Ohm's Law Problem

[Mickey]

RussB has a point about the current and voltage directions. Because it is a DC circuit BUT no polarity markings are shown on the Ammeters and Voltmeters, then we can only calculate the relative size of the currents and voltages (the size of the voltage and current vectors, but not the direction). I think we can assume that the ammeter and voltmeter leads are probably connected to read the same direction, and most likely will read positive values. We know that A1 reads a +4 Amps, and V2 reads a +36 volts, so most likely all the other instruments are also connected to read upscale or positive.

That 116 volt battery must be one more rare device. I don't think I have run across one of those. I guess you could use two 48 volt forklift batteries and two 12 volt car batteries all in series to get close enough.

Battery shows polarity.

 

[maxketcham]

RussB has a point about the current and voltage directions. Because it is a DC circuit BUT no polarity markings are shown on the Ammeters and Voltmeters, then we can only calculate the relative size of the currents and voltages (the size of the voltage and current vectors, but not the direction). I think we can assume that the ammeter and voltmeter leads are probably connected to read the same direction, and most likely will read positive values. We know that A1 reads a +4 Amps, and V2 reads a +36 volts, so most likely all the other instruments are also connected to read upscale or positive.

That 116 volt battery must be one more rare device. I don't think I have run across one of those.

It is all moot, this is a classic electrical problem using a battery. No magic, no smoke and mirrors, what goes in comes out, nothing more nothing less. Trying to read more into the problem will only confuse the issue.

it says 4 amps in, polarity is moot can be plus or minus... 4 amps plus or minus comes out period. there is only one source voltage E=I*R, I=E/R , R=E/I that is OHM's law. The only thing to remember is that current flows out and will be the same polarity (direction) going in, this is DC, not AC or AC+DC. if A1 = +4 Amps and A2= -4 amps then one of them is connected in opposite polarity of the other. if they are connected in the same polarity then they will both read the same current and the same polarity.

Trying to read more into than that is confusing the issue.

Also who said that the battery supply in a text book question was ever going to be a standard battery? they use whatever comes out even in the math to make it easier for the beginning student to grasp the problem, IE, DO IT IN YOUR HEAD, something the calculator generation seems not to be able to do nay more.

 

[Lancie1]

Yes, the battery shows its polarity, but each meter can be connected two ways to read either a + value or a - value. Without markings, we only really know the polarity of the 2 given meters, A1 and V2. We have to ASSUME that the other meters are also connected to read positive values. What if they are not?

Yes, imposing a real-life situation on this obviously imaginary ideal-world student problem is getting too techincal. But I can't count the number of times that I got wrong data because an ammeter or voltmeter was connected backwards.

The moral of this story is that in the student world, everything is always connected correctly and comes out smelling like roses. In my world, often it came out smelling like burned plastic or worse.

Also who said that the battery supply in a text book question was ever going to be a standard battery?

I wonder why that is? Could it be that many of the professors that write those books have very little practical experience? I remember those type problems, and I think the least they could do is make up a problem that somewhat resembles reality. Pie-in-the-sky stuff never cranked my tractor.

 

[Mickey]

What if they are not?

We would get something less then an "A" on our homework.

 

[Lancie1]

We would get something less then an "A" on our homework.

Ouch! That reminds me of the final exam in a Thermodynamics course. 5 questions, 20 points each. One question was about putting two trays of water in a 0-degree freezer with a 1/4" of frost, one at 150 degrees, one at 65 degrees. Given the heat transfer rates of the ice trays and the freezer metal, calculate which one would freeze first. I ran out of time and guessed the 65-degree one would freeze first. I was wrong and flunked the course and had to take it again.

 

[maxketcham]

Yes, the battery shows its polarity, but each meter can be connected two ways to read either a + value or a - value. Without markings, we only really know the polarity of the 2 given meters, A1 and V2. We have to ASSUME that the other meters are also connected to read positive values. What if they are not?

Yes, imposing a real-life situation on this obviously imaginary ideal-world student problem is getting too techincal. But I can't count the number of times that I got wrong data because an ammeter or voltmeter was connected backwards.

The moral of this story is that in the student world, everything is always connected correctly and comes out smelling like roses. In my world, often it came out smelling like burned plastic or worse.

