# Problem with short circuit

Comparing with short circuit how does a current flow in parallel circuits with different resistances. Shouldn't it just choose the one with lesser resistance?

• It is choosing such a route, that maximum current will flow overall. If it can flow two ways instead of one, it will. – Eugene Sh. May 16 '16 at 14:03
• If you have a water tower with 2 different diameter pipes coming out the bottom, does water only flow through the bigger pipe? – CHendrix May 16 '16 at 14:03
• @CHendrix While I always use the pipes analogy, it is not always obvious, why this analogy is good.. – Eugene Sh. May 16 '16 at 14:14
• @EugeneSh. Water molecules = electrons. Or would that be positrons? The analogy breaks down when you directly compare electrons to water molecules, because electrons flow opposite of current direction. I just do the direct comparison, but remember that electrons flow opposite of current direction. – CHendrix May 16 '16 at 14:33

Resistance is defined as the ratio of voltage across an object (the 'resistor') to current flowing through it. It doesn't matter if other components in a circuit are drawing more current, when a particular voltage occurs across a resistor, a certain current will flow through it - always.

So long as the voltage remains constant, adding other components in parallel makes no difference to the current an individual resistor draws. Consider the 3 circuits below:-

simulate this circuit – Schematic created using CircuitLab

In circuit A, R1 draws 5v/5Ω = 1 Amp. In circuit B, R2 draws 5V/1Ω = 5 Amps. In circuit C, V1 and V2 are replaced by a single 5V supply, and R3 and R4 are in parallel. Both resistors get 5V, so R3 draws 5V/5Ω = 1 Amp and R4 draws 5V/1Ω = 5 Amps.

In a 'short circuit' situation the total current may be limited by other components in series, or by the power supply itself. As a result the voltage drops and the lower value resistor 'steals' some of the available current from the other one. However the current will still split between them in proportion to their resistances.

Typically the resistance of conductors increase with temperature (they have a positive temperature coefficient.) What this means is that there is an "auto-balancing" effect between conductors that are connected in parallel, since as a conductor conducts more current, it begins to heat up, and become more resistive- meaning that electrons will begin to flow in the other, cooler conductor until they reach some kind of equilibrium.

Follow this link to find the proper way to calculate the total resistance of multiple conductors connected in parallel, as well as some basic information about how such circuits behave: https://www.swtc.edu/ag_power/electrical/lecture/parallel_circuits.htm