understanding concept of voltage and current

Discussion in 'General Electronics Chat' started by sharanbr123, Sep 29, 2014.

1. sharanbr123 Thread Starter Member

Sep 29, 2014
49
0
Hello All,

I studies Circuit theory almost 2 decades back and hardly applied it in my work. Mostly I tend to work in digital area.

I am getting a lot confused with the concept of voltage & current. If I may explain with an example.

I have 2 series resisters of 2 ohm and 3 ohm connected to a 10 v source. The voltage drop across first resister is 4 V and across second resister is 6 V.

Since the current flowing through 2 ohm and 3 ohm resister is same, how should I interpret what exactly voltage is?. Is it the rate at which current flows through R1 and R2 is different and is related to voltage drop?

Similarly, if I keep increasing the number of resisters in the series then the current comes down. Does that mean that current supply reduces as resistance load on a voltage source increases?

Similarly, if number of series resisters are decreased than current increases. Does that mean that rate at which the current flows across the resister increases as the number of resisters decreases?

Thanks a lot in advance ...

2. MrChips Moderator

Oct 2, 2009
12,636
3,454
Stop using the term "rate".
Rate means a change per unit time.

Go back to Ohm's Law:
The current I through a resistor R is directly proportional to the applied voltage V and inversely proportional to the resistance.

In mathematical terms:
I = V/R

In each of your examples above, simply apply Ohm's Law.

For a fixed voltage V, calculate the current I in a series circuit. Knowing the current I in each resistive element, you can determine the voltage across each element by applying a corollary of Ohm's Law, V = IR.

3. Kermit2 AAC Fanatic!

Feb 5, 2010
3,851
967
voltage is a force.
current is a flow.
the greater the resistance, the more force it takes for current to flow.

4. sharanbr123 Thread Starter Member

Sep 29, 2014
49
0
Thanks, MrChips & Kermit2. Slowly getting hang of it ...

Instead of passive components, assume I am using a MOS type of transistor. As we know, when transistor is ON, the current flows through the channel.
Now, if I increase the channel width of the transistor keeping eveyrthing else same, my intuition tell me that current increases.

So, what happened in this case, did the resistance decrease due to increase in the channel width?

5. crutschow Expert

Mar 14, 2008
13,501
3,375
It's all a matter of Ohm's law. Thus if you have a 2 ohms and 3 ohms in series with a 10V source, the current would be I = V/R = 10V / (2Ω+3Ω) = 2A. Then the voltage across the 2Ω resistance is V = I*R = 2A x 2Ω = 4V and across the 3Ω resistor is 2A x 3Ω = 6V. The sum of those is 10V, equal to the supply voltage of course.

From that you should be able to answer the rest of your questions.

6. MrChips Moderator

Oct 2, 2009
12,636
3,454
Yes, the resistance decreases.

But when you get to semiconductor devices, a transistor for example, you cannot apply Ohm's Law. That is because the device is non-linear, i.e. the current does not vary proportionately with applied voltage. Another way of looking at it, the resistance is not constant. The resistance changes with applied voltage.

Nov 30, 2010
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