Ohm’s Law – Series Circuits and Parallel Circuits

There are two basic types of electrical circuits, series circuits and parallel circuits. In this article we’re going to talk about the electrical differences between the two circuits and in what situations they are individually suited for. But before we delve into these circuits I’m going to briefly go over a fundamental electrical relationship called Ohm’s law.

Ohm’s Law

Ohm’s law describes a relationship between a circuit’s voltage, current, and resistance in the form of a simple algebraic equation:

ohms law equationswhere V is voltage, I is current, and R is resistance.

As you can see from the above equations, this relationship can be expressed interchangeably when solving for voltage or resistance as well. A common illustration used for easier commitment to memory is the Ohm’s law triangle. Cross out V when solving for V and multiply I and R; cross out R when solving for R and divide V over I; cross out I when solving for I and divide V over R.

Ohms Law Triangle

  • The term “resistor” will be used in this article to refer to any load or resisting component (besides of course, the small resistance of the wire itself) along a given electrical circuit, such as a light or similar electrical device.

Series Circuits

A series circuit is a circuit whose electrical components are connected end-to-end, or through each other. In series circuits, current remains uniform across all resistors while voltage and resistance are calculated as the sum of the individual values across each resistor in the circuit. With this principle in mind one can use Ohm’s law and calculate an unknown variable as long as the other two values are known.

  • Remember that when resistors in a given circuit aren’t uniform, the voltage will vary across the differing resistors. Ohm’s law can only be used to define a variable in relation to the given set of points.

One thing to remember about series circuits is that even though the current remains uniform, due to the fact that each resistor in the circuit uses a portion of the available voltage, each additional resistor will cause a subsequent decrease in voltage across any one resistor. What this practically means is that 2 lights connected in series will each give off only one-fourth the brightness of one – if it were connected alone.

Diagram explaining Series Circuits

The brightness of a light depends on a property called power. Power is the rate at which work is done, is measured in watts, and is the product of the voltage and the current. By connecting two lights in series, you’re essentially halving both the voltage and the current, and as per the definition of power, the result is a decrease by a factor of 4 when compared to a single light.

While the voltage in a series circuit is the sum total of voltages across each resistor in the circuit, making it easy to see why voltage across a given resistor will be halved with a second resistor, you may be wondering how the current gets halved in the same way as it’s supposed to be equal across the entire circuit. Good question.

Provided that the second resistor has equal resistance as the first, the current will be halved due to the increase in resistance across the circuit. As per Ohm’s law, current is equal to voltage over resistance. As such, current is inversely proportional to resistance, that is, it decreases in proportion to an increase in resistance.

Parallel Circuits

Diagram explaining Parallel Circuits

Parallel circuits are those in which resistors are connected parallel to each other, as opposed to end-to-end. Instead of the the circuit continuing only in one direction through any resistors there may be, it splits into two paths when it reaches a resistor, one going to the resistor and the other continuing on to the next resistor.

A fundamental difference between a parallel circuit and a series circuit is that the voltage across a parallel circuit remains constant. So using the above light analogy, you would be able to connect as many lights as you want within the capacity of your circuit breaker and have full brightness on each light. Constant voltage however, isn’t the only difference.

Both the current and resistance have different values when compared with a series circuit. Firstly, the current of a parallel circuit is the sum of the currents across each of the resistors as opposed to being equal as in a series circuit. Secondly, the total resistance of a parallel circuit is the reciprocal of the sum of reciprocals of each resistor – as follows:

Equation to calculate Resistance for Parallel Circuit


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