The Physics Behind Generating AC: Why Can AC Be Transformed?

In a previous article, we learnt the importance of alternating currents (also known as AC) and its role of transmitting electricity over long distances. In this article, we will learn the physics behind generating AC. Then, we will explore how transformers work and we can answer the question that was left hanging in the previous article: why can AC be transformed, and not direct current (DC)? 

How is AC generated? 

The process of AC generation relies on the conversion of mechanical energy to electrical energy using the concept of electromagnetic induction. The process first starts off from a mechanical source, such as steam turbines or combustion engines, where there is mechanical energy for the purpose of rotational motion (BYJU'S, 2024). This rotational motion would usually be used for the purpose of spinning a magnet, as you would find out later on. 

Now, the next question would be: what is electromagnetic induction? Discovered by Michael Faraday, this principle tells us of a relationship between magnetism and electricity. 

Let us illustrate it with an experiment: Imagine you have a coiled piece of wire in the shape of a solenoid and a magnet. The wire could be connected to an analog ammeter to observe the amount of current flowing through the wire. Then, place a spinning magnet next to the coil of wire. You would notice that the needle on the ammeter will oscillate about the zero marker. This is actually electromagnetic induction in action, but what exactly is going on? 

Before going into the explanation, you should first be aware of the existence of magnetic fields. They are basically where the magnetic force acts on, as shown in the image below:

Image of the magnetic field lines of a bar magnet (Science Facts, 2022). The magnetic field lines represent the magnetic force, where the arrows point in the direction of the magnetic force.

Another term to note would be magnetic flux, which refers to the total magnetic field that passes through an area. If the lines are more concentrated (like at the poles or ends of the magnet), the magnetic field strength is larger, leading to the magnetic flux to be larger. Conversely, if the lines are less concentrated (like in the middle of the magnet), the magnetic flux would be smaller due to a smaller magnetic field strength. 

When the field is weaker, there would be a smaller magnetic flux on the solenoid if the middle of the magnet is near the coil of wire. Likewise, when the field is stronger, there will be a greater magnetic flux on the solenoid. As such, as the magnet rotates beside the solenoid, the magnetic flux will change as the different parts of the magnet would be nearer to the solenoid at different times.. This change in magnetic flux would cause the electrons in the wire to move in a particular direction, which causes the electrons to flow in a similar way as a current (The Engineering Mindset, 2021). Thus, this leads to current being induced onto the wire and electricity is "created"! (Note that we cannot create electrical energy, but rather this is due to mechanical energy being converted to electrical energy, which leads to electricity in the coil of wire). 

What makes the current “alternating” would be due to the magnet rotating. As the magnet rotates, the North and South pole would take turns to be near the solenoid. From the image above, you can see that the arrows at the two poles of the magnet are pointing in different directions. These arrows refer to the direction of the magnetic force. As the two poles have the magnetic force pointing in different directions, the induced current would swap its direction flow periodically, thus forming the alternating current. 

Why can AC be transformed? 

After understanding how AC is generated, we can take a look into how transformers work.

An image of a step-up transformer (BYJU’S, 2023)

Transformers are devices that are able to change the voltage of electricity to a higher one (also known as stepping up) or to a lower one (also known as stepping down). Its core principle is once again, the use of electromagnetic induction. 

In essence, there is a particular current flowing through the primary coil. Then, this current would generate a magnetic field, which induces a current on the secondary coil, allowing for the secondary coil to have an induced current flowing. If the secondary coil has fewer turns than the primary coil, the transformer would be a step-down transformer as the voltage was stepped down from a higher to a lower one. Likewise, if the secondary coil has more turns than the primary coil, the transformer would be a step-up transformer as the voltage was stepped up from a lower to higher voltage (GeeksforGeeks, 2025). 

However, as mentioned above, for currents to be induced, the magnetic flux must be changing. Only changing currents are able to produce a changing magnetic flux onto the secondary coil (MEP Academy Instructor, 2022). Therefore, only AC can be used in transformers to be stepped up or down. 

Then what about DC? Unlike AC, direct current is steady and unchanging. As such, there will be no changing magnetic flux produced that will induce a current onto the secondary coil. Without such a property, transformers would not be able to work as intended.

Conclusion 

The process of generating electricity can be summarised in one sentence: converting mechanical energy to electrical energy via Faraday’s law of electromagnetic induction. What makes generating AC different as compared to DC is that the polarities of the magnet that faces the wire switches constantly, so the induced current in the wire is not flowing in only one direction, but rather it flows in alternating direction. With AC being a current that changes periodically, the magnetic field produced by AC would also be changing, allowing AC to be able to work in transformers so that they can be stepped up and down. This property allows AC to be used as the more efficient current to be used to transmit electricity over long distances since AC can be easily transformed from high voltage to low voltage, and vice versa. 

Works Cited 

BYJU'S (2024). AC Generator - Principle of Working, Parts and Schematic Diagram. [online] BYJUS. Available at: https://byjus.com/physics/ac-generator/. [Accessed on 6 Nov. 2025]

BYJU'S (2023). Transformer - Definition, Types, Working Principle, Diagram. [online] BYJUS. Available at: https://byjus.com/jee/transformer/ [Accessed 10 Nov. 2025].

GeeksforGeeks (2025). Step-Up and Step-Down Transformer. [online] GeeksforGeeks. Available at: https://www.geeksforgeeks.org/electrical-engineering/step-up-and-step-down-transformer/ [Accessed 10 Nov. 2025].

MEP Academy Instructor (2022). How Electrical Transformers Work. [online] MEP Academy. Available at: https://mepacademy.com/how-electrical-transformers-work/ [Accessed 7 Nov. 2025].

Science Facts (2022). Magnetic Field Lines: Definition, Direction, & Properties. [online] Science Facts. Available at: https://www.sciencefacts.net/magnetic-field-lines.html [Accessed 7 Nov. 2025].

The Engineering Mindset (2021). AC Electrical Generator Basics - How electricity is generated. [online] www.youtube.com. Available at: https://www.youtube.com/watch?v=WhATjUHgzxQ. [Accessed 10 Nov. 2025]