The formula for calculating the force per unit length between two parallel wires of +/- X volts is F = k * (q1 * q2) / d^2, where F is the force, k is the Coulomb's constant (9 * 10^9 Nm^2/C^2), q1 and q2 are the charges of the wires, and d is the distance between the wires.
The voltage of the wires does not directly affect the force per unit length between them. However, the voltage determines the magnitude of the charges on the wires, which in turn affects the force per unit length according to the formula mentioned above.
The force between the wires is always attractive, regardless of the polarity of the wires. This means that the force will always act towards the opposite wire.
The force per unit length between two parallel wires is inversely proportional to the square of the distance between them. This means that as the distance increases, the force per unit length decreases, and vice versa.
The SI units for force per unit length between two parallel wires of +/- X volts are newtons per meter (N/m).