At the same temperature, two wires made of pure copper have different resistances. The same voltage is applied at the ends of each wire. The wires may differ in.

a. length. b. cross-sectional area. c. resistivity. d. amount of electric current passing through them

a) length b) cross-sectional area

Explanation:

The resistance of a wire, assumed to be an homogeneous one, of a resistivity ρ, length L and cross-sectional area A, is given by the following expression:

       

At the same time, the current flowing through the resistor, is related to the voltage applied at the ends of the wire, by Ohm's Law, as follows:

       

So, for a given voltage, the current is proportional to the voltage, being the proportionality constant the resistance of the resistor.So, if the material of both resistances is the same, the wires can differ in the length L and / or in the cross-sectional area A.

The answer is 4f======since electrostatic force is given by    we let f be the electrostatic force when  r = 1 m    if we let  denote the electrostatic force when the distance is decreased so that r = 0.5m, then    we established earlier that f = kq₁q₂. therefore    the force will now be 4f.