# Understanding current flow in example

When I have series circuit with 3 light bubbles the voltage in second should be less than in first and the voltage in third should be less than in second. Conventional current flow is from positive to negative while electron flow is from negative to positive. Which light bulb will get the lowest voltage - the one closest to negative charge of battery or the one closest to positive charge of battery? (correct me if my reasoning is wrong)

• The current in a series circuit is the same everywhere. The bulb with the highest resistance will have the highest voltage, regardless of its position. – Dave Tweed Aug 1 '14 at 3:39

## 1 Answer

Assuming that the three light bulbs have identical characteristics, the voltage in their terminals will be exactly the same for each one. In this way, the sum of the voltages of the three bulbs will be equal to the voltage of the source, as follows

Vs = Vb1 + Vb2 + Vb3

where: Vs = Voltage source (battery) Vb1...3 = Voltage of bulb 1...3

The difference in voltages when measuring (e.g. with a voltmeter) depends on the terminal that you use for reference (negative, ground reference). If you measure with respect to the source's negative terminal (typical measurements), the bulb closer to the positive terminal will measure the maximum voltage, then the second and the lower voltage for the third.

The bottom line is that when measuring the reading is a sum of the voltages across the device's terminals, but the voltage in each device must be exactly the same.

• When I measure voltage in a wire after third light bulb it shows 0. Does it mean current doesn't flow in that part of a wire? (I thought electrons always get from one side of battery to another) – bvbfbgf Aug 1 '14 at 4:47
• When wire the three bulbs and then connect them to the battery there will always be a current (electron flow). The current (I) and the voltage (V) in a circuit are related by the resistance (R), through the circuital ohm's law: I=V/R. If you try to measure voltage across two points of wire you always will get a 0[V] reading because the resistance of such a piece of wire is really small. Special equipment, such an electrometer, is needed to measure voltage in common metal wires. Nonetheless, you can still measure current across the whole circuit. – bnolo Aug 1 '14 at 5:04