# Fundamental questions

Discussion in 'General Electronics Discussion' started by ramit36, Oct 9, 2012.

1. ### ramit36

17
0
Oct 9, 2012
Current is defined by “rate of change of charge”. If this is the only definition of current then how come direct current is not always 0 according to the definition of current because in dc rate of change of charge is 0 i.e. it doesn't change with time?

2. ### ramit36

17
0
Oct 9, 2012

Ans: The answer is actually very simple. Let’s use an analogy and that is speed which is similar to current by basic definition i.e. these both are rate of change of something.

Speed = distance/time
For example: speed = 40 km/hr

That means, say your bike is covering the distance with the above speed. So, it will cover a distance of 40 km in one hour. Now, to understand the concept completely let’s go to the 2 hour. If we see the 2nd hour then we will find out that your bike has covered now total of 80 km and that happened in the period of 2 hour. So, the total distance covered is doubled i.e. from 40 km to 80 km but if you see you will find that the speed is still constant no matter how many hours have passed i.e. the rate of change of distance with respect to time is constant while total distance covered is changing with time with the constant speed.
So, it is only the total distance which is actually changing with time and not the speed. Similarly, in electronics it’s the total charge which actually changes (total charge i.e. total number of electrons passed through a wire) with time with the constant current in dc.

3. ### BobK

7,682
1,689
Jan 5, 2010
Think of it like counting the cars that are going by on a highway. You could express this as cars per minute. Current is the the same thing, you count the coulombs of charge that go through a point in a circuit and cuirrent is coulombs per second.

Direct current has the cars going in the same direction all the time. Alternating current has them turn around and go the other way after a certain time period. In either case, you can measure the number of cars that go by in a time period.

Bob

4. ### ramit36

17
0
Oct 9, 2012
That was a nice difference between ac and dc. And by the way Bob can you see the answer that i wrote for my own question, i mean is that visible to you. Since i am a new user that's why i am just confirming whether my post in which i answered my question is visible to every body or not. And thank you for the reply to my question.

5. ### BobK

7,682
1,689
Jan 5, 2010
Yes, I can see it now, but it was not there yet when I was writing my response, we were writing at the same time.

Bob

6. ### ramit36

17
0
Oct 9, 2012
Yes, that could be the reason but nice to hear that it is visible.  