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Topic: voltage drops... across a resistor. (Read 994 times) previous topic - next topic

cjdelphi

May 25, 2013, 05:51 am Last Edit: May 25, 2013, 08:56 am by cjdelphi Reason: 1
5v circuit, now stick a 100ohm resistor in...

the voltage drop is.... 5v?

5/100
i = 0.050 (50ma)

if this was true, there would be 50 (edit) ma current flowing through the circuit, how's that a 5v drop across the resistor?  

Graynomad

Rob Gray aka the GRAYnomad www.robgray.com

cjdelphi



This is what I was (am?) struggling with to understand.

- The net voltage in any closed path circuit must be 0v   .. this is a constant then.

so let's take a 1mohm resistor and a 1v power supply, the voltage drop has to be 1v regardless of resistor used then?

Grumpy_Mike

Quote
so let's take a 1mohm resistor and a 1v power supply, the voltage drop has to be 1v regardless of resistor used then?

Yes.

Regardless of what value of resistor you use as well, the battery keeps pumping current into it until it is 1V.

Of course in practice the battery has an internal resistance that can be thought of as a series resistor inside the battery. That is why the voltage output of the battery will drop for some values of load resistor.

cjdelphi


Grumpy_Mike

Have a look at this
http://www.thebox.myzen.co.uk/Raspberry/Understanding_Outputs.html
It talks about internal impedance of batteries and any power source in general.

fungus


50 (edit) ma current flowing through the circuit, how's that a 5v drop across the resistor?  


Because of the laws of the universe (and in particular Ohm's law), that's how.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

cjdelphi



50 (edit) ma current flowing through the circuit, how's that a 5v drop across the resistor?  


Because of the laws of the universe (and in particular Ohm's law), that's how.




Ohms law may not one day may not "apply" to all.

BillHo


Ohms law may not one day may not "apply" to all.

Ohms law was correct all the time but the world was not perfect, voltage source had internal impedance, even the connection wire that you used had resistance.

tack

You have to remember real life isn't the same as the 'perfect' circuit.

As Mike says, a power source may have an internal impedance.

If measuring using an instrument then that will have an impedance too.

Both of these can affect what you may measure in a real-life circuit vs what you calculate.

You have to be aware of these factors to understand why initial calculation and measurements may not agree. Once you understand the extra factors then you can add those into your calculations and find agreement, within certain inherent tolerances.

majenko

Get 10% off all 4D Systems TFT screens this month: use discount code MAJENKO10

pito

I wish I have 18 multimeters in my hamshack handy.. :)

majenko

Yeah.  I have to make do with one DMM, and a 2 channel o'scope.  I could do with a couple more bits of measurement kit really.  And a dedicated shelf to keep them all on ;)
Get 10% off all 4D Systems TFT screens this month: use discount code MAJENKO10

Grumpy_Mike


I wish I have 18 multimeters in my hamshack handy.. :)

I think there is something slightly suspect about some one with that many multimeters.

fungus



I wish I have 18 multimeters in my hamshack handy.. :)

I think there is something slightly suspect about some one with that many multimeters.


Two is often useful - see volts and amps at the same time. Even cheap $5 meters will do for watching voltages.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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