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Question about ideal voltage and current source.

An ideal voltage source has zero resistance to produce a constant voltage.
An ideal current source has infinite resistance to produce a constant current.

The question is why does the resistance have to be assumed zero to produce a constant voltage? And why does the resistance have to be infinite to produce a constant current?

I am trying to think of it in terms of V=IR to see how that is possible.

For Ideal Voltage Source, if R = 0, V = IR, then V = 0.
For Ideal Current Source, if R = infinity, the I = V/R, so I = 0.

How does that make it produce the constant amount of quantity it needs to?

Also, when it says resistance, does it refer to the internal resistance of the source?

I read that for non-ideal sources, the source would have a finite non-zero resistance, wherein it is in series for a voltage source and parallel for a current source. If it is referring to the internal resistance, it makes sense if it is in series, but how can an internal resistance be parallel to it? Shouldn't it be intrinsic to the source?
(edited 10 years ago)
Reply 1
Avatar for Joinedup
Joinedup
20
Original post by Alpha-Omega
An ideal voltage source has zero resistance to produce a constant voltage.
An ideal current source has infinite resistance to produce a constant current.

The question is why does the resistance have to be assumed zero to produce a constant voltage? And why does the resistance have to be infinite to produce a constant current?

I am trying to think of it in terms of V=IR to see how that is possible.

For Ideal Voltage Source, if R = 0, V = IR, then V = 0.
For Ideal Current Source, if R = infinity, the I = V/R, so I = 0.

How does that make it produce the constant amount of quantity it needs to?

Also, when it says resistance, does it refer to the internal resistance of the source?

I read that for non-ideal sources, the source would have a finite non-zero resistance, wherein it is in series for a voltage source and parallel for a current source. If it is referring to the internal resistance, it makes sense if it is in series, but how can an internal resistance be parallel to it? Shouldn't it be intrinsic to the source?


the property of an ideal voltage source is that it has the same pd across it's terminals regardless of the quantity of current going through it.
What amount of resistance has a guaranteed voltage drop of zero at any current?

The properties of an ideal current source is that it has the same current going through it regardless of the pd across it's terminals. What amount of resistance doesn't pass different amounts of current at different voltages?

Since ideal current source is slightly harder to grasp, I'll give another hint. you might want to contemplate how you could make a 0A ideal current source. i.e. Always 0A , any pd. (no fancy components required)
Reply 2
Avatar for natninja
natninja
21
Original post by Alpha-Omega
An ideal voltage source has zero resistance to produce a constant voltage.
An ideal current source has infinite resistance to produce a constant current.

The question is why does the resistance have to be assumed zero to produce a constant voltage? And why does the resistance have to be infinite to produce a constant current?

I am trying to think of it in terms of V=IR to see how that is possible.

For Ideal Voltage Source, if R = 0, V = IR, then V = 0.
For Ideal Current Source, if R = infinity, the I = V/R, so I = 0.

How does that make it produce the constant amount of quantity it needs to?

Also, when it says resistance, does it refer to the internal resistance of the source?

I read that for non-ideal sources, the source would have a finite non-zero resistance, wherein it is in series for a voltage source and parallel for a current source. If it is referring to the internal resistance, it makes sense if it is in series, but how can an internal resistance be parallel to it? Shouldn't it be intrinsic to the source?


The resistance has to be 0 or otherwise the voltage would change depending on the current through the source i.e. the source has no internal resistance. Similar for a current source.

for the last bit, that's how you would model them using ideal components. In the real component yes it is intrinsic to the source but we don't like that. The way they teach circuits at school leads to a lot of confusion here... You cannot have an ideal circuit that consists of a voltage source and a wire connecting the terminals with nothing in between for example.

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