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If I have a one volt D.C. battery and a resistor of .0005 ohms, is the current really 2,000 amps? Like enough to kill someone.

Qmechanic
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Krits
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  • You need to know the internal resistance of the battery, too, to accurately calculate the current. – S. McGrew Apr 16 '20 at 23:45
  • Ok, well I was just thinking of V = IR, and I was thinking no way can some little wire have 2,000 amps. So the batteries internal resistance will be well over my .0005 ohms? – Krits Apr 16 '20 at 23:57
  • 2K amps for a microsecond? What battery do you have in mind? AAA-type, or tractor? – Cosmas Zachos Apr 17 '20 at 00:07
  • Ya like a basic battery, I was wondering that does it actually supply that current but just fro some really small time – Krits Apr 17 '20 at 00:10

3 Answers3

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If you have a voltage source of 1V and the circuit has a resistance of 0.005 ohm, then indeed you will run 2000A through the circuit. But there are caveats

  • Most voltage sources are not ideal. They are modeled as having an "internal resistance" which is part of the circuit if they are part of the circuit. For an AA battery, it is on the order of 0.5 ohms. So you can't use a AA in the circuit the way you want... the result would be a circuit reistaance of 0.5005 ohm or something like that. And if you short the circuit like this, it is highly likely that you will start to push against kinematic limits of the reactions in the battery, and the voltages will dip, rather than being held constant at 1V.
  • Wires that carry large amperage heat up. As conductors heat up, their resistance increases. This is the fundamental way lightbulbs work. When you turn them on, their resistence is very low, so a lot of current flows. This heats the filament up until there is a balance between the power being put into the filament and the power dissipated as heat.

That being said, what you describe is exactly how MIG welding works. A MIG welder typically outputs something on the order of 12V, but a large number of amps.

Cort Ammon
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  • Thank you, I kind of was asking the question sarcastically because I knew it couldn't be right. I guess I'm just thinking if you have a like 5V battery and the resistance is .5 ohm, isn't 25 amps still very dangerous. Or does the heat from the wire create a resistance so that you would never experience the 25 amps?Thank you – Krits Apr 17 '20 at 00:20
  • @ChrisBolig That depends very much on the battery. Consider that this 21/23 battery and this car/truck battery are both 12V – Cort Ammon Apr 17 '20 at 00:22
  • But the result of shorting them will be obviously very different. I don't worry about a 21/23 very much, but most car mechanics are very careful to remove their wedding rings when working on a car, because a short through the ring could heat the ring up enough to do permanent damage to your finger. – Cort Ammon Apr 17 '20 at 00:23
  • I was thinking like the battery you find in a drawer – Krits Apr 17 '20 at 00:25
  • And then there's MIG welding, that as I mentioned earlier, is also on the order of 12V. Welding 1/4" plate steel with a MIG welder uses about 70A (which means you can calculate the resistence of that circuit at .17ohm) – Cort Ammon Apr 17 '20 at 00:25
  • Batteries you find in the drawer probably can't reach 25A. Though I have personally melted one wire onto a battery while experimenting with this myself. – Cort Ammon Apr 17 '20 at 00:26
  • So what makes the 21/23 battery so different than the car/truck battery. I know obviously their size, but does like 21/23 battery breakdown with higher currents. Sorry if I'm miss using words – Krits Apr 17 '20 at 00:32
  • Basically, size matters. There's several effects in play. One is that when you get to high currents, you start pushing the limit of the two half-equations governing the battery chemistry. It starts to get hard to physically push the positive ions from one terminal to the other, so its hard to maintain the electric field at 1V. Also, the large battery tends to have a lower internal resistance. This is by design: starting a car takes a lot of amperage, so the battery is desinged to support it. – Cort Ammon Apr 17 '20 at 00:41
  • Ok thank you so much, I got a better handle on it – Krits Apr 17 '20 at 00:48
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That would be the current in the circuit created by the .0005 Ohm resistor and the 1 volt battery, yes, but only within that circuit. In order for a 1 Volt battery to result in 2,000 amps of current through a human body, the human body would need to also have a resistance of .0005 Ohms.

