Resistance is a hindrance to the electrons while passing through a conductor. The resistors are made from insulating materials like carbon or plastic. These materials have variable structural properties based on which they offer variable resistance. Conductors have very few electrons affixed to voids and mostly the electrons are free to move around within their crystalline structure.
The electrons which can move, pass on the current through the conductor. Whereas, insulators have crystalline structures that have all its electrons fixed to a location. Therefore, there is no free electron to move and carry the current.
In the same manner, batteries are made from a material having non-zero resistivity. The internal resistance is produced due to the negligible resistance of the material of which battery is made of. Metals also offer some negligible resistance because of impurities present in them or bulk collision of electrons on randomized heating. This brings us to a conclusion that there is no practical workable battery with 100% voltage source.
This voltage, also known as the electromotive force, is represented by ε in series with a small internal resistance of r. On connecting an external resistance “R” in the circuit series to complete the circuit, we obtain the following through Ohm’s Law: