: Apr 16, 2018
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  3. Q-reeus Valued Senior Member

    Regarding 'speed of electricity', and additionally actual conduction charge motions inside a typical metal, here is a cut&paste from a much earlier thread:
    [page number was wrong - rather go from ~ p38 to p45]
    Beyond that, the secondary windings are all immersed in essentially the same E = -dA/dt solenoidal emf field owing to the primary windings harmonically exciting the ferromagnetic core magnetically. Hence there is no lengthy delay as you imagine. No lag somehow preventing an initial following of the primary frequency. Without significant non-linearity, there can be no harmonics hence no issue with a frequency mismatch.
    Last edited: Apr 16, 2018
    ajanta likes this.
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  5. Q-reeus Valued Senior Member

    Forgot to add the following in #2:
    That secondary voltage response faithfully follows primary assumes an open circuit situation where current flow in the secondary is zero or at least negligible.
    But typically the secondary will be coupled to a load that in general will be partly resistive and partly reactive. Additional to the secondary's own inductive reactance and resistance (capacitive reactance being negligible for mains frequency transformer windings). In that case the startup response of the secondary circuit current will be quite different to the steady state situation that fapp may take a few cycles to achieve. See e.g.: https://en.wikipedia.org/wiki/RLC_circuit#Transient_response
    What's shown there is similar to AC driven case except one superposes that shown to a steady-state final sinusoid.
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