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Homework Help: What does the symbol d mean?

  1. Mar 18, 2007 #1
    What does the symbol "d" mean??

    I've seen the symbol "d" many times and in a physics lecture. For example: the professor would write: [tex] I = \frac {dq}{dt} [/tex] instead of: [tex] I = \frac {Q}{t}[/tex] I think I've also seen it in calculus equations such as derivatives and such. I'm only in Pre-calculus so I haven't not yet gone over anything related to calculus. But I'm interested in finding out what the "d" means in the formulas. Thanks. :smile:
     
  2. jcsd
  3. Mar 18, 2007 #2

    mjsd

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    "d" means change... in the limit that the change is infinitestimal. in your particular exapmle: [tex]I=Q/t[/tex] really means "average" current because that's total change in charge over total change in time: in fact it means
    [tex]I_{av.}=\frac{\delta Q}{\delta t}=\frac{Q_f-Q_i}{t_f-t_i}[/tex], now when in the limit of very small change.... ie. [tex]\delta t \rightarrow 0[/tex] this becomes [tex]\frac{dQ}{dt}[/tex], the advantage of this quantity is that you can now specify "I" at any instance.

    anyway, calculus means two things in essence: chop things up into small bits or adding small bits togeter.
     
  4. Mar 18, 2007 #3
    Oh ok, I knew it was related to delta [tex] \Delta [/tex] thanks!
     
  5. Mar 18, 2007 #4

    cepheid

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    Yikes, this is not the best way of learning what dq/dt means.

    Note that an "infinitesimal change" as a quantity is NOT a well-defined term mathematically. Mjsd's comment about calculus is good as a conceptual way to look at things only. Therefore, although physicists do it all the time, dq and dt really shouldn't be treated as quantities, and dq/dt shouldn't be treated as a ratio. It is the limit of a sequence of such ratios:

    [tex] \frac{dq}{dt} = \lim_{\Delta t \to 0} \frac{\Delta q}{\Delta t} = \lim_{\Delta t \to 0} \frac{q(t + \Delta t) - q(t)}{(t + \Delta t) - t} [/tex]​

    To reiterate: the derivative of the function q(t) is given by the limit as [itex] \Delta t \rightarrow 0 [/itex] of the above sequence of ratios. A limit IS a well-defined concept in mathematics, and you will learn what it means when you take calculus. It is used to define a derivative rigorously and formally. As a result, d shouldn't be thought of as a symbol, if you want to be mathematically proper. Instead, [tex] \frac{d}{dt} [/tex] should be thought of as a symbol that represents the operation of differentiation. When this d/dt acts on a function, the operation of differentiation with respect to time is carried out on that function to produce the first dervative of the function with respect to time.

    [tex] \frac{d}{dt}q(t) = i(t) [/tex]​

    In this example, the derivative of the function, denoted by dq/dt, represents the instantaneous rate of change of q(t) (i.e. the instantaneous current, as opposed to the average current over some finite time interval).
     
    Last edited: Mar 18, 2007
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