If not, then, in experiments, why must a wire always be straight?
no, not really, perhaps a fraction of a percent if the cross-section narrows down at the bend. And wires can be any shape in an experiment. Who told you they had to be straight? I once told kids in a lab that you used the striped wires for AC and the solid colored ones for DC. A few believed me.
Bending might increase the resistance very slightly by damaging the crystal structure, but the effect would be tiny. The only reason to want straight wires is neatness!
The resistance of a wire results from: (1) the conductivity,σ, of the material used to construct the wire; (2) the length, L, of the wire; and (3) the cross-sectional area,S, of the wire. R = L / σS Bending the wire might theoretically crimp the wire and affect the cross-sectional area. However, as a practical matter, this is not a factor. Most likely, you are required to keep the wires straight for the sake of neatness. It makes it easier for your instructor to follow the logic of your circuit.
