# Quick Question about ohms law and resistors

This may have been asked before, but I'm currently I'm an argument about whether or not passing 12v through a .1 ohm resistor would result in 120 amps due to Ohm's Law. (I=V/R) Providing that the power source has a negligible internal resistance

• if you have a very good battery with, say 1mohm internal resistor, hypothetically, then yes, the current will be 120 amps or 118.81 to be precise. We don't call it "passing through" 12 V, we call it applying 12 V "across" – Umar Feb 20 '17 at 3:41
• Go for it, @Umar. Upvote. Now of course what would happen next would depend on the power supply and wattage rating of the resistor. This little experiment may not last very long. – SDsolar Feb 20 '17 at 3:47
• Current compliance of the voltage source is a problem. Applying a $1\:\textrm{A}$ current source to a $8\:\textrm{k}\Omega$ resistor is a quick way to generate $8\:\textrm{kV}$ as well as cook a turkey dinner. But compliance matters and the voltage compliance of the current source will likely get in the way. Theory is great. But then practical reality impinges. – jonk Feb 20 '17 at 5:19
• A current source and a MegaOhm resistor is the solution to the worlds energy problems! – Christian Feb 20 '17 at 15:55

In theory, yes, exactly.

Provided the 12V source has negligible internal resistance and provided the 0.1 ohm resistor does not change resistance (or melt or explode) from dissipating 1.44kW.

• A 'simple' resistor from the local electronics shop can usually handle less than 1W. If anyone thinks of tying 10 1R resistors together in parallel, that's not going to cut it. Not by a long shot. – Mast Feb 20 '17 at 10:37
• @Mast 5 pieces 0.5$\Omega$ 300W Vishay FVE030020ER500KE in parallel would do it- and can be had overnight anywhere in Canada/US (239 in stock at Digikey). Might want to spring for a fan. – Spehro Pefhany Feb 20 '17 at 10:45
• That's not the local electronics store. My lab supply can only handle 40A, so can't test them here... – Mast Feb 20 '17 at 14:25

If you assume internal resistance is negligible then yes that is exactly what will happen!

Of course you may start a fire or explode your resistor at that level of power...

If you can maintain 12v across a 0.1ohm resistor, then yes, it will conduct 120 Amps.

At these power levels and currents in the real world, we'd normally flip Ohm's Law around, and say that if a current of 120A was passing through a resistance of 0.1ohms, then it would drop 12v across it.

So for instance if you put your arc welder on the 120A setting, and used cables with a total resistance of 0.1ohms to connect to the torch and workpiece, 12v would get dropped on the cables, reducing the voltage available at the arc. The cables would get quite warm as well.

Rather than passing a voltage through a resistor (or circuit) it is more usual to describe it as applying a voltage. The term passing is more usually used in relation to current. i.e. apply a voltage and pass a current. The short answer is that according to Ohm's Law, V = I * R, so applying V = 12 V across a resistance of R = 0.1 Ohm, the resulting I must be 120 A. Likewise passing 120 A through a resistance of 0.1 Ohm must result in a voltage of 12 V across it.

As others have indicated, there are various practical considerations related to a real-world application of this particular scenario. The theory, however, is incontrovertible.

If the power of 0.1 ohm resistor > 1.44kW then you are right.

But based on my understanding, there is no such a big power resistor.

• For resistances in that range (power and resistance) you'd be looking at a chunk of thick wire or a piece of metal. Look up current shunts intended for large currents. A 10A shunt looks like a piece of coat hanger wire. – JRE Sep 29 '17 at 11:06