Question

what causes resistance in a wire?

Answer 1

It takes energy to make an electron move. A current is effectively a flow of electrons. Hence it takes work to make a current flow.

We can see that mathematically in the equations: As you know,
\[ work = VI = I^2R. \] If R were zero, then it would take no work to generate a non-zero current I.

Answer 2

i meant like what factors cause resistance...not the mathematical equation

Answer 3

The degree to which electrons are bound to the atoms in the material you are trying to use a conductor.

Metals are typically good conductors because they have electrons in the outer shells of their atoms which are loosely bound. Something else like plastic or wood are terrible conductors, because it takes relatively a much greater amount of energy to make the electrons move.

Answer 4

That's the answer at a high level. For more detail, look at this:
http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance

or watch this:
http://ocw.mit.edu/courses/physics/8-02-electricity-and-magnetism-spring-2002/video-lectures/lecture-9-currents-resistivity-and-ohms-law/