The discussion centers on the concept of DC equivalent circuits, specifically the idea of replacing resistors with cells. Participants explore the reasoning behind this equivalence, the implications of voltage drops across resistors, and the nature of power sources versus dissipators in electrical circuits.
Participants express differing views on the equivalence of resistors and cells, with some suggesting conditions under which they may behave similarly, while others assert that they are fundamentally different components. The discussion remains unresolved regarding the practical application of these concepts.
Participants highlight the importance of initial conditions and the specific context in which the equivalence is considered, indicating that the analysis may not hold universally across different scenarios.
vk6kro said:Maybe you should give an example.
Resistors are not the same as voltage sources, but there are similarities.
If a resistor has a large current passing through it, and you place a much larger resistor across the first one, it will have almost no effect on the voltage across the first resistor and will draw a current as if it had been placed across a voltage source.
vk6kro said:You really need to accept the initial conditions for this to work.
Suppose you had a radio that normally ran off 6 volts from 4 "C" cells.
All you had was a 12 volt battery from a car and two 12 volt rear lamps, also from a car.
The lamps draw 3 amps if placed in series across the 12 V battery. So they have a resistance of 4 ohms total or 2 ohms each.
Across each of them is 6 volts. Now if you connect the radio across one of the lamps, it may draw 50 mA or 10 mA depending on how loud the sound was.
If it drew 50 mA at 6 volts, it has a resistance of 6 V / 0.05 A = 120 ohms.
If you put this across one of the lamps, the resistance of the pair would be 1.97 ohms.
This would cause the voltage to drop from 6 volts to 5.95 volts.
So, the car lamp is behaving just like a source of EMF and giving a very stable voltage.
It is very inefficient to do this, but it gives a result.
amaresh92 said:please could you tell me that ,In dc equivalent circuit the resistors are replaced by cell.why is it so?though the resistor gives voltage drop where cell provides a emf then how does these are equivalent?
thanks.
sophiecentaur said:They are not really equivalent at all - despite the fact that you can measure a potential difference across them both. A cell is a source of power and a resistor is a dissipator of power. All you can say is that, in any circuit, under a specific set of conditions, the values of voltage and current in that circuit will be the same IF you replace the resistor by a cell with the same voltage. It can even apply for AC, if your 'voltage source' has the right signal on it.
What I'm saying is that the concept of 'equivalent circuit components' is for analysis and not really intended to be applied practically. I remember, when I was very young, looking at a complicated equivalent circuit for a transistor, with all sorts of current and voltage sources in it, and being laughed at (much derision from fellow students, of course), when I suggested you could "build" a transistor that way.