Ohm Law and units

[daok]

have a simple schematic with a 4 volts as input. The current is 0.4 amper. The resistance of the circuit is 980 ohm.

My question is :

Why if V = I * R, when I do 980 * 0.4 = 392 instead of 4 volts.

I guess the 392 mean 3.92 volts so what do I do wrong with the unit?

Thank you in advance

 

[crutschow]

Your units are fine, but one of your values is incorrect. 4V with a current of 0.4 ampere requires a resistance of 10 ohms, not 980 ohms. Alternately, 4V across 980 ohms would give a current of 4.08mA (.0048A).

 

[daok]

Your units are fine, but one of your values is incorrect. 4V with a current of 0.4 ampere requires a resistance of 10 ohms, not 980 ohms. Alternately, 4V across 980 ohms would give a current of 4.08mA (.0048A).

Hi,

Thank you for the fast answer.

Unfortunately, I have checked again with the multimeter and the number I gave you seem to be good.

Here is the schematic. In black all wire and resistance. In blue are the multimeter probe and value taken to show you what I see.

**broken link removed**

Hope you find the mistake I do I am struggling about that simple problem since few hours.

 

[bychon]

You must be having trouble measuring. If .8 amps are flowing and half of that is flowing through R2, the other half must be flowing through R1. R1 must be equal to R2, and they can not be 980 ohms. All 3 resistors are 10 ohms. Check the battery in your meter. That can cause the false reading in the ohm scale but not affect voltage or current readings.

 

[crutschow]

Are you sure there's not a decimal point in front of the 980 ohm reading?

 

[daok]

I put the multimeter at 20k and I have .980. I have triple check. I'll try to get the color of the band when I come back from work.

.4 Amp through R1 and 0.4 Amp through R2 and 0.8 Amp through R3 seem ok from my point of view. And voltage seem very logic too.

So from your point of view the resistance are 10 ohms?

 

[crutschow]

Ok, so are you sure you aren't measuring mA instead of A. What scale is the ammeter set to?

 

[ericgibbs]

Ok, so are you sure you aren't measuring mA instead of A. What scale is the ammeter set to?

hi Carl,
I make the current 8mA, I believe you are correct regarding his meter scaling.

 

[daok]

Hi,

I think I can tell you something more with all your replies guys.

First, I think you are right about mA instead of ampere. I have notice that in the multimeter, the red wire is connected to the mA port ! So the mesure should me in mA

But, the problem is that when I put the MultiMeter to the DC Current at 20 (I guess it's the good rande because the next one is higher at 200m) I clearly see 0.4 mA

If I do the calculs 4V/980ohm I get 0.004A, this is in fact 4mA... why the multimeter give me 0.4mA and not 4mA?

 

[crutschow]

When you use the 20A input for current, the switch needs to be placed in the position for that measurement (which is usually indicated by a small 10 or 20 on the switch). Otherwise it will display in the wrong units.

Sounds like you need a short tutorial on how to use a multimeter.

 

[MrAl]

Hi,

I can offer a little suggestion since you seem to be having problems with this circuit. You can calculate the total current out of the battery with this equation:
I=((R2+R1)*V)/(R2*R3+R1*R3+R1*R2)

First check what the current I should be, then measure again.

 

[daok]

Hi,

I can offer a little suggestion since you seem to be having problems with this circuit. You can calculate the total current out of the battery with this equation:
I=((R2+R1)*V)/(R2*R3+R1*R3+R1*R2)

First check what the current I should be, then measure again.

This gave me 0.00816

In the AC->DC power adapter it's written 12v-1amp for the ouput.

0.00816 is 8.16mA isn't? I do not understand why it doesn gave 1 amp?

 

[bychon]

I've got to quit expecting people that write questions here can read the labels on their meter. Maybe then I could get the answer right.

 

[ljcox]

This gave me 0.00816

In the AC->DC power adapter it's written 12v-1amp for the ouput. This means the voltage is 12 Volt and the MAXIMUM current is 1 Amp. If you applied a load that drew more than 1 AMP, the power adapter would overheat and possibly catch on fire!

0.00816 is 8.16mA isn't? I do not understand why it doesn gave 1 amp?

The actual current depends on the resistance of the load.

In your curcuit the resistance is 980 + 980/2 = 1.5 * 980 = 1470 Ohm

Thus for 12 Volt, I = 12/1470 = 0.00816 Amp which is 8.16 mA as you calculated.

 

[MrAl]

This gave me 0.00816

In the AC->DC power adapter it's written 12v-1amp for the ouput.

0.00816 is 8.16mA isn't? I do not understand why it doesn gave 1 amp?

Hi again,


That's not the way electricity works. The adapter is stamped with 12v, 1 amp, because it
puts out 12v at a maximum of 1 amp...note the word "maiximum". That means it doesnt
always put out 1 amp. With smaller loads it can put out anything under 1 amp.
With heavier loads it may actually go above 1 amp but the voltage usually sags down then too.