InkTide
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  • So when people say 2-3 amps can kill someone, they don't mean touching a wire that has that current? – Krits Apr 17 '20 at 00:12
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    And human body's resistance is about in order of 1 M ohm. As well as with that much high current the heat produced in the resistor should enormous and may meet the melting point of the metal. So circuit may be open earlier. If not then resistance of the resistance should increase as well due to thermal expansion of resistance. Limiting the current. And as well as 5 v batteries can handle 10 amps current in maximum . So soon after completing the circuit the battery should blow off. – Nobody recognizeable Apr 17 '20 at 00:16
  • @ChrisBolig provided the resistance of human body is about 10^6 ohm then you need 2-3 Mv to do that. Your phone adapter gives 5 amps of current. Try touching it . Don't scare it'll not even get close to hurt you. However AC is more dangerous as our body's reactance is low. – Nobody recognizeable Apr 17 '20 at 00:20
  • Ok I must be confusing touching 2-3 amps with 2-3 amps going through your body. Thank you. I know it was a silly question – Krits Apr 17 '20 at 00:22
  • You mentioned A.C. if the body has 10^6 resistance, then a 120V wall outlet will put like .0001 amps through the body. How is it so dangerous? – Krits Apr 17 '20 at 01:03
  • @ChrisBolig see ac can pass through body cause our body has high capacitance so low reactance. And second 120 v rms means 1.4*120 v which is high – Nobody recognizeable Apr 17 '20 at 08:36
  • @ChrisBolig please refer to :"https://physics.stackexchange.com/questions/59359/why-is-ac-more-dangerous-than-dc" – Nobody recognizeable Apr 17 '20 at 08:38
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There are really three separate issues here.

  1. How much current the battery can actually deliver. All real batteries have internal resistance. You would have to know what the internal resistance of the one volt battery is. Then you need to add that to the load resistance of .0005 Ohms and divide the battery emf (open circuit voltage) by the total to get the maximum possible current to the load. That current may only exist for a short period of time. For example, a lead acid car battery can deliver several hundred amps to a short circuit for a brief period of time.

  2. To determine how much current the battery can deliver to the body, you need to know the body's resistance. At low voltages the body resistance is quite high under dry contact conditions, on the order of thousands to 10's of thousands of Ohms. So even if the battery had zero internal resistance, for a 1 volt battery we are talking about hundredths or thousandths of a milliampere. Way below that needed for a lethal electric shock

  3. If a battery were to harm someone, it generally wouldn't be due to the battery delivering current through the body, since as already noted the current through the body would typically be very small due to the high resistance of the body. Injuries from high battery currents typically are due to touching a metal part that shorts the battery terminals. In other words, the injury would be burns due to touching metal that is hot because of resistance heating. In the safety business we call these "energy hazards" to differentiate them from "electric shock hazards". While severe burns can occur, the injury is not likely to be fatal. Fatal electric shock requires current through the heart generally requiring a higher voltage source of sufficiently low source impedance. Moreover, the vast majority of electric shocks involve ac sources, not dc.

Hope this helps.

Bob D
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  • Supper helpful, sorry I just keep thinking of questions. How does a car battery injure someone at only like 12 volts DC if the human body has such a high resistance ? – Krits Apr 17 '20 at 01:56
  • @ChrisBolig Because the injury it causes is not due to current through the body, but due to current through metal parts that short circuit the battery and get hot. Then the person suffers skin burns by touching the hot metal part. This sometimes happens to auto mechanics when a tool in their hand (e.g., a metal wrench) accidentally shorts the battery terminals. – Bob D Apr 17 '20 at 02:00
  • Gotcha, so it's a burn and not a shock. So if you held the red alligator clip in one hand and the black in the other would current travel through your body? – Krits Apr 17 '20 at 02:07
  • @ChrisBolig Correct, it is a burn and not a shock. If the red and black alligator clip were connected to a low voltage battery, such as a 12 volt battery, current would travel through the body but it would be extremely low because the body resistance is very high for low voltages. The amount of current would typically not even be perceptible if the hands are dry. You may feel a tingle if the hands are soaking wet, but the current would not be large enough for a lethal electric shock. – Bob D Apr 17 '20 at 02:13
  • Wow so you could potentially hold the ends of alligator clips from a car battery and only feel a tingle. I will not try this, but that is very interesting thank you. – Krits Apr 17 '20 at 02:15
  • @ChrisBolig Yes, but that is because the main source of resistance is the skin which is very high at low voltage. However if the skin is compromised, for example if it were cut, then the resistance becomes the body internal resistance which is much lower and more current can flow. So yes, you should never try it. Car batteries are high energy sources deserving respect. – Bob D Apr 17 '20 at 02:24