Magnetic field coil of wire problem

1. Mar 23, 2005

leolaw

This is the problem taken from Giancoli Chapter 20, number 80:
The magnetic field B at the center of a circular coil of wire carrying a current I is :
$$B = \frac{\mu_{0}NI}{2r}$$, where N is the number of loops in the coil and r is its radius. Suppose that an electromagnet uses a coil 1.2m in diameter made from square copper wire 1.6mm on a side. The power supply produces 120V at a maximum power output of 4.0kW.
(a) How many turns are need to run the power supply at maximum power?
(b) What is the magnetic field strength at the center of the coil?
(c) If you use a greater number of turns and this same power supply (so the voltage remains at 120V), will a greater magnetic field strngth result?

First of all, I cannot picture what a "square copper wire" is.

(a) P = VI, so 4* 10^3 = 120V (I), I = 33.3333
then I set the above forumla into:
$$\frac{B2r}{\mu_{0}I} = N$$
But then I am having problem finding the value of "B", which is the magnetic field of the coil. And this is also the problem for the next question

(c) I think the greater number of turns would result a greater magnetic field strength because when you make more loops for the coil, you are increasing the magnetic field. And when you add up all the magnetic field, it will ultimately greater then the original strength.