# Video: Calculating the Total Resistance of Resistors in Series and Parallel

Three resistors, each with a resistance π, are connected in different ways, as shown in the diagrams (1)-(4). Which figure gives the smallest value of resistance between π and π?

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### Video Transcript

Three resistors, each with a resistance π, are connected in different ways as shown in the diagrams one through four. Which figure gives the smallest value of resistance between π and π?

In figures one, two, three, and four, we see these resistors arranged between points marked π and π in each diagram. To solve for the figure which gives the smallest value of resistance between these two points, we can redraw each one. Figure one can be redrawn to show π then a single resistor followed by two resistors arranged in parallel then point π.

Figure two can be redrawn as π and π separated by two parallel branches one with resistance π and the other with resistance two π. In figure three, π and π are separated by a resistance of π. And there is a resistance of two π that leads to a break in the circuit. And finally, in figure four, we can combine all three of the resistors in series so that π and π are separated by three π of resistance.

Now that weβve drawn simplified circuits for each diagram, we can calculate the resistance between points π and π in each one. In diagram one, π ππ is equal to π, the value of the resistor in series, plus the value of the combined parallel resistors which is π squared, their product, divided by two π, their sum. In this fraction, one factor of π cancels out so that π sub ππ is equal to π plus π over two or three-halves π. Thatβs for diagram one.

For diagram two, π sub ππ is equal to the product of the two resistances in parallel: two π times π all divided by their sum which is three π. Once again, we see a factor of π cancel from numerator and denominator. In total then, π sub ππ for diagram two is two-thirds π.

Moving on to diagram three, in this case, π and π are separated by one resistor of resistance value π. We write down that value then for π sub ππ for this diagram and move on to our final diagram number four. In this final diagram, π sub ππ is equal to three π.

Comparing these four values, we find that π sub ππ equalling two-thirds π is the smallest of the bunch. That means the figure with the smallest resistance between points π and π is figure two.