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    I just want to know that if you was given a question like e) do you think that they are asking for the induced emf across the inductor when (ii)the power is going to the inductor and then (iii) when the power is cut off.

    (i) time constant= L/R
    would I use the equation for (ii) Induced voltage= L di/dt
    would I use the equation for (iii) Induced voltage= -L di/dt
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    (Original post by picklesmartyn)
    I just want to know that if you was given a question like e) do you think that they are asking for the induced emf across the inductor when (ii)the power is going to the inductor and then (iii) when the power is cut off.

    (i) time constant= L/R
    would I use the equation for (ii) Induced voltage= L di/dt
    would I use the equation for (iii) Induced voltage= -L di/dt
    You don't need to use the differentials unless you intend to derive the equations from first principles. This is something you would not need to do until undergraduate level on an engineering or physics course.

    It's exactly the same as you did before but be careful with the V and -V terms. i.e. when the switch is closed the inductor will try to oppose the change in it's magnetic field by producing a back emf that tries to reach infinity but then decreases exponentially towards zero.

    When the switch is changed over, the same thing happens but this time the voltage developed across the inductor tries to maintain the magnetic field and increases to -ve infinity but then exponentially decays towards zero.

    The equations are:

    V_{L_{(t)}} = V_S(e^{(\frac{-tL}{R})}) for the t=0 initial connection to the supply.

    V_{L_{(t)}} = -V_S(e^{(\frac{-tL}{R})}) for the t=0 after steady state above achieved when the components are shorted.

    Be careful with the initial conditions because you need to consider what happens a very short instant after t=0 to see what's going on.

    i.e. consider what happens when t \Rightarrow 0 in reverse time.


    EDIT. Typo with the time constant should be L/R. Corrected now. Apologies for the error.
 
 
 
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