question about voltage and also resister measuring

[nirVaan]

hi guys

i have two questions

1. technically can a given voltage provide infinite number of current values provided the required resistance is available

for example can a voltage of 1 volt produce a current of 1000 Amperes if we are able to provide with a resistance of 1 mili ohms.

does this mean a value of one volt can actually produce a current that could kill a person

2. i have seen in books thats say when measuring resistance using a ohm meter to make sure your hands are not touching the leads specially when measuring high resistance resisters.
is this because when measuring a small resistance the current go through that resistance and not through the body (because the body resistance is so hing compired to the small resistance) but when measuring a large resistance sum current does go through the human body ?

does both hands neet to touch the two leads ,what if just one hand touchers?

thankz




[/GVideo]

 

[siongboon]

1) It should not be able kill the person. Maybe burn the person touching it.
The low ohm conductor will be either hot/warm or melt down if the conductor is thin enough. The current have to somehow pass through our body organs to kill us. Our body have high impedance, so the effective current from the 1 volt source will be very small.

2) Yes. I think you are right. Our body have high resistance. For example imagine measuring a 1000Mohm in parallel with our body (assume body resistance is 1000Mohm). Effectively you get the result of 500Mohm, which is half of what the actual resistance value would be. Now imagine measuring a 1 ohm in parallel with our body (again assume 1000Mohm), the result would be close to 1ohm, therefore our body resistance is insignificant.

You got to do more hands. You will learn faster this way.
Try out simple experiment yourself, you will understand.
Happy learning.

Best Regards,
Siong Boon

www.siongboon.com

 

[nirVaan]

thankz regarding to the 1st question what i really wanted to know was technically (may be not practically) can certain voltage provide any amount of current for example 1V provide 1000 A when a 0.001 ohm reister is connected ?

 

[Miguel Gaspar]

The answear to your question is depend of the energy source. You if you have a small battery only a small amount of current will be permited.

For your case even a battery car can't give you the 1000A.

 

[Phytech]

Question 1:

1 V will never kill a person due to the body resistance. Actually, 36 V is considered safe for a person to handle.

 

[nirVaan]

"The answear to your question is depend of the energy source. You if you have a small battery only a small amount of current will be permited. "

how does this relate to V=IR , if V=1 and R=0.001 ohms then current should be 1000 A right.so then why doent this holds true

 

[FvM]

can a voltage of 1 volt produce a current of 1000 Amperes if we are able to provide with a resistance of 1 mili ohms

Clearly yes, but the 1 milli ohms must include the internal resistance of the voltage source. As said, you may have difficulties to find a suitable voltage source, but it is technically possible and there are real applications using higher currents and lower voltages at the same time.

 

[nirVaan]

thankz FvM

so where does w=VI fit into whole scheme of things i have seen in books
" if the source cannot supply 10w. its out put voltage drops with the excessive current load.Then the current is reduced"

what exactly does this mean how does this relate to V=IR (i know how the equations relate but why does these equation relate ,why is w=-VI etc...)

and how can a voltage source like a battery reduce its out put voltage

thankz

Added after 1 minutes:

ooops w=-VI should be W=VI

 

[siongboon]

Hi,

I think that the equation stills hold in certain sense.
If you expand the equation further,
every equipment be it battery or supply source,
models a different sets of characteristic.
The characteristic can be fundamentally describe
by what we learn from the formula and equation.

It is possible that the equation can be more complex than V=IR.
We have to model the equation through experiencing.
The basic electrical would still be valid.
Just that we are still not aware, what is actually happening to the equipment.
Every equipment equation would be different.
And the equation would be defined to model it's characteristic.

I think that V=IR is meaningless by itself.
Understanding & experiencing the system itself would brings in the meaning.

It is a good point you brought out.
I couldn't manage to explain it in a concrete sense,
as I have limited knowledge also.

