11155Views18Replies

# Infinity divided by 0?

Does infinity divided by zero theoretically equal any number other than 0 or infinity?

I heard something about ohm's law applied to a circuit with an ideal voltage source (which is only theoretically possible) and having the terminals shorted out and at 0 ohms, resulting in the current going up to infinity?

## Discussions

10 years ago

NaN

Answer 10 years ago

The practical point of the lim R->0 game is to figure out what kind of over current protection and over-voltage protection the device is required to have based on its carry load.

Don't knock lim. lim makes magic into science.

In an ideal crunching game, the answer is infinity. (all fanboys clap your little hands)

In practical electronics or electrical engineering terms, the actual short circuit current is mediated by the intrinsic resistance, capacitance, and inductance of the hardware.

However, we buy a relay that says "10 amps, 220 VAC"...but what happens if you're running a nice inductive motor from that?

Let's say the motor is operating at a quiescent current of 9 amps.

Now. turn off the power circuit. fun begins in microseconds.

the inductor, because of Ldit/dit, does not wreally want to go from N amps to 0 instantaneously...In fact, it with get rather peaviish...its temper will rise, and rise and rise, until it towers over you, until it dispels that energy by flowing into some other path or into the switch.

So the device is rated for 220V right? but the kick back voltage generated by the flywheel...oops, I mean, inductor, can reach significantly higher than that number. Which is why there are relays made for motors and one made for "not motors". in a semiconductor, the voltage can get so high that it literally burns a hole thru the semiconductor junction, one itsy bitsy, totally pissed off lighting bolt encase in black plastic

ramble mode off...I forgot what I was saying.

oh...well the use of the "approaches infinity" idea comes in handy, because it reduces the indeterminacy of governing equations when its results are used to strip variables from the calculation. (called reducing the degrees of freedom.very useful in modeling)

10 years ago

You are dealing with two different constructs. One is mathematical, the other is electrical theory.

Division by zero is mathematical. In mathematics division by zero is undefined. From a practical standpoint you can't divide an orange (or or pie, anything else) by zero. You can divide by 2, 1, .5 .000001, but not zero.

The answer to the short-circuit question is electrical theory, and "theoretically" the current would approach infinity. One form of Ohm's law to calculate current is: I=E/R. I=current, E=voltage, and R= Resistance. From this simple formula as R gets smaller, I (upper case "i" or current) gets larger. As R approaches 0 (zero), I (upper case 'i') approaches infinity. That's the theoretical side. The practical side shows that the amount of current that will be provided by a short-circuit is limited by the circuit breaker, or device that has the lowest power rating. In a power supply typically a fuse will blow before a component in the power supply.

Answer 10 years ago

No, the limiting current in a circuit is NOT limited by the circuit breaker. The MAXIMUM current that CAN flow in a mains circuit can be 10 thousand amps or more: the breaker has to interrupt.

10 years ago

Don;t do it. You'll go back in time.

Answer 10 years ago

He'll only have trouble if he does it on two adjacent calculators - if the power leads cross as you hit the "=" keys, the LHC will materialise in your living room and give you a good hard slap for playing silly buggers with the fabric of reality.

Answer 10 years ago

I say we blame Mr. Kelsey.

Answer 10 years ago

EDIT:

Kelsey.Dr.10 years ago

W/ref the circuit, superconductors will build massive currents, but you're limited but the parts of the circuit that

aren'tsuperconductors. You can have the super-conductor in a loop and charge by induction (still limited by your non-superconducting charger though) and build mighty electromagnets:11.7 Tesla!L

10 years ago

It's still zero.

Answer 10 years ago

Nope. It's formally undefined. The limits give inconsistent results.

Answer 10 years ago

4, 5, 6... I dont know. :)

10 years ago

Anything divided by zero equals zero. Here is a wikipedia article that explains about dividing a number by zero.

Answer 10 years ago

Nope, sorry. Read Steve's followups, look up whatever terms you don't recognize, maybe borrow a freshman calculus book (limits are covered pretty early).

Answer 10 years ago

OK, OK, I was wrong. My mistake.

Answer 10 years ago

. heehee I may not understand infinity, repeating nines, or just about anything having to do with Calculus, but I

understand "undefined." :)canAnswer 10 years ago

I suggest you read the whole article you have quoted. I assume you are familiar with the theory of limits and L'hopital's work ?

Steve

Answer 10 years ago

FTFA.

To understand division by zero, we must check it with multiplication: multiply the quotient by the divisor to get the original number. However, no number multiplied by zero will produce a product other than zero.To satisfy division by zero, the quotient must be bigger than all other numbers, i.e., infinity. This connection of division by zero to infinity takes us beyond elementary arithmetic (see below).10 years ago

As Orksecurity said, it's an undefined quantity, because the limits give inconsistent results depending on the order in which you evaluate them.

In any practical problem, you cannot actually reach

eitherof the two limits: a short is not zero resistance, just small (typically milliohm or so), and all current sources have a maximum output.What happens with a dead short, obviously, is that the current output goes to its maximum and the voltage source is quickly drained, or overheats (that pesky Ohm's law, again, P = I

^{2}R).10 years ago

Infinity divided by zero is undefined. Among other things, you get into the question of which infinity you're talking about. (Yes, there are different orders of infinity)