Resistor

The resistor is a lineair component in electronics which is the exact opposite of a conductor in behaviour. The behaviour of the resistor is described by Ohm's law namely

$$U = I R$$

Where U is the voltage drop over a resistor, I the current through the resistor and R the resistorvalue measured in Ohm's. This same relation also works as a function of time where the current is also dependent of time, assuming that the resistance is constant.

$$U(t) = I(t) R$$

Series of resistors
In the case of a series of resistors across a voltage drop, the series of resistors will act as if they are only one resistor with a resistance equal to the sum of resistor values inside the series. Mathmatically that looks like this.

$$R_r = \sum_n R_n = R_1 + \dotsb + R_n $$