Physics 101: Learning Ohm’s Law Made Easy

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In 1827, a scientist named Georg Ohm discovered the potential difference between two points in a circuit is directly proportional to the electricity flowing through the resistance and directly proportional to the circuit’s resistance. Or if put into a formula, V = IR. This principle is known as Ohm’s law.

At its core, Ohms law is about the connection between current, voltage, and resistance. This is a nothing but quick summary of it. It is much deeper than that. Here, we will explain Ohms law in detail. We will start from electrical charge, voltage, current, resistance, units of measurement and end it with the law.

Electrical Charge

Three particles made an atom: proton, neutron, and electron. Proton is the particle that has a positive electric charge, neutron has no electric charge and electron has a negative electric charge. Proton and neutron are packed together, forming the core of an atom while the electron moves around in the shell.

With enough energy, electrons can move from one shell to another. This movement of electrons is what is known as electricity. Electrons produce charge which can be harnessed to do work. Your smartphone, refrigerator, light bulb and any other electrical devices all harness this electrical energy to function.

When we talk about the core of Ohm’s law, which is about the relationship between voltage, current, and resistance, we are describing the charge’s movement. In other words, the electrons’ behavior. We will get into voltage, current, and resistance deeper later but here are the basic definitions of them:

  • Voltage: the potential difference between two points
  • Current: the rate at which electricity is flowing through the resistance
  • Resistance: the tendency of a given material to resist the charge flow

Voltage

Voltage is defined as the potential difference between two points on a circuit (note: a circuit is a closed-loop which allows charge to move from one point to another). In the circuit, the charge of a point is higher than the other. The difference in charge between these two points is what is known as voltage.

If it is difficult to understand, just imagine a water tank. In this water tank analogy, the amount of water is the charge, the water pressure is the voltage, and the water flow is the current. The bottom of the water tank is connected to a hose and the tank is placed several meters above the ground.

Now, the end of the hose will have pressure. This represents voltage. The water stored within represents charge. This means if there is more water inside the tank, there is more charge. Consequently, the pressure at the end of the hose will be increased as well.

The tank here can be likened to a battery as it is a place where there is a certain amount of energy stored within which then gets released. In the analogy, if you are to drain a certain amount of water from the tank, the pressure at the end of the hose will decrease. And less pressure means less flow.

Current

What does current here mean? Let’s continue our analogy to make it easier to understand. You can think of the current as the volume of water that flows through the hose. We can quantify the flowing water volume in a given period of time. The same applies to electricity.

Let’s add another water tank in the analogy. Imagine you have two water tanks. Both tanks have the same shape and amount of water. Each tank has a hose at its bottom, allowing water to flow through. The difference between the tanks is the hose with one tank has a narrower hose than the other.

Which tank have more flow rate? The answer: the tank that has a wider hose. To put it into electrical terms, the wider hose has more current than the narrower one. If you want the same flow rate (current) from the tanks, you must increase water volume (charge) inside the tank that has a narrower hose.

As the amount of water is increased, so is the pressure. This results in increased flow rate. In electrical terms, increased charge results in increased voltage and increased voltage results in increased current. Notice the interdependence in the quantities.

Now you see the connection between current and voltage. Although the previous water tank analogy is good enough to explain, there is a factor missing: the width of the hose. This is the resistance, which we will explain shortly.

The water tank analogy is now like this:

  • Volume of water = Charge
  • Pressure = Voltage
  • Flow rate = Current
  • The width of the hose = Resistance

Resistance

Moving on to the third factor: resistance. Let’s continue using the two water tanks analogy. Again, one tank has a narrow hose while another a wider hose. Both tanks have the same amount of water inside. The difference between the two lies in the width of the hose.

Since the volume of water is the same, the pressure is the same. However, due to the difference in width, the volume of water flowing through the hose is different. We can say that the narrower hose “resists” the water flow even if the pressure of the tanks is the same.

This situation can be represented by two circuits that have equal voltage but unequal resistances. The circuit that has lower resistance allows more charge to flow, resulting in a higher current. Conversely, the circuit that has higher resistance allows less current to flow, resulting in less current.

Units of Measurement and “Symbols”

If we are to make any meaningful statement regarding Ohm’s law, voltage, current, and resistance, we will need some electrical units of measure so we can describe the quantities of the three. Indeed, this is the same thing that we do to quantify length, volume, mass, temperature and just about any physical quantity.

For example, for temperature, we quantify it using degrees Celsius or Fahrenheit, kilogram or gram for mass, and so on. To quantify voltage, current, and resistance, we use the following:

  • Voltage: Volt (abbreviation: V)
  • Current: Ampere or Amp (abbreviation: A)
  • Resistance: Ohm (abbreviation: Ω, which is called omega but pronounced Ohm)

These units of measurements are named after famous scientists who experimented with electricity. The volt is named after Alessandro Volta, the amp after Andre M. Ampere, and the Ohm after Georg Ohm.

