WHY does source voltage drop with the increase in current?

Beetle_X

Joined Nov 2, 2012
52
There are a lot of guides that state that this is caused by internal resistance of the source but not why. Here's the formula V = emf − Ir.Does increasing current increase resistance so the resistance is not a constant?

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Audioguru again

Joined Oct 21, 2019
1,306
Increasing the current causes a higher voltage drop across the internal resistance which reduces the source voltage.
Some resistances increase their resistance when the current is increased caused by heating.

Papabravo

Joined Feb 24, 2006
13,560
No. The source resistance is an inherent property of the voltage source. It is nearly constant, and as the current increases, that constant resistance drops more of the source voltage. Look at the typical discharge curves for different types of batteries and notice what happens as the battery reaches it's end of life.

Beetle_X

Joined Nov 2, 2012
52
Thanks for both of the replies! I can't find a clear reason for this with batteries. What about a generator?

nsaspook

Joined Aug 27, 2009
7,192

Notice he says electrical (electric field) potential energy not current potential energy.

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Papabravo

Joined Feb 24, 2006
13,560
Thanks for both of the replies! I can't find a clear reason for this with batteries. What about a generator?
The reason is right in front of you but you refuse to acknowledge it. There is no such thing as an ideal voltage source. The source resistance is the deviation of a real source from ideality. In this regard, generators are no different than batteries.

Papabravo

Joined Feb 24, 2006
13,560
Do you really think he will believe his lyin' eyes as he watches the video?

nsaspook

Joined Aug 27, 2009
7,192
Do you really think he will believe his lyin' eyes as he watches the video?
We can only hope and lead them to water, you can't make them drink.

GetDeviceInfo

Joined Jun 7, 2009
1,702
Thanks for both of the replies! I can't find a clear reason for this with batteries. What about a generator?
Same, but the term becomes impedence.

WBahn

Joined Mar 31, 2012
25,590
Thanks for both of the replies! I can't find a clear reason for this with batteries. What about a generator?
WIth batteries, the underlying reasons is in the details of the chemical reactions. With no load, the terminal voltage builds up to its open circuit value when it is sufficiently high enough to suppress further chemical reactions in the cell. But without the chemical reactions, no current can flow. So for there to be any current, the voltage has to drop so that the chemical reactions can start up again. With a small voltage drop, only a small amount of chemical reaction can take place (most of it is still suppressed). So to get more current, the voltage has to be even lower. Hence, more current, lower voltage. This happens naturally. Let's say that you have a battery that has an open circuit voltage of 100 V and that it has an internal resistance of 1 Ω. That 100 V is the result of a charge separation resulting from the chemical reactions and at 100 V those reactions stop and the charge separation on the terminals maintains that 100 V. Now lets say you put a 9 Ω load across the terminals and let it sit there and settle down. You will have 10 A flowing and a terminal voltage of 90 V. What happens in the load resistance goes up a bit? The current will drop but the chemical reactions are still producing the 10 A of current it can produce with a terminal voltage of 90 V, so the excess current will build up on the terminals, thus raising the terminal voltage to a bit more than 90 V. If the load resistance were to go down, the load would draw more than the 10 A being produced by the cell with a terminal voltage of 90 V and the terminal voltage will drop, but this will result in more chemical reaction occurring and the output increasing as a result. The amount of charge associated with the charge separation the produces the terminal voltage is truly minuscule, so these changes occur almost instantly.

With generators it generally with the physical resistance of the components in the generator. You can design active power supply circuits that monitor the output voltage and increase or decrease the driver signal in order to keep the voltage at the terminal constant regardless of current draw. If the supply uses a symbol proportional controller to do this, then the effective output impedance will be small, but not zero because an error signal is needed in order to produce the drive signal. But if the controller has an integral term, the error will be driven to zero and the supply will look like it has zero output impedance, at least in steady state.

MisterBill2

Joined Jan 23, 2018
5,804
Thanks for both of the replies! I can't find a clear reason for this with batteries. What about a generator?
Effective source resistance is found in ALL voltage sources. The amount varies, but it is always present. In some situations it may not matter, but source resistance is intrinsic. A well designed regulated power supply can compensate for it's internal source resistance and provide an effective equivalent of a zero source resistance, but that is as close as you can get, and it is not simple.

Audioguru again

Joined Oct 21, 2019
1,306
The 9V alkaline battery used in my smoke detector produces 8V with a current of 100mA when new. After being used for 1 year its voltage drops to 6V when its current is only 15mA.
Therefore its internal resistance is (9V - 8V)/100mA= 10 ohms and its internal resistance after 1 year is (9V - 6V)/15mA= 200 ohms.

WBahn

Joined Mar 31, 2012
25,590
The 9V alkaline battery used in my smoke detector produces 8V with a current of 100mA when new. After being used for 1 year its voltage drops to 6V when its current is only 15mA.
Therefore its internal resistance is (9V - 8V)/100mA= 10 ohms and its internal resistance after 1 year is (9V - 6V)/15mA= 200 ohms.
What is it's open-circuit voltage after one year?

Audioguru again

Joined Oct 21, 2019
1,306
An Energizer 9V alkaline battery has a guaranteed 5 years shelf life. After only one year its open circuit voltage is 9V.
A Chinese Super-Heavy-Duty 9V battery has no guarantee and is seen leaking in its package in the store after a couple of weeks in the boat.

MisterBill2

Joined Jan 23, 2018
5,804
An Energizer 9V alkaline battery has a guaranteed 5 years shelf life. After only one year its open circuit voltage is 9V.
A Chinese Super-Heavy-Duty 9V battery has no guarantee and is seen leaking in its package in the store after a couple of weeks in the boat.
"Heavy Duty" in a battery name means it is able to deliver more current for a while. Sort of the opposite of long life. And of course there are always the really poor quality batteries that will leak and give other sorts of grief even before you buy them.

Audioguru again

Joined Oct 21, 2019
1,306
"Super-Heavy-duty" batteries from Asia use 110 years old carbon-zinc technology. They have low cost, low current and short life.

WBahn

Joined Mar 31, 2012
25,590
An Energizer 9V alkaline battery has a guaranteed 5 years shelf life. After only one year its open circuit voltage is 9V.
A Chinese Super-Heavy-Duty 9V battery has no guarantee and is seen leaking in its package in the store after a couple of weeks in the boat.
But you were talking about a battery that had been in use for a year, not sitting in a package on the shelf.

shortbus

Joined Sep 30, 2009
7,623
But you were talking about a battery that had been in use for a year, not sitting in a package on the shelf.