Basic Circuit Theory

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vani4373

Joined Jun 30, 2005
1
If we assume two ideal cells of 1 V connected in parallel with 1 ohm resistance and calculate the current flowing through the resistance through following two methods:
(i) Thevenin's Theorem
(ii) Superposition Theorem
The results obtained are 1 A and 0 A respectively. Logically it should be 1 A but where is the anamoly. Please help me to figure it out?
 

Smack

Joined Jun 25, 2005
6
Originally posted by vani4373@Jun 30 2005, 09:15 AM
If we assume two ideal cells of 1 V connected in parallel with 1 ohm resistance and calculate the current flowing through the resistance through following two methods:
(i)  Thevenin's Theorem
(ii)  Superposition Theorem
The results obtained are 1 A and 0 A respectively. Logically it should be 1 A but where is the anamoly. Please help me to figure it out?
[post=8838]Quoted post[/post]​
voltage sources in parallel aren't cumulative, so superposition doesn't work?
 

n9xv

Joined Jan 18, 2005
329
When you use Thevinins theorem, you (mentally) short the voltage source and open the load end of the circuit and compute the resistance back towards the "shorted" voltage source. You then look at the voltage across the open load. In this case, the only resistance was the 1-ohm load resistor across the voltage source which "got shorted" when you applied Thevinins theorem. In effect, you shorted the source and the load at the same time. You cant look at the voltage across the open load because the load and the source are effectively the same thing. Your mathimatical computations are indeed correct but the theorem in this case does'nt apply because there can be no thevinin voltage across a thevinin resistance when only one resistance exist in the circuit to begin with and that one resistance is connected directly across the voltage source.
 

mozikluv

Joined Jan 22, 2004
1,437
hi,

let me add something about "superposition"

it is known that in a network with 2 or more sources the current or voltage for any component is the algebriac sum of the effects produced by each source acting seperately. so for this theorem to work, all components must be linear and bilateral in order for you to superimpose current & voltages.

it is also known that by being linear it means the current is proportional to the applied voltage. hence the current computed for different source voltages can be superimposed. by being bilateral the current has the same amount for opposite polarities of the source voltage. then we can combine algebraically the values for opposite directions of current.

networks containing resistors, capacitors and air core inductors are generally linear & bilateral. likewise these are also passive components hence they can't amplify or rectify.

hope this helps. :D

moz
 
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