# What determines a load's current requirements?

Apologies if this question has been asked before. I'm sure it has but hard to find as wording of question may not bring up the duplicate question.

Let us say we have a 10 ohm resistor as a load. Under a power source of 5V, it will draw a 500mA current. In this scenario, we had two "givens": the resistance and the voltage.

Now here is what I am wondering. Does a 10 ohm load think (using the above resistor as a figurative character)

"Ok, I never know how much current I need. For me to know how much current I need, I need to know what the voltage is since I already know my resistance. If your voltage is 5V, then I will take 500mA. If your voltage is 10V, I will need 1A. But don't give me more than 20V because I won't be able to handle that much of a voltage drop and 2A of current!"

So does this mean a load in series will always draw what it needs to comply with Ohm's law rather than preserve itself from taking on too much current? (i.e. taking the above example, "Load sees that the voltage is 30V "Uh oh, I need to take on 3A. This will destroy me but I must draw it anyway to comply with Ohm's law. So long!"?"

• Loads do not "think" anything. A resistor is a carbon-metal sandwich. Whether it self-destructs is more to do with how readily the surrounding environment carries away heat. – pjc50 Oct 13 '14 at 9:22
• @Ho Gil: +1 and congratulations on a perfect analogy. :-) – EM Fields Oct 13 '14 at 11:05
• @pjc50: Well, he did say he was using the resistor as a figurative character, much like the rhetorical "sandwich" to which you allude. – EM Fields Oct 13 '14 at 11:16

## 2 Answers

Using the water analogy, a 10 mm pipe will only allow a certain amount of water to pass, and it will depend on the applied pressure. If you double the pressure, the flow will double.

A certain material will require a certain voltage to flow a certain current. Quoting a nice answer in Physics.SE about gravitation:

Physics [Ohm's law] does not answer existential problems. It gathers data and observations and models them with mathematical equations and functions, and then can explain the data with the model and predict new observations.

• Needed to say, Ohm's law for electric current is quite precise. It's analog for water pipes not so quite. But for the idea of what's going on, it probably doesn't matter :) – yo' Oct 13 '14 at 13:03
• @tohecz: The rules for water pipes are also quite precise, but have more complications depending on whether your flow is turbulent or laminar, and other factors. But for laminar flow, it is much like ohms law. – whatsisname Oct 13 '14 at 18:36
• Note for future me (and others): the linked question is closed, the link may be broken when you read this. :) – clabacchio Oct 14 '14 at 17:10

Sometimes it's easier to think of a resistor in terms of conductance. For instance, a 1 ohm resistor has a conductance of 1 amp per volt - if you put 5 volts across it the current will be 5 amps. A 100 ohm resistor has a conductance of 0.01 amps per volt etc..

Conductance is the reciprocal of resistance.