As per Ohms law , V = IR OR I = V/ R

So if I set R to 0 , then V should be equal to I ?

However when I connect 9 Volt battery poles to each other with multimeter in between the reading says 0.2 A.

| improve this question | | | | |
  • \$\begingroup\$ your 9v battery is flat. \$\endgroup\$ – Jasen Jul 30 '19 at 8:00
  • 3
    \$\begingroup\$ Mathematics 101. \$\endgroup\$ – Andy aka Jul 30 '19 at 8:04
  • \$\begingroup\$ @jasen what is flat ? \$\endgroup\$ – Mr Coder Jul 30 '19 at 8:05
  • 5
    \$\begingroup\$ Your math is wrong. I = 9/0 which is "infinity" or cannot be computed. However, resistance is never zero. And there is a concept you did not address, which is the internal resistance of a battery. \$\endgroup\$ – mkeith Jul 30 '19 at 8:09
  • 2
    \$\begingroup\$ @MrCoder: "101" from a common numbering system for college/university level courses. 100 is the base level, usually first year. 101 is typically the first year introductory course for a given subject. "Math 101" is therefore just a fancy way of saying "basic math." \$\endgroup\$ – JRE Jul 30 '19 at 8:12

Fortunately for you, batteries have internal resistance. If they hadn't then you would have either blown your meter's current limit fuse or destroyed your meter.

Never short-circuit a battery or power supply with an ammeter. Always connect it in series with the load.

You can model your battery as an "ideal" 9 V battery with a series resistance. From you measurement you can calculate that the battery's internal resistance. \$ R = \frac {V}{I} = \frac {9}{0.2} = 45 \ \Omega \$.

From the comments:

So does it mean if I short circuit 220 V (in house voltage), then reading should be 220 amps, since now there is no battery hence no internal resistance?

If there is no internal resistance on your 220 V supply then \$ I = \frac {V}{R} = \frac {220}{0} = \infty \ \text A \$.

Never short out a supply like that. Explosive amounts of energy can be released. (Look up "arc flash" on YouTube.) If you try it with your multimeter the results could be lethal.

| improve this answer | | | | |
  • \$\begingroup\$ so does it mean if I shortcircuit 220V (in house voltage) , then reading should be 220 Amps , since now there is no battery hence no internal resistance ? \$\endgroup\$ – Mr Coder Jul 30 '19 at 8:34
  • 1
    \$\begingroup\$ Sigh 2.0........ \$\endgroup\$ – Bart Jul 30 '19 at 8:43
  • \$\begingroup\$ @MrCoder See mkeith's comment above. Your math is incorrect. \$\endgroup\$ – HandyHowie Jul 30 '19 at 8:43
  • \$\begingroup\$ @HandyHowie thanks I made an error in calculations. \$\endgroup\$ – Mr Coder Jul 30 '19 at 9:19
  • \$\begingroup\$ @MrCoder: See the update. \$\endgroup\$ – Transistor Jul 30 '19 at 9:24

First, as already said- don’t ever attempt to deliberately short any power source unless you are trying to obtain a UL rating for a device- you run a high risk of fire or (perhaps minor) explosion! I once worked for a power supply manufacturer and was assigned to do this sort of work one week- the supply was behind a blast shield for this reason.

Second, the ammeter itself has resistance in addition to the wires connecting it as well as the battery itself. There’s no way in a practical circuit to achieve zero resistance however close you might get and your current reading would only be 9A if the total resistance was exactly 1 ohm.

For internal resistance of typical battery types, see this article (the table therein shows a 9V zinc carbon battery is typically 35 ohms):


| improve this answer | | | | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.