How to Calculate the Voltage of a Resistor

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Before you can calculate the voltage of the resistor, you must first determine what type of circuit you are using. If you need to review the basics or need a little help understanding electrical circuits better, start with the first part. If not, skip it and go to the section about the type of circuit you need to deal with.

Table of Contents

Understanding electrical circuits

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Learn about circuits. Imagine the circuit in the following way: imagine you are pouring a bag of corn kernels into a bowl. Each corn kernel is an electron (electron), and the flow of particles into the bowl is an electric current. ^{[1] X Research Resources Serway, RA and John W. Jewett, Jr., Physics for Scientists and Engineers with Modern Physics. 8th edition. California: Brooks/Cpe. 2010. Ebook} When talking about flow, you describe it by saying how many particles are moving per second.

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Think about electricity. Electrons carry a “negative” charge. That is, they attract (or move toward) the positively charged object, and repel (or move away from) the negatively charged object. Because they are all negative, the electrons are always trying to repel each other, scattering whenever possible.

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Understand voltage. Voltage is the difference in charge between two points. The larger the charge difference, the stronger the two ends attract each other. Here is an example of a typical battery:

In a battery, a chemical reaction takes place and electrons are accumulated. These electrons go towards the negative end, while the positive end remains in a near-empty state (They are called cathode and anode). The longer this process takes, the greater the voltage between the two ends.
When wiring between the cathode and anode, suddenly, the electron at the cathode has room to go. They shoot towards the anode, creating an electric current. The higher the voltage, the more electrons move towards the anode per second.

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Understand the concept of resistors. Resistor is exactly what its name implies. The higher the resistance of an object, the harder it is for electrons to move through the object. It slows down the flow of electricity, because now, in each second, less electrons can move through.

A resistor is anything that belongs to a circuit and adds resistance to the circuit. You can buy a real “resistor” at an electrical store, but in circuit problems, resistance is usually represented by a light bulb or any other resistive object.

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Remember Ohm’s Law. There is a very simple relationship that exists between amperage, voltage and resistance. Write it down or memorize it – you’ll use it often when solving circuit problems:

Amperage = voltage divided by resistance
It is usually written as: I = ^V / _R
Think about what happens when V (voltage) or R (resistance) is increased. Does it match what you learned in the explanation above?

Calculate the voltage of the resistor (series circuit)

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Understand what is a series circuit. Series circuits are easy to identify. It’s just a coil, with everything lined up in a row. Current moves around the entire coil, passing through each resistor or component that makes up the circuit in turn.

The current is always the same at every point in the circuit. ^{[2] X Research Source}
When calculating voltage, the position of the resistor in the circuit does not matter. You can take and change the resistor position, the voltage of each resistor will remain the same.
Consider the example circuit with three resistors in series: R ₁ , R ₂ , and R ₃ . This circuit is powered by a 12V battery. We will find the voltage of each resistor.

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Calculate the total resistance of the circuit. Add up all the resistor values in the circuit. The answer obtained is the total resistance of the series circuit.

For example, the three resistors R ₁ , R ₂ , and R ₃ have resistances of 2 Ω (ohm), 3 , and 5 Ω respectively. The total resistance of the circuit is 2 + 3 + 5 = 10 ohms.

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Find the amperage. Use Ohm’s law to find the total current in the circuit. Remember that in series circuits, the amperage is the same everywhere. Once we have calculated the line in this way, we can use it for any calculation.

Ohm’s law says that amperage I = ^V / _R . The voltage across the circuit is 12 volts, and the total resistance is 10 ohms. The answer is I = ¹² / ₁₀ = 1.2 amp .

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Transform Ohm’s law to find the voltage. With basic algebra, we can transform Ohm’s law to find voltage instead of amperage:

I = ^V / _R
IR = ^V R / _R
IR = V
V = IR

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Calculate the voltage of each resistor. We know the resistance value, we know the amperage and we have the equation. Change numbers and solve. For the example problem, we have:

Voltage of R ₁ = V ₁ = ( 1.2A )( 2Ω ) = 2.4V.
Voltage of R ₂ = V ₂ = ( 1.2A )( 3Ω ) = 3.6V.
Voltage of R ₃ = V ₃ = ( 1.2A )( 5Ω ) = 6.0V.