When you probe in deeper like this,
you are actually into the process of finding how the thing function.
If a battery source reduces it's output, when you connect it in a certain manner,
you will have to accept it as a characteristic of the battery.
For further detail you have to move in deeper.
It might be chemical reaction...etc....
Example: fuel cell characteristic. No gas fuel, means no voltage, no electron supply.

This is how I think.

 

[nirVaan]

yes and i have also seen in books

"the power needed to produce the current"

so what i can think of is

1. in a ideal perfect situation V=IR holds true and a certain V can produce any amount of current depending on the resistance.

2. but in practical situation there is a limit to the current that can be produced and that depend on the energy that can be provided by the source.

3. different sources with the same voltage (ex. 1 volt) may have have different levels of energy stored in them

4. which means even thought both have same voltage the maximum current it can generate will vary.

5. so when designing a circuit you 1st figure out the total current needed by the circuit and the total resistance and then figure out how much voltage is needed.

6.then you figure out the total power (w=VI) and find a voltage source of the desired voltage that can also produce this much of power.

7. and also you make sure the resisters that are used can with stand this power with out getting heated and melted.

thats what i can think of i do not know whether this is correct so can some with experience and knowledge tell me whether this is correct or incorrect and if it is incorrect why is that so ?

thanks

 

[siongboon]

Hi nirVaan,

I agreed with you your point 1 to 4.
For the rest, my thinking direction is quite different from how you think.

5) & 6) 1st. Figure what voltage your circuit will be using. It depend on your circuit design. Varies IC requires certain voltage to operate correctly. 2nd. Know how much current your circuit will draw or rather, consume. To find out how much your circuit consume, you may need to add up individual circuit design. Example IC1 uses a maximum of 0.999A. Your LED in series with the resistor uses 1mA. Assume your circuit uses 5V. Add all this up, your circuit will need at least 1A 5V supply. Make sure your power supply can regulate 5V without fail, when supplying the 1A. To define a rating for your power supply in terms of wattage, it will be a minimum of 5V 5Watt power supply.
I would choose a power supply of 10W, to be on the safe side not to push the power supply to it's limit.
Let say your voltage regulator circuit or power source can support 2A of load. Drawing the 1A would definitely of no problem. If your design consume 10A. The source will be overloaded and starts to fail. It will not be able to sustain it's duty to regulate the voltage at 5V. It will drop. The voltage drop will cause the rest of the circuit to malfunction.
This is the way I always think when designing the circuits.
Start of with the circuit requirement. Voltage and the current it draws.
Find a power supply with the correct rating. 5V 1A.
Some equipment likes to use wattage to define how much it can supply.
In this case 5 watt.
Choose a power supply with higher rating. Safer but usually more expensive.
I prefer to see the power consumption in terms of current.

7) Use a resistor with the correct wattage, such that it will not burn off. Higher wattage resistor is of course fail safe. The bigger wattage the safer. But usually we don't select a resistor with too high wattage because they are bigger in size and usually more expensive. A rating at least 2x than what you expect would be good.
If you expect 1A to flow through the 2ohm resistor, be sure to get a minimum of 2watt resistor.

It is all about voltage and current in electronics.
I see the term power as another form of dimension to represent voltage and current.


Best Regards,
Siong Boon

www.siongboon.com

 

[nirVaan]

yea i agree with you

and when you say "It will not be able to sustain it's duty to regulate the voltage at 5V. It will drop. The voltage drop will cause the rest of the circuit to malfunction"

does it mean that if you have a voltage source with a given voltage of 1V this means that if you connect it with a 1 ohm resistance it is guaranteed to provide 1A of current right.

but if you increase the resistance current will drop
and if u decrease the resistance the current will increase. but this increase has a limit and its decided by the power of the voltage source, and in situations when this occurs either the voltage source stop functioning or with some sort of chemical reaction or voodo black magic it drops its voltage down so ultimatly V=IR holds true right

 

[FvM]

If I understand right, one of your original questions was regarding the validity of ohms law in voltage source behaviour. You also quoted the reservation made for an (apparently) electronic voltage source that has a power limitation.

