The lines of magnetic field are closer between the poles and so the field is stronger there.
The wire is wound round the core of the electromagnet to give two opposite poles at the two ends of the U.
This is one of the most useful forms of electromagnet.
Making an electromagnet stronger
An electromagnet be made stronger by:
(a) more coils
(b) more current
(c) coils closer together
(d) putting an iron core in the centre of the coil
(e) bending the poles of the magnet closer together to make a U shaped magnet