Besides the units of measurements and abbreviations, internationally recognized “symbols” are also used. The “symbol” give for voltage, current, and resistance is the standard alphabetical letter. For voltage, the symbol is E or V (volts), current is I, and resistance is R.

All the symbols are meaningful as well. The E represents electromotive force, the V represents voltage (E and V are interchangeable as both symbolize volts) and the R is for resistance. The I is a bit different as many disputed what the letter symbolizes. That said, many people think it represents Intensity (of flow)

Discovery of Ohms Law

Georg Ohm discovered the mathematical connection between voltage, current, and resistance in 1827. Only after many experiments and considerable effort did Georg Ohm find the connection. He then managed to develop a law, which later on is known as Ohm’s law, named after him to honor his work.

Ohm’s Law: What Is It All About?

Now that we have explained the major circuit quantities, their units of measurements and symbols, let’s get to the Ohm’s law. What is Ohms law all about and why does it matter? This law is considered among the most important and fundamental laws governing electronic and electrical circuits.

How important is the law? Very important. Firstly, the law involves three major circuit quantities: voltage, current, and resistance. Secondly, the law is used within every electrical and electronic science branch. Thirdly, it is applicable anywhere current flows. Such is how important Ohms law is.

Application

Ohm’s law can be used to determine a quantity be it current, voltage or resistance of a linear electric circuit (like DC circuit, for example). That is, so long as the other two quantities are known. Because of this law, calculating power is easier.

Any device, material or component that follows Ohm’s law (in other words, having current that is proportional to the voltage) are called “Ohmic” in nature. Likewise, any device, material or component component or device that does not obey the law is called non-ohmic devices.

Ohm’s Law Formula and Triangle

Ohms Law Triangle

Ohm’s law formula is V = I x R, where

  • V is the voltage
  • I is the current
  • R is the resistance

If you want to find current (I), the Ohm’s law formula is I = V / R.

If you want to find resistance (R), the Ohm’s law formula is R = V / I.

Ohm’s law formula

You can use a triangle to memorize Ohms law easier. Imagine a triangle with one part at the top and two parts of the bottom. The top part is voltage or V, while the bottom parts are current or I and resistance or R.

The relationship between the parts is like this:

  • If the quantities of the top part and a bottom part are known, the top part is divided by the bottom part. So the formula is either I = V / R or R = V / I
  • If the quantities of the bottom parts are known, the quantities of the parts are multiplied with each other. The formula is V = I x R
Ohms Law Relationship

Examples

Understanding a new concept is a lot easier if we have examples to apply the said concept. This, of course, applies to Ohm’s law as well. Below, we have some examples to help you better understand the law.

Example 1

A circuit has a current of 0.1 A with a distance of 100 Ω. How large is the voltage?

Solution:

Since V = I x R, then 0.1 x 100 = 10
The voltage (V) is 10 volts.

Example 2

A circuit has a voltage of 25 volts with 0.1 A of current flowing through. How large is the resistance?

Solution:

Since R = V / I, then 25 / 0.1 = 250 Ω
The resistance (R) is 250 Ω.

Example 3

A circuit has a voltage of 10 volts with resistance of 500 Ω. How large is the current?

Solution:

Since I = V / R, then 10 / 500 = 0.02 A
The current (I) is 0.02 A.

Back to the Analogy

Let’s return to our water tanks analogy again. Remember, the two tanks have the same volume of water thus the same pressure but different widths of hose. Is the flow of water of the tanks the same? Or are they different? Let’s find out the answer.

Let’s call the tank with the wider hose is tank X and the one with the narrower hose is tank Y. Let’s say that the pressure of water is 1 volt, the wider hose has a resistance of 1 Ω while the narrower hose 2 Ω. How large is the current of each tank? I = V / R is the correct Ohm’s law formula here.

So, in the case of tank X:

Since I = V / R, then 1 / 1 = 1
Thus, the current is 1 A.
While in the case of tank Y:
Using the same Ohm’s law formula, 1 / 2 = 0.5
Thus, the current is 0.5 A

Due to the difference in the width of hose, the flow of water of the two tanks is different despite the same amount of pressure. In electrical terms, due to its larger resistance, tank Y has less current flowing through it. Conversely, tank X has a larger current flowing through it due to its smaller resistance.

Ohms Law Limitations

  1. Ohms law can’t be applied to unilateral networks as such networks allow the current to flow in a single direction. Unilateral networks consist of elements such as a transistor, diode, etc.
  2. Ohm’s law also can’t be applied to nonlinear elements. Nonlinear elements are elements whose current is not directly proportional to the applied voltage. In other words, the amount of resistance depends on the quantity of current and voltage. If the two quantities change, so is the resistance.