Their answer only makes sense if you consider the thickness of the
wire, which they told you not to do.

If the wire has a large thickness, well then they have assumed it
takes the entire length of the reel to wind it on.

You assumed the wire had zero thickness, and so it would take no
lateral space on the reel.

You are correct.

On 26 March 2015 at 02:10, CDB <colin@btech-online.co.uk> wrote:
>  I was recently given a maths question as part of test.
>
> The question.
>
> You have a round coil core 10mm thick  and 50 mm long. You wrap a copper
> wire exactly 5 times around. How long is the wire? (don t consider the
> thickness of the wire)
>
>
> My answer along with crap diagram was - find how much wire one turn
> required.
>
> So circumference =3D pi * diameter =3D 3.142 * 10mm =3D 31.42mm =3D 1 tur=
n length.
>
> 5 turns =3D 31.42 * 5 =3D 157mm of wire.
>
> Their answer was completely different, so obviously my thinking is awry.
>
> They worked it out using 3-D triangulation.
>
> Imagine a rectangle with the vertical side =3D L =3D 50mm, horizontal sid=
e =3D W
> =3D 10mm
>
> They then drew a right angled triangle top left of rectangle and bottom
> right to represent an unwrapped cylinder.
>
> They then stated the wire runs between two opposing corners, therefore th=
e
> wire length (one turn) =3D the standard sqr (L^2 + C^2), they then went o=
n to
> show a series of equidistant triangles along the length of the core, 5 in
> total for the turns.
>
> So finally they have N * sqr((L/N)^2 + (pi * W)^2) =3D 165mm.
>
> Now I need to get myself a 10mm * 50mm (min) rod and test this out.
>
> Is their version the only correct solution or just an over engineered one=
?
> Slightly rhetorical question there.
>
> Colin
> --
> cdb,   25/03/2015
>
> --
>
>
>
>
> colin@btech-online.co.uk
>
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.