An iron core is needed in an electromagnet because it serves to increase the magnetic field created. It has a ferromagnetic properties. <span>An </span>electromagnet<span> is essentially just a battery attached to a coil of wire wrapped around a metal</span>core<span>. </span>
Answer:
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The force is greatest when the wire is perpendicular to the magnetic field.
In fact, the expression for the magnetic force is

where I is the current, L the length of the wire, B the magnetic field intensity and

is the angle between the directions of I and B.
We can see that F is maximum when

, so when

, so when the wire and the magnetic field are perpendicular to each other.