Physics Lesson 16.17.2 - Transformers

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Welcome to our Physics lesson on Transformers, this is the second lesson of our suite of physics lessons covering the topic of Power in an Alternating Circuit. Transformers, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.


It is already known that electric power is the product of current and voltage. As explained earlier, electric power P, is the amount of electrical energy W produced or delivered in the unit time. Thus, we have

W = P ∙ t = I ∙ ∆V ∙ t

Since power is related to energy, we often use terms like "power station" or "power source" instead of "energy station" or "energy source". Therefore, we will henceforth use the term power but keeping in mind the energy.

Electricity is produced in power stations through generators. It must be transported from power stations to the end user (at our homes for example). Mains electricity transportation system is used for this purpose. It includes cables, poles, cabins and especially transformers, which are passive electric devices used to change the value of voltage in a circuit. Transformers use electromagnetic induction to transport electricity. We will explain below why transformers are so important in electricity transportation system.

Power stations produce large amounts of energy in the unit time. Typical values range from hundreds of megawatt hour (MWh) to several gigawatt hour (GWh) of electricity. To carry this huge amount of energy throughout the transportation system, we can use two methods:

  1. At high current and low voltage
  2. At high voltage and low current

The first method is not suitable, as high current increases the temperature of cables and a lot of electricity is lost during the transportation because it is dissipated into the environment as heat energy. Therefore, we must increase the value of voltage during the transportation and then decrease it near the end user premises to make the electricity useable. Hence, during this process, we use two types of electric transformers: step-up transformers which are voltage increasers and step-down transformers, which are voltage reducers. Let's have a closer look at both of them.

Electricity is usually generated at 11kV in power stations although the generation voltage may vary in the range between 11kV and 33kV. Then the voltage increases during transportation to 220kV, 400kV or 765kV depending on the country. Step-up transformers are installed at high voltage poles to achieve this. Then, the voltage decreases to 20kV near residential areas (in electric substations) using step-down transformers and then, the voltage decreases further to 110V or 220V in electric cabins (using again step-down transformers) depending on the country. This is the value of voltage used in most part of daily activities.

Some electrical appliances require even lower voltages. For example, a TV needs 12V to operate. Hence, step-down transformers are also installed in specific appliances that require low operating voltages.

You have reached the end of Physics lesson 16.17.2 Transformers. There are 3 lessons in this physics tutorial covering Power in an Alternating Circuit. Transformers, you can access all the lessons from this tutorial below.

More Power in an Alternating Circuit. Transformers Lessons and Learning Resources

Magnetism Learning Material
Tutorial IDPhysics Tutorial TitleTutorialVideo
16.17Power in an Alternating Circuit. Transformers
Lesson IDPhysics Lesson TitleLessonVideo
16.17.1Electric Power as a Rate of Electric Energy Transfer
16.17.3How does a Transformer Work?

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