I wonder why that is? Could it be that many of the professors that write those books have very little practical experience? I remember those type problems, and I think the least they could do is make up a problem that somewhat resembles reality. Pie-in-the-sky stuff never cranked my tractor.

Again it is a standard DC problem, the battery, if you know the schematic gives you the polarity, by knowing the battery polarity you know the direction of current flow ( minus to positive) (electron theory) there is no question to that, A first year electronics student should be able to ID and know both. the Idea about the question is to be able to think out the problem, knowing that current going into a circuit is the same current going out of the circuit. Having been in this field for 50 years now, I remember when there were batteries that could put out up to 300Vdc (portable tube radio batteries) so what you think isn't standard or actual life, may have been 60-70 years ago when the book was originally written. yes it may have been "updated" to include transistors, IC's, etc but why change the parts that are already proofed and right?

the reason I graduated 4.0 out of 4.0, I didn't try to read more into the problem than what it actually showed. To understand DC you have to understand current flow and ohms law, some very simple math, not the trig and calculus you'll need for the later subjects.

KISS

keep is simple S......

 

[Lancie1]

so what you think isn't standard or actual life, may have been 60-70 years ago when the book was originally written.

You are getting back close to when I was born, and still I don't remember any 116 volt batteries, for tubes, transisitors, or anything else. Neither does Google. I would hope that their books are a little bit newer than that anyway.

If you know the schematic gives you the polarity, by knowing the battery polarity you know the direction of current flow ( minus to positive) (electron theory) there is no question to that,

Right, no question about that. The only question is about the devices that are MEASURING that ideal electron theory current. How are they connected - to read upscale or downscale? Maybe the professor put a fine-print note in the front of the book that says "Assume that all meters in all diagrams are always connected to read the correct polarity". It will make a difference in every world except Star Trek. There the magic tricorder always reads the correct direction. Back in the old days we put a big dot on the + side of each ammeter lead or terminal. I suppose nowdays the students get an A if they make an attempt at a poorly written problem.

 

[RussB]

I agree with maxketcham.

4 amps thru 20 ohms is 80 volts plus 36 volts is 116 volts.

You may be correct it is generally my policy to use a simple SWAG when doing someones homework for them as I find fewer repeats with that rather than using the standard WAG.

 

[maxketcham]

You are getting back close to when I was born, and still I don't remember any 116 volt batteries, for tubes, transisitors, or anything else. Neither does Google. I would hope that their books are a little bit newer than that anyway.

Right, no question about that. The only question is about the devices that are MEASURING that ideal electron theory current. How are they connected - to read upscale or downscale? Maybe the professor put a fine-print note in the front of the book that says "Assume that all meters in all diagrams are always connected to read the correct polarity". It will make a difference in every world except Star Trek. There the magic tricorder always reads the correct direction. Back in the old days we put a big dot on the + side of each ammeter lead or terminal. I suppose nowdays the students get an A if they make an attempt at a poorly written problem.

http://en.wikipedia.org/wiki/Battery_(vacuum_tube)
read it, 120V b battery, I don't know it's actual voltage may have been close to 116, but it really wouldn't matter and as far as the meter, does it say anywhere what it's polarity is? most ammeter, movement type would read 0-XXX amps, connect it wrong and the meter would peg at zero, so how could you connect it wrong and read a negative reading? Again trying to read too much into a problem, doesn't matter what the meter reads (+/-) build the circuit and test if you are in doubt.

 

[TConnolly]

When I was about 14 I had a 72v battery that looked like a giant transistor battery about the size of a thick papaerback novel. I don't recall how I got it, but I do recall some of the "creative" uses I found for it. Lets just say that a kid in Jr High should not have access to something like that and leave the rest of the ignoble tale be.

 

[maxketcham]

When I was about 14 I had a 72v battery that looked like a giant transistor battery about the size of a thick papaerback novel. I don't recall how I got it, but I do recall some of the "creative" uses I found for it. Lets just say that a kid in Jr High should not have access to something like that and leave the rest of the ignoble tale be.

I had the same experience, found 10 B 90v batteries for a dollar at an Army surplus store, man did I have fun with those through Jr. High and high school. loved to make neon bulbs flash, and other things.