What is the relationship between voltages in a series circuit?

The rule for voltage in a series circuit is that the signed sum of the voltage drops around the circuit add up to zero. This is Kirchoff's voltage law.

Take a simple circuit of a 1.5V battery and light bulb in series with each other. Starting at the battery negative terminal, the voltage from negative to positive is +1.5V. Starting at the top of the light bulb (the end connected to the battery positive terminal), the voltage from top to bottom is -1.5V. The two drops (rises, in this case), +1.5V and -1.5V add up to zero.

Note that the simple example above, is also a parallel circuit. The implication is that elements of a parallel circuit have the same voltage across them.

Lets add a second bulb in series with the first. The voltage going up across the battery is still +1.5V, while the voltage going down across each bulb is -0.75V. Again, the sum is zero.

You can take this to any level of complexity during circuit analysis. Find the series or parallel elements and calculate drops around the series part or find the equivalent voltages across the parallel parts, combine that with other techniques such as Kirchoff's current law, and Norton and Thevanin equivalents, and you can analyze any circuit.

ANSWER: In a series circuit since the current remains the same for every items the voltage will vary according to ohm law