# How to teach concepts of AC & DC?

### What is DC?

When a teacher explains about DC, he generally says that it is a smooth and uniform voltage or current without any variations, w.r.t. time. For example, the battery voltage, etc.

### Simple explanation of DC

In DC, there is nothing between ‘D’ & ‘C’ in terms of English alphabets. It means that there is no ** variation or nothing or no change** between D-C, hence we say that DC is smooth…! So, DC means – Disturbance Cancelled. Just saying! Actually DC means Direct Current…. You know that!

### What is AC?

AC means alternating current. AC voltage means alternating voltage. AC current means alternating current – its just a convention to say like that! It means that it changes its polarity with respect to time. Also it doesn’t remain steady i.e. its amplitude always changes with respect to time.

### Simple explanation of AC

There is a ‘B’ between ‘A’ & ‘C’. ‘B’ means bumpy. Bumpy means having variations. So AC always contains ups and downs i.e. its magnitude and polarity always changes with respect to time. How simple to understand! So the behaviour of AC is opposite to DC.

### How to explain KCL?

Do you remember, Newton was sitting in a garden and an apple fell down and he got the idea of gravity. The idea of KCL can be explained in the same way.

Consider a busy, crowded square with large traffic with number of roads connected to it. If the square is treated like a ‘node’ in KCL terminology, then we can say that the sum of all the vehicles coming towards the square will be always equal to the sum of the vehicles going away from the square. This is because number of vehicles will come towards the square and then they will bifurcate on different roads.

This idea is very much equivalent to the fundamental idea of Kirchhoff’s Current Law. I think, just like Newton’s idea, generated from the garden event of falling apple, Gustav Robert Kirchhoff, might have got this idea of KCL, while, perhaps, sitting at a busy square. चौकात बसण्याचा हा ही एक फायदा असु शकतो…! *(Well, it’s only a joke about Kirchhoff. We respect Kirchhoff, very much.)*

**Note:**

*In KCL terminologies, node is a point in a closed circuit, which is connected to at least three or more than three branches or wires or conductors through which different currents are flowing in different directions.*

### How to explain KVL?

This can be explained with a simple example of changing the pencil cells of a TV remote control. Have you ever changed the discharged pencil cells of your TV remote control with new cells? How do you insert the new cells in remote control?

It’s like connecting them in series, i.e. positive-negative-positive-negative. When you connect number of pencil cells in series (like in remote control, see pictures above), their total effective voltage becomes 1.5V + 1.5V = 3V. The connecting style of the pencil cells in above example gives us an idea about KVL.

Thus the basic and simple concept of KVL is like this – When number of voltage sources, say V1, V2, V3… are connected in series, i.e. following the order, positive-negative-positive-negative, then the total effective voltage will be V1 + V2 + V3 … = V.

The sequence ‘+-+-‘ means that you must follow the order of polarities of the cells. Suppose you connect two pencil cells *(with equal individual voltage)* in anti-series i.e. in this style:

**positive-negative-negative-positive**

Then the total effective voltage will be V1 + (-V2) = 0

Nice explanation

Proud to have been your student, sir…

Thanks.

Great explanation.

Thanks sir