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Check your answers. In a series circuit, the total voltage across all the resistors must be equal to the total voltage of the circuit. ^{[3] X Research Source} Add up all the voltages you have calculated and see if we get the full circuit voltage. If that doesn’t work, go back and find the error.

In our example: 2.4 + 3.6 + 6.0 = 12V, which is the total voltage of the circuit.
If the sum of the voltages is slightly lower (such as 11.97 instead of 12), you’ve probably rounded the number somewhere. Your answer is still correct.
Remember that voltage measures the difference in charge, or number of electrons. Imagine you are counting the number of electrons you see as you move along a circuit. If you count correctly, you will eventually get the total charge present in the electrons from start to finish.

Calculate the voltage of the resistor (parallel circuit)

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Understand what is a parallel circuit. Visualize an electrical cord with one end located at the battery, the other end being split into two separate wires. These two wires run parallel to each other, and then connected again before reaching the other end of the battery. If there is a resistor on the left and right wires, then the two resistors are connected “in parallel”. ^{[4] X Research Sources}

Parallel circuits can have an arbitrary number of wires. This instruction still holds true for circuits that are divided into a hundred wires and then reassembled.

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Think about how current flows in a circuit. In a parallel circuit, current flows through every line it is supplied with. It will run through the left wire, pass the left resistor, and reach the other end. At the same time, it will also run through the right wire, through the right resistor, and to the other end. No part of the current flows back to or through both resistors in parallel.

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Use the full circuit voltage to find the voltage of each resistor. When the voltage of the whole circuit is known, it is surprisingly easy to find the voltage of each resistor. Each parallel wire has the same voltage as the voltage of the whole circuit. ^{[5] X Research Source} Suppose a circuit with two resistors in parallel is powered by a 6V battery. The voltage of the left resistor will be 6V and the voltage of the right resistor will also be 6V. It doesn’t matter how big the resistor value is. To understand why, let’s review the series circuit mentioned above:

Remember that in series circuits, the total voltage of the circuit is always equal to the sum of the voltages of each voltage drop.
Think of each line path as a series circuit. The same holds true: when you add up the voltages of the whole resistor, you will eventually get the full circuit voltage.
Since current flows through each wire through only one resistor, the voltage of that resistor must be equal to the total voltage.

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Calculate the total current in the circuit. If the problem doesn’t show the full circuit voltage, you’ll have to complete a few more steps. Start by finding the amperage going through that circuit. In a parallel circuit, the total current in the circuit is equal to the sum of the currents passing through each parallel branch. ^{[6] X Research Resources Serway, RA and John W. Jewett, Jr., Physics for Scientists and Engineers with Modern Physics. 8th edition. California: Brooks/Cpe. 2010. Ebook}

In mathematical terms: I _sum = I ₁ + I ₂ + I ₃ …
If it’s confusing, imagine a water pipe split in two. Total water flow is simply the amount of water flowing through each pipe added together.

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Calculate the total resistance of the circuit. In parallel circuits, resistors are not as effective because they only hinder the flow of current through a wire or bypass. In fact, the more circuits there are, the easier it is for the current to find its way to the other end. To find the total resistance of the circuit, solve the following equation and find the _total R:

¹ / _R_sum = ¹ / _R₁ + ¹ / _R₂ + ¹ / _R₃ …
Take for example a circuit with 2 ohm and 4 ohm resistors installed in parallel. ¹ / _R_sum = 1/2 + 1/4 = 3/4 → 1 = (3/4)R _total → R _sum = 1/(3/4) = 4/3 = ~1.33 ohms.

Advice

If you have a complex circuit with series and parallel resistors, choose two that are close together. Find their combined resistance using the appropriate series or parallel resistance rule. Now you can think of them as a single resistor. Do this until you get a simple circuit with resistors either in parallel, or in series. ^{[7] X Research Sources}
The voltage of a resistor is often called a “drop in voltage”.
Understand the term:
- Circuit – consists of electrical circuit components (such as resistors, capacitors and inductors) that are connected by wires and where current can flow
- Resistor – part that can reduce or hinder current
- Amperage – the flow of charge entering the wire, unit: Amp, A
- Voltage – work done to move a charged particle; Unit: Volt, V
- Resistance of an object – measures its resistance to electric current; Unit: Hugs,

Steps

Understanding electrical circuits

Calculate the voltage of the resistor (series circuit)

Calculate the voltage of the resistor (parallel circuit)

Advice

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