Help with simple electronics circuit

Discussion in 'General Electronics Discussion' started by blablae, Feb 20, 2013.

1. blablae

2
0
Feb 20, 2013
Hello
I am taking introductory course in electronics and I am having some difficulties understanding some things.

The voltage source in a electronic circuits provides energy to drive electrones around the circuits which is what we call current.

How I see it :

In circuit A (I attached simple circuit drawing)

The current goes from the negative pole of the voltage source all the way through the resistor to the positive pole. And it keeps doing that untill the voltage source is emptied of its energy.

In circuit B I cant understand what is happening. How is the current supposed to reach the resistor ? The current goes from the negative pole and then meets a sign/symbol which I dont quite understand what is yet.

I believe this symbol is "ground"? But I havent yet understand what its purpose is.

Also I am confused with definitions with current/electrones and which direction they go in a circuit. I read in an article on the internet that by definition the elctrones in a circuit go from the negative to the positive pole, but the current goes from the positive to the negative pole.

However in my electronics fundamentals electronics course book the current is depicted in the circuit as going from the negative pole to the positive. What is correct? Did misunderstand ?

Any help is much appreciated

Thank you

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2. BobK

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Jan 5, 2010
The ground symbol indicates a common connection. I.e. all things that connect to a ground are connected together. So the two circuits you show are identical, just represented differently. The symbol you show is actually an earth ground, which means they should be connected to rod driving into the earth, but the use of this symbol is abused and it generally does not really mean an earth ground, just a common connection point, which is usually the negative side of the power supply.

The convention (sometimes not followed) is that current goes from postive to negative. The fact that electons are actually moving in the opposite direction is irrelevant, just stick with one diretion or the other always, and things will work out.

Bob

3. Harald KappModeratorModerator

11,513
2,651
Nov 17, 2011
In any schematic you can find signals (nets) that seem to be unconnected. If these signals have a name, then by convention all sgnals having the same name are connected, even if there is no visible line on the schematic.

The ground symbol is only a special case of this, as Bob has explained.

4. blablae

2
0
Feb 20, 2013
I think understand this, Thank you.

I have more things to ask. I am doing a task in which I am given a circuit which I dont quite understand. The circuit is "circuit A" in the picture I attached.

Can circuit A be drawn in a different way? Same as earlier I dont see how the current circuits, and in circuit A there are two voltages, where voltage V0 is kind of alone at the edge/cornor dont understand what it is doing there.

I dont understand how this circuit is supposed to work or what the idea behind it is.

How does current flow in circuit A ?

In my task I am told that V1 is the entrance voltage that is quickly changed from 0 Volt to 5 Volt. The resistance R works against the current that will go from V1 to V0. Here I dont understand why it goes from V1 to V0 ?

It also says that the current through R will add or remove charge from the capacitor and then something more is written which I dont understand. The task is about the charging the capacitor and measuring voltage with respect to time : V0(t) = Vs(1 - e^(-t/RS)) where Vs is V1 I believe. This formula is also written in the book but is given as v = Vf(1 - e^(-t/RS)) where Vf is final voltage.

So circuit A I dont understand.

However I think I understand how to charge and uncharge a capacitor in a simple circuit with one voltage source and one capacitor. A simple circuit such as circuit B in the picture I attached.
In this circuit electrons flow from one of the plates through the voltagesource to the other plate. The capacitor will be charged when the voltage in the capacitor is equal to the source voltage. And the formula v= Vf(1 - e^(-t/RS)) is easy to use in this circuit. Simple the way I see it but in circuit A not so simple.

Most important questions I would like answered:

How does current/electrons flow in circuit A? Like the earlier circuit it is not connected.

How the electrons flow from one of the plates of the capacitor to the other in Circuit?

Can circuit A be drawin in a different way? Is circuit A the same circuit as circuit A-2 ?

For me it would be alot simpler if the V0 voltage was removed from circuit A and V1 was connected to earth ground. What is purpose of having two voltage sources and what is V0 purpose ? I am confused with multiple voltagesources in a circuit.

any input on circuit A is much appreciated

Thank you

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5. davennModerator

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Sep 5, 2009
They dont, its an open circuit. In a DC circuit as you have shown, you get a build up of electrons on the lower plate and a defficiency of electrons on the upper plate ( the one connected to the battery +) you would see a brief current flow as the capacitor charges
as the electront move into one plate and away from the other plate

No, in A, the capacitor is going to ground ... aka the negative of the power supply
in A2 the capacitor is in parallel with the resistor

both circuits are missing some vital info, that for an experienced technician would be assumed

this is a better representation

Thank you [/QUOTE]

its really showing you that V1 and V0 are different, I suspect this would have been better represented as Vin and Vout

Dave

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