Popular Courses
Beginner – Linear Regression
Introduction
Beginner – Decision Trees
Introduction
Beginner – Neural Networks
Introduction

Sign in to Curvingo

or

New Account     Reset Password     

Plans

Details
Alternating Current
Introduction
AC Voltage Applied to a Resistor
Representation of AC Current and Voltage by Rotating Vectors — Phasors
Practice: RMS Values and Phasors
AC Voltage Applied to an Inductor
AC Voltage Applied to a Capacitor
Practice: Reactance of an Inductor and a Capacitor
AC Voltage Applied to a Series LCR Circuit
Practice: Series LCR Circuit and Resonance
Power in AC Circuit: The Power Factor
Transformers
Practice: Power Factor and Transformers
Summary
Curvingo · Made in India

Share

Send us Feedback

Please let us know if you have any feedback and suggestions, or if you find any errors and bugs in our content.

Sorry, your message couldn’t be submitted. Please try again!

Thanks for your feedback!

Reset Progress

This will delete your progress and chat data for all chapters in this course, and cannot be undone!

Glossary

Select one of the keywords on the left…

Alternating CurrentIntroduction

Reading time: ~10 min

It is a monsoon evening in Riya's flat. The tube light glows, the fan turns, her phone charges and her grandfather is tuning his old radio to his favourite station. Every one of them is plugged into the same socket in the wall, and that socket does something no battery does.

So far in this course, every source has given a direct current (dc): a current that does not change direction with time. The mains supply in our homes and offices is different. Its voltage varies like a sine function of time. Such a voltage is called an alternating voltage (ac voltage), and the current it drives is an alternating current (ac current). Most of the electrical devices we use today run on ac.

Here is the mains voltage in slow motion.

Move through time and find these instants.

t = ${t} ms

The blue curve repeats itself every ms, so it goes through complete cycles every second: the mains frequency in India is 50 Hz. A bulb gets the most power at each peak and none at each zero, a hundred times a second, far too fast for your eye to notice.

Why Not Just Use DC?

At the end of the nineteenth century there was a famous quarrel. Thomas Edison wanted the world wired for dc. George Westinghouse was convinced that ac was the key to the electrical future; he founded the company named after him and hired the inventor Nikola Tesla, who gave us the rotating magnetic field, the induction motor and the polyphase system of ac power. The SI unit of magnetic field, the tesla, is named in his honour.

Ac won, and nearly all the electrical energy sold by power companies is transmitted and distributed as ac. What is the main reason?

Ac is safer to touch than dc
Ac voltages can be stepped up or down easily and efficiently with transformers
Ac can be stored in batteries
Ac travels through wires faster than dc

A transformer (Section 7.8) changes an ac voltage from one value to another with very little loss, and that lets electrical energy be sent economically over long distances. Ac circuits also have properties we use every day: whenever you tune a radio to a station, you are using one of them (Section 7.6).

In this chapter you will find out

  • how an ac behaves in a resistor, and what its rms value is,
  • how rotating arrows called phasors keep track of phase,
  • why inductors and capacitors limit an ac without using any power,
  • what happens when all three are joined in series, and what resonance is,
  • what the power factor means, and how a transformer works.

Note

Read literally, "ac voltage" means "alternating current voltage" and "ac current" means "alternating current current", which is odd. But the abbreviation ac, for any electrical quantity that varies like a sine, is so universal that we use it too. And "voltage" always means the potential difference between two points.

Sina