It's only remedial if you SHOULD know the answer but don't.
Otherwise it's educational.

To find the answer, use the ideal opamp assumptions:
1.  No current will flow into either terminal of the opamp.
2.  The opamp will slew its output in an attempt to minimize
the voltage difference between the two input terminals.

OR, if you prefer:
1.  A virtual open exists for the input terminals (no current
flow).
2.  A virtual short exists for the input terminals (the voltage
at both terminals is identical).

Put a voltage at the "open" input in your drawing, and then
analyze the two circuits and see what the output will do.
After you've done that, you'll know more about opamps
than you do now.

Mike H.

On 3/12/07, Jake Bush <jake.e.bush@gmail.com> wrote:
> Hello,
> Wondering why I see an extra resistor in a lot of circuits. For example...
>
> -------------------------------------------------------
>        __________
>    ----|                 |
>        | Op Amp    |--------|
>    ----|__________|     _ |__
>    |                           |      |
>    |                           | R1 |
>    |                           | __ |
>    | ________________|
>                                _|__
>                                |      |
>                                | R2 |
>           A                   | __ |
>                                 _ |_
>                                 \    /
>                                Ground
> -------------------------------------------------------
>        __________
>    ----|                 |
>        | Op Amp    |--------|
>    ----|__________|     _ |__
>    |                           |      |
>    |                           | R1 |
>    |                           | __ |
>    | ________________|
>
>               B
> -------------------------------------------------------
>
> What's the difference between circuit A and B? There must be a reason
> for putting the extra resistor. I know R1 and R2 make a voltage
> divider. But doesn't just going from R1 back into the component do the
> exact same thing?
>
> Sorry if this is pretty remedial and the reason is just staring me in the face.
> --
> Jake E. Bush
>
> "For small erections may be finished by their first architects; grand
> ones, true ones, ever leave the copestone to posterity."
> - From "Moby Dick" by Herman Melville
> --
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