An electronic voltage source basically isn't governed by ohms law. It can have any arbitrary voltage-current-characteristics, in most cases it's non-linear, this already contradicts ohms law. If the external circuit is a pure resistive network, it's behaviour still follows the law, but the source voltage is neither constant nor linear current dependant.

A battery in contrast is nearer to an ideal voltage source (with an internal resistance), but also not exactly. Usual battery models are mostly linear, but have to reflect the time-dependant behaviour with a RC ladder circuit. As a result: You won't find a real voltage source, that exactly follows ohms law. A thermocouple is probably one of the few examples of a pure resistive voltage source (with a very small voltage), also a DC generator (except for the armature ínductance).

 

[nirVaan]

Thankz FvM

But what i have said over here

yes and i have also seen in books

"the power needed to produce the current"

so what i can think of is

1. in a ideal perfect situation V=IR holds true and a certain V can produce any amount of current depending on the resistance.

2. but in practical situation there is a limit to the current that can be produced and that depend on the energy that can be provided by the source.

3. different sources with the same voltage (ex. 1 volt) may have have different levels of energy stored in them

4. which means even thought both have same voltage the maximum current it can generate will vary.

thanks

and

does it mean that if you have a voltage source with a given voltage of 1V this means that if you connect it with a 1 ohm resistance it is guaranteed to provide 1A of current right.

but if you increase the resistance current will drop
and if u decrease the resistance the current will increase. but this increase has a limit and its decided by the power of the voltage source, and in situations when this occurs either the voltage source stop functioning or with some sort of chemical reaction or voodo black magic it drops its voltage down so ultimatly V=IR holds true right

are these correct statements or does it at least have some truth to it ?

 

[FvM]

The first statement is absolutely correct. It describes an ideal voltage source without internal resistance. As the term ideal suggests, it doesnt exist in real world,

The others surely have some truth to it, but I would prefer an exact description of a voltage source, that includes the nature of possible current or power limitations. The first step is to define an internal resistance for a voltage source. It would be basically sufficient to found statement two to four. If I replace the term energy temporarily by power, also a maximum power delivery could be calculated from internal resistance, it is achieved at halved no-load voltage. This is the case for an ideal source with internal resistance and can be completely described by linear equations respectively ohms law. As an advantage, the behaviour can be predicted exactly also with external load for this type of source.

Limited energy content in contrast would involve time-dependant behaviour, e. g. a battery voltage decreasing during discharge. As previously mentioned, more complex models are necessary to reflect time-dependant properties.

Furthermore non-linear behaviour can be present in a voltage source. This is always the case with electronic voltage sources (although they may have partial linear operation ranges). A typical example is a laboratory DC supply with current limit, but also constant power or current foldback characteristics are possible. Additionally, the electronic control creates a complex time-dependant behaviour of these sources.

Also the said chemical reaction in a battery could create non-linear behaviour, obviously if the electrolyte is boiling and spilled in the enviroment...

 

[nirVaan]

so i guess at a higher level what i said is accurate but if you dig deeper there are lots of other things to consider.

and since im starting out in electronics i hope the higher level description will be enough to build and analise and to understand simple circuits.

Added after 11 minutes:

as a side note according to what you have said if we have a ideal voltage source with internal resistance , lets say 4V and 2 R .then its power would be

w=v^2/2r
w=4

which means the maximum current it can provide is

w=VI
4=4I

thus I equals 1 A. and if you have external circuit with a resistance thats connected to the voltage source that would theoretically enable a current flow that is higher than 1A the voltage will droop in the voltage source

Added after 2 minutes:

so that v=IR holds true

 

[siongboon]

Not quite get what you mean....

 

[nirVaan]

hey siongboon

the question i last posted was in responce to what FvM said