Polarity of voltage drops

Chapter 2 - Ohm's Law

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When using conventional current flow, we can trace the direction of the current in the same circuit by starting at the positive (+) terminal and going to the negative (-) terminal of the battery, the only source of voltage in the circuit. From this, we can see that the current is flowing clockwise, from point 1 to 2 to 3 to 4 to 5 to 6 and back to 1 again.

As the current encounters the 5 Ω resistance, voltage is dropped across the resistor’s ends. The polarity of this voltage drop is positive (+) at point 3 with respect to point 4. We can mark the polarity of the resistor’s voltage drop with negative and positive symbols, in accordance with the direction of current; whichever end of the resistor the current is entering is positive with respect to the end of the resistor it is exiting:

polarity of the resistors voltage drop

We could make our table of voltages a little more complete by marking the polarity of the voltage for each pair of points in this circuit:

Between points 1 (+) and 4 (-) = 10 volts                     
Between points 2 (+) and 4 (-) = 10 volts  
Between points 3 (+) and 4 (-) = 10 volts 
Between points 1 (+) and 5 (-) = 10 volts                        
Between points 2 (+) and 5 (-) = 10 volts                       
Between points 3 (+) and 5 (-) = 10 volts  
Between points 1 (+) and 6 (-) = 10 volts 
Between points 2 (+) and 6 (-) = 10 volts                         
Between points 3 (+) and 6 (-) = 10 volts

While it might seem a little silly to document polarity of voltage drop in this circuit, it is an important concept to master. It will be critically important in the analysis of more complex circuits involving multiple resistors and/or batteries.

Polarity has Nothing to do with Ohm’s Law

It should be understood that polarity has nothing to do with Ohm’s Law: there will never be negative voltages, currents, or resistance entered into any Ohm’s Law equations! There are other mathematical principles of electricity that do take polarity into account through the use of signs (+ or -), but not Ohm’s Law.

REVIEW:

  • The polarity of the voltage drop across any resistive component is determined by the direction of current flow through it: positive entering, and negative exiting.

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