For us to better understand and explore the world of electricity and electronics, we need to understand the basics of voltage, current and resistance. Now these are the three main building blocks that we require to manipulate and fully utilize electricity, and because we cannot see/touch them, they can be somewhat difficult to comprehend. In this tutorial, we’ll understand how voltage, current and resistance relate to each other. We’ll also cover what Ohm’s law is and how to use it to understand how electricity works with a simple experiment that demonstrates the concepts in action.
Take a look at the previous tutorials;
Lesson 1: Electricity and how it works
Lesson 2: Circuits basics – Open Vs Closed
Lesson 3: Current – AC Vs DC
The three basic principles of this tutorial can be explained using electrons or to be more specific, the charge they create. Let’s briefly define these principles:
- Voltage: This is the difference in charge between two points.
- Current: This is the rate at which charge is flowing.
- Resistance: This is the material’s tendency to resist the flow of charge which in this case is current.
Beginning with Voltage, let’s try and explain the above principles in further details to better comprehend how they relate to each other.
We define Voltage as the amount of potential energy between two points in a circuit. For every circuit, one point has more charge than the other. Voltage is measured in “volts” and is represented in equations and schematics by the letter “V”. A “volt” is technically the potential energy difference between two points that will impart one joule of energy per coulomb of charge that passes through it. The unit “volt” was named after the Italian physicist Alessandro Volta who invented what is considered the first chemical battery.
When describing the relationship between voltage, current and resistance, we’ll use a common hydraulic analogy or water tank analogy. In this analogy, the water amount represents charge, the water pressure represents the voltage and the water flow represents current.
So in brief:
- Water amount = Charge
- Water pressure = Voltage
- Water flow = Current
Let’s consider a water tank filled with water at a certain height above the ground and a hose pipe with a restrictor connected to the bottom of the tank as shown below:
The water pressure measured in Pascals (PSI) is the analogy of voltage because establishing a water pressure difference between two points along a horizontal pipe causes water to flow.
Let’s think of the water tank as a battery where we store a certain amount of energy and then release it. If we drain our tank to a certain level, the pressure created at the end of the hose goes down and we can think of this as a decreasing voltage. There will also be a decrease in the amount of water in the tank that will flow through the hose which literally means, less pressure thus less water is flowing and this leads us to current.
From the hydraulic analogy, we can think of the amount of water flowing through the hose from the tank as current. Here, the higher the pressure, the higher the flow and vice versa. Current is measured in Amperes (usually referred to as “Amps”) and is represented in equations and schematics by the letter “I”. An “Amp or Ampere” is defined as one coulomb (6.241 x 1018 Electrons) per second passing through a point in a circuit. The unit “Ampere or Amp” was named after the French mathematician and physicist Andre-Marie Ampere. With water, we would measure the volume of the water flowing through the hose over a certain period of time but with Electricity, we measure the amount of charge flowing through the circuit over a period of time.
From the water tank, when water begins to flow, the flow rate of the water through the narrow part of the hose pipe will be less than the flow rate of the water in the wider part of the hose pipe. In electrical terms, the current through the narrow (restrictor) is less than the current through the wider part of the hose pipe. When we add more water in the water tank, this increases the pressure (voltage) at the end of the pipe restrictor and this pushes more water through the pipe. This is analogous to an increase in voltage that causes an increase in current.
So at this point we’re starting to see the relationship between voltage and current, but there is a third factor we need to consider, the width of the pipe (pipe restrictor) which represents Resistance.
By now our model is as follows:
- Water amount = Charge (measured in coulombs)
- Water Pressure = Voltage (measured in volts)
- Water Flow = Current (measured in Amperes or Amps)
- Pipe Restrictor = Resistance (measured in ohms)
Considering the water tank analogy, with the hose pipe that has a restrictor in between. The flow restrictor in this case represents resistance. We therefore say that the rate of water flow through an aperture restrictor is proportional to the difference in water pressure across the restrictor. Similarly, the rate of flow of electrical charge that is electric current, through an electrical resistor is proportional to the difference in the voltage measured across the resistor. This also means that a circuit with high resistance will allow less current to flow and vice versa.
Resistance is measured in “ohms” and is represented in equations and schematics by the Greek letter “Ω” called omega but pronounced as “Ohm”. The unit “Ohm” was named after a Bavarian Physicist, George Ohm who defines the unit of resistance of “1 Ohm” as the resistance between two points in a conductor where the application of 1 Volt will push 1 Ampere, or (6.241 x 1018) electrons.
George Ohm combined the three elements i.e. Voltage, Current & Resistance and came up with a formula that we call “Ohm’s Law”.
V = I x R ………………… (i)
Where V = Voltage (volts),
I = Current (amperes)
R = Resistance (ohms)
Ohm’s Law states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the Resistance. The Resistance here is a constant independent of the current.
Ohm’s Law Experiment:
Let’s now look at how we can apply Ohm’s Law and see how it works in an electric circuit. Consider the circuits A and B below:
In circuit A, the LED will blow up because it requires 3v & 20mA and we are supplying 5v to it. So to prevent this, we shall have to introduce a resistor in the circuit. We then need to calculate the resistance that we need to introduce to the circuit in order to meet the conditions of the circuit as to have the LED to light up at 3v and 20mA.
This is why we need to use Ohm’s Law, and upon using the Ohm’s law triangle,
We have, R = V/I = (Change in Voltage) / Current
= (Source Voltage – Forward Voltage) / Current
= (5 – 3)v / 20mA
R = 100Ω
Therefore, we need a resistor of 100Ω to take away the 2v leaving the 3v for the LED.
To show ohm’s law in action, if we increase the Resistance, the current decreases and the LED is barely lit and if we decrease the Resistance, the current increases and this means we are providing way too much current to the LED and if we decrease the resistance a lit bit more, the LED will blow up. This is the inverse part of Ohm’s law. Keeping the Resistance the same, and adjusting the voltage from say 5v to 6v, the current also increases. Likewise, if we decrease the voltage, to say 4v, the current also decreases to 10mA and the LED is barely lit.
By now, you should be able to understand the concepts of voltage, current, resistance and how all the three relate to each other. The majority of the equations and laws for analyzing circuits that we are yet to cover in the coming tutorials can be derived from Ohm’s law as this is the basis for the analysis of any electrical circuit.
In the coming tutorials, we are going to learn about the use of Ohm’s law in more complex applications and also learn how electrical circuits are designed, but first let’ s look at a few components such as Resistors, Capacitors, LEDs, Batteries,…. Etc.