You are correct in that the question refers only to a non-specific connection. Since it states the condition that current remains unchanged even though the resistance increases, implies it is in a circuit... but with constant current, the rest of the circuit has no bearing on the relationship with resistance and voltage (drop) across the connection. The only possible fallacy in this theory is if the current remains zero.... in which case, the voltage drop would also remain zero.
ohms law proportional
- Thread starter johnstanleyelectric
- Start date
- Status
As part of an international corporation I have to be a little cognisant of world time zones.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I can agree with that situation.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Which just to confuse us Yanks, stands for British Summer Time, not British Standard Time.
Tapatalk!
John120/240
Senior Member
- Location
- Olathe, Kansas
Besoeker When do you start and stop British Summer Time ? Here it is March 9 to November 2. Except in Arizona, Hawaii, & parts of Indiana.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Actually it is not a specific date like you mentioned, those maybe were the days for this year. The change is always at 02:00 on a Sunday - second Sunday of March and first Sunday of November.
It's always on a Sunday morning at 02:00 so it will be a different date each year. I think the "spring forward" is a week or so earlier than US EST and "fall back" is the same weekend.
The changing time difference keeps you on your toes.............
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
140609-0751 EDT
The question in the OP is perfectly clear. It states the current is a constant. That means a constant current source and it does not matter what the circuit is that generates the constant current.
In a practical circuit that most of you work with you assume the source is a constant voltage, but in the real world in an electrical distribution system it is not real constant, but for convenience of many calculations it is assumed constant. If this practical circuit is an ideal constant voltage of 240 V with a load resistor of 240 ohms and a series switch contact with a voltage drop of 0.01 V, a contact resistance of about 0.01 ohms, then if the contact resistance increases to about 0.02 ohms what is the change in current? What is the change in voltage across the contact?
The question in the OP is a very good question and doesn't even require close reading to understand it. It is stated in black and white that the current is constant, and it represents an approximation to a real world problem.
.
The question in the OP is perfectly clear. It states the current is a constant. That means a constant current source and it does not matter what the circuit is that generates the constant current.
In a practical circuit that most of you work with you assume the source is a constant voltage, but in the real world in an electrical distribution system it is not real constant, but for convenience of many calculations it is assumed constant. If this practical circuit is an ideal constant voltage of 240 V with a load resistor of 240 ohms and a series switch contact with a voltage drop of 0.01 V, a contact resistance of about 0.01 ohms, then if the contact resistance increases to about 0.02 ohms what is the change in current? What is the change in voltage across the contact?
The question in the OP is a very good question and doesn't even require close reading to understand it. It is stated in black and white that the current is constant, and it represents an approximation to a real world problem.
.
K8MHZ
Senior Member
- Occupation
- Electrician
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
In the real world, yes. On a test question where stated explicitly as a problem condition it happens all the time.
Tapatalk!
K8MHZ
Senior Member
- Occupation
- Electrician
In the real world, yes. On a test question where stated explicitly as a problem condition it happens all the time.
Tapatalk!
How well I know.
In my apprenticeship, we were taught that the 'correct' answer what we were looking for on a test. Not the right answer, the most accurate answer or the most likely answer. Only the answer deemed correct by the author of the test would give us points toward our score. Along with real world scenarios we were also taught how to answer tests. Some think the two would go hand in hand, but anyone that has taken many multiple guess tests can tell you otherwise.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
140609-1703 EDT
In the example I suggested, 240 V with a 240 ohm load in series with a low resistance contact, the calculated current values are 0.999968335 A and 0.999916674 A. The change in current is 0.000042 A, or a fractional change of 0.000042, and a percent change of 0.0042. Most common digital ammeters do not have sufficient resolution to display the change. Thus, for most purposes this is a constant current source.
If the contact resistance changed to 10 ohms from the 0.01 ohms the change in current is only about 0.999968335 - 0.959961692 = 0.04 or 4%. As an approximation this is a somewhat constant current source.
In the real world you encounter this type of circuit in most switched applications. Very low resistance in the switching component, relatively a much larger resistance or impedance in the switched load. Thus, the OP question was a good real world illustration.
.
In the example I suggested, 240 V with a 240 ohm load in series with a low resistance contact, the calculated current values are 0.999968335 A and 0.999916674 A. The change in current is 0.000042 A, or a fractional change of 0.000042, and a percent change of 0.0042. Most common digital ammeters do not have sufficient resolution to display the change. Thus, for most purposes this is a constant current source.
If the contact resistance changed to 10 ohms from the 0.01 ohms the change in current is only about 0.999968335 - 0.959961692 = 0.04 or 4%. As an approximation this is a somewhat constant current source.
In the real world you encounter this type of circuit in most switched applications. Very low resistance in the switching component, relatively a much larger resistance or impedance in the switched load. Thus, the OP question was a good real world illustration.
.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Motor circuit would be one exception.
K8MHZ
Senior Member
- Occupation
- Electrician
I know where you are going with that, and have seen it mathematically, but not in 'the real world'. By that I mean I have never actually measured a motor circuit that had an incipient contact failure and seen exactly the same current as it would have with that contact in perfect condition. For that to happen, there would have to be a perfect balance between heat loss caused by reduced REMF and the opposition to current flow happening at the contact point. Even it that could be achieved, the contact resistance would increase over time due to plain physics, drastically reducing the current to the point of eventual failure and the loss of current altogether.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Usually current goes up when there is resistance in the supply to a motor. It certainly never remains the same. A lightly loaded motor may defy the rules to some extent, but one that is heavily loaded (for it's rating) will draw more if you lower the voltage.
I should add this may not be true for all motors, but should be the general rule for AC induction motors, which is what I run into most of the time.
Last edited:
But Gar is right. Answer the question asked. Not the one you think was asked.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Understand that this is a theoretical question, and not necessarily what would happen in the real world. Most supplies of power are voltage sources (like batteries, for example) where increasing the resistance lowers the current and the voltage across the network remains virtually the same.
There are, however, power sources which behave as current sources for much of their range; they strive to keep the current constant and let the voltage vary. One such source is a PV (photovoltaic) module. Over most of a PV module's power output range, given a constant flux of irradiance, increasing the resistance will increase the voltage across the resistor while the current remains about the same. See http://sargosis.com/articles/scienc...ting-the-iv-and-pv-curves-for-a-solar-module/
Last edited:
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
140617-1051 EDT
Going back to the original post the question reads ---
"If the contacted resistance of a connection increases, and the current of the circuit (load) remains the same, the voltage dropped across the connection will ________."
The question is about the contact resistance of a connection. For most applications this means the contact resistance is quite small relative to the load resistance, and in this case the load resistance can be considered nearly constant. Assuming the source voltage is moderately constant, then for reasonable changes is the contact resistance, the series current thru the connection can be considered reasonably constant. Thus, voltage across the connection increases with increasing contact resistance.
This is not a theoretical question, but a real world situation. For example two wires twisted together, or wire to screw terminal connection.
.
Going back to the original post the question reads ---
"If the contacted resistance of a connection increases, and the current of the circuit (load) remains the same, the voltage dropped across the connection will ________."
The question is about the contact resistance of a connection. For most applications this means the contact resistance is quite small relative to the load resistance, and in this case the load resistance can be considered nearly constant. Assuming the source voltage is moderately constant, then for reasonable changes is the contact resistance, the series current thru the connection can be considered reasonably constant. Thus, voltage across the connection increases with increasing contact resistance.
This is not a theoretical question, but a real world situation. For example two wires twisted together, or wire to screw terminal connection.
.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
- Status