Series Circuit Wiring
A circuit that is wired completely in series is just a single loop, which means that there is only one path through which current can flow. The current across each element is therefore the same because it never gets split across two different paths. In this circuit diagram, two resistors are wired in series:
R1 & R2: Resistors
V: Voltage Source
Voltage is the electric potential difference between two points. Therefore, when we talk about voltage, we are always talking about the voltage in one location relative to another. In a circuit, the reference point is always zero. For instance, in the series circuit drawing above, if the voltage source were a 1.5 V battery, the voltage at the positive terminal would be 1.5 volts relative to the negative terminal or ground, where the voltage would be 0 volts. Therefore, the voltage supplied by the battery will drop over the circuit. In the case of a light bulb connected to the battery, essentially the entire voltage drop occurs across the terminals of the light bulb since it represents the only meaningful resistance in the circuit.
R: Light bulb
In a circuit with multiple resistors in series, the voltage will drop incrementally across each. For instance, if two resistors of the same resistance are wired together in series, then the voltage drop across each resistor will be the same - and will be equal to half the total voltage drop across the circuit.
If the two voltage sources in the series above are 3-Volt photovoltaic modules, the total voltage across the resistor would be 6 Volts. In this configuration the positive (red) lead of one module would be wired to the negative (black) lead of the next. This is shown in the schematic above where the two voltage sources are connected negative side to positive - essentially stacked. In this instance, since both voltage sources have the same voltage (3 Volts) the overall potential difference between the topmost positive terminal and the bottommost negative terminal is twice as large - making it 6 Volts. Below are three 3-Volt photovoltaic modules wired in series, making the total voltage drop in the circuit 9 Volts.
Vtotal = V1 + V2 + V3 + ... + Vn
As previously mentioned, in a completely series circuit there is only one path through which current can flow. Current will be conserved in a circuit because charge must be conserved. Electrons move through a circuit, but they don't disappear, and as each electron propels the next forward, a current is created. If you imagine the electrons moving through a wire as a traffic jam on the highway, the rate at which traffic moves is the current. A car can't move faster than the car in front of it, and so the speed of each car is the same as every other car. Obviously, this is an imperfect metaphor, but the key point is that if there is only one loop through which electrons can flow, the rate at which they flow - which is to say the current - will be the same at every point in the circuit.
Itotal = I1 = I2 = I3 = ... = In