The spec's have to be taken the right way, and lots of people think exactly the same thing you
have thought when they read the label...i hear this all the time, so dont feel bad, just keep trying
to understand it and you will soon.

One way of thinking of voltage and current is as follows...

The Voltage is the potential to force Current to flow. The voltage forces current to flow, but the resistance of the circuit
limits just how much current the voltage can actually force. In this way you can have say 12v, but if you have different
resistance in different circuits those circuits will all have different currents flowing because their resistance limited the
current flow. It's a balance of sorts, where V=I*R, and many circuits work mostly based on voltage and they draw
whatever current they need (due to their resistance).

For example, if you have a 12v wall wart and you connect a light load, the current may be only 0.010 amps, but if you
connect a heavy load the current may be as high as 1 amp. If you try to connect a load that is GREATER than 1 amp
however that higher current will draw the voltage down to less than 12v in most cases. What this really means is that
you should use that wall wart with a device that requires 12v but does not draw more than 1 amp of current.
So basically you can start to see that the current depends on the load, and it is really the MAX current that is
stamped on the wall wart, not the current that ALWAYS flows when you use the wall wart.

 

[daok]

Hi again,


That's not the way electricity works. The adapter is stamped with 12v, 1 amp, because it
puts out 12v at a maximum of 1 amp...note the word "maiximum". That means it doesnt
always put out 1 amp. With smaller loads it can put out anything under 1 amp.
With heavier loads it may actually go above 1 amp but the voltage usually sags down then too.

The spec's have to be taken the right way, and lots of people think exactly the same thing you
have thought when they read the label...i hear this all the time, so dont feel bad, just keep trying
to understand it and you will soon.

One way of thinking of voltage and current is as follows...

The Voltage is the potential to force Current to flow. The voltage forces current to flow, but the resistance of the circuit
limits just how much current the voltage can actually force. In this way you can have say 12v, but if you have different
resistance in different circuits those circuits will all have different currents flowing because their resistance limited the
current flow. It's a balance of sorts, where V=I*R, and many circuits work mostly based on voltage and they draw
whatever current they need (due to their resistance).

Thank you for your patience, I feel that some other members of this community are less understable about new people that try to join the world of the electricity. Thank you.

I have played with the multimeter and I have 2 places to use the probe (20Amp and 1mA) I use 1mA. I also have 4 possibilities for the switch concerning the DC current. I have 2m, 20m, 200m and 20. I used 20 when I got the 0.4am. Now, I try 20m and I have the 4am, I guess I have used the wrong switch. I thought tha the m was meaning "million" but it seem that it means "miliamp".

I continue to read and to do some practical exercices. Will come back if need some help again Thank you very much for all your inputs, really helped me to learn.

 

[ljcox]

Thank you for your patience, I feel that some other members of this community are less understable about new people that try to join the world of the electricity. Thank you. You're welcome.

I have played with the multimeter and I have 2 places to use the probe (20Amp and 1mA) I use 1mA. I also have 4 possibilities for the switch concerning the DC current. I have 2m, 20m, 200m and 20. I used 20 when I got the 0.4am. Now, I try 20m and I have the 4am, I guess I have used the wrong switch. I thought tha the m was meaning "million" but it seem that it means "miliamp".

The abbreviation in the metric system are M = Mega = million, and m = milli = 1/1000.

For example, in the case of linear measurements:- millimetre is mm & megametre is Mm.

 

[MrAl]

Thank you for your patience, I feel that some other members of this community are less understable about new people that try to join the world of the electricity. Thank you.

I have played with the multimeter and I have 2 places to use the probe (20Amp and 1mA) I use 1mA. I also have 4 possibilities for the switch concerning the DC current. I have 2m, 20m, 200m and 20. I used 20 when I got the 0.4am. Now, I try 20m and I have the 4am, I guess I have used the wrong switch. I thought tha the m was meaning "million" but it seem that it means "miliamp".

I continue to read and to do some practical exercices. Will come back if need some help again Thank you very much for all your inputs, really helped me to learn.

Hi again,


You are welcome

I see that you have a 5.5 digit multimeter then? That's a little rare as these meters are very expensive (although not as rare as it used to be) so i ask about that. On the 20 amp scale to measure 0.4ma you would have to see this on the meter:
00.0004

or at least:
_0.0004

(first digit blanked out).
Most multimeters under about 100 dollars will read this:
00.00 or _0.00

and so would not be able to see something as low as 0.4 ma on that 20 amp scale. A meter that could resolve 400ua on the 20 amp scale would probably retail for over 500 US dollars these days, so i have to ask you how you measured 400ua on the 20 amp scale with your meter.


Another thing to think about eventually is the internal resistance of the current meter. On different ranges they usually use different internal resistance values to make measurements, and sometimes these values have to be considered as the meters are not perfect.
Typical values for the 20 amp range would be 0.010 ohms, and progressively higher as you go down in scale, such as 0.1 for 2 amp range, 1.0 for 200ma, even 10 ohms for the 20ma range, but these are only typical and your meter may be slightly different.