3
$\begingroup$

In my world, people are able to use magic to produce piezoelectric materials of a high strength, making it a viable material for use in weapons and armour. For the purpose of this question you can assume the material to be equal to steel in all other regards, unless a different set of properties is required to answer the question.

As I understand it, piezoelectricity is a phenomenon in which a material produces a charge when undergoing elastic deformation. This leads me to believe blunt weapons such as hammers would produce the greatest amount of electricity as they would smash into an opponent, as opposed to sharp weapons which should cleave through an opponent.

I also imagine that even if a notable charge could be produced, leather or thick cloth armors would block it from shocking the opponent, meaning they would need to be unarmoured or wearing metal armour. You can assume them to be naked when answering.

Assume a medieval level of technology. For the purposes of this question, my magic system has no relevance beyond its ability to produce strong piezoelectric materials.

I was unable to confidently find the efficiency of piezoelectric materials and generators, with figures online ranging from 7% to 90%. If you have knowledge of the subject, use your best judgement. If not, let's go with an efficiency of 30% (I assume that's 30% of energy that goes into causing elastic deformation in the weapon?).

Assume a 'notable shock' to mean a shock that causes significant pain/surprise, or which can cause the body (or a body part) to jolt.

$\endgroup$
25
  • 2
    $\begingroup$ Take a step back and read what I'm actually saying. I never ever said to make your hammer use magic to make shocks in a magical way. I don't care how you made the hammer. Magic, nanotech, divine material from the gods, whatever. I'm saying you're already making up an imaginary piezoelectric material with super strength and that no one is going to care if you also just make it really piezoelectric enough for your purposes. It's in for a dime, in for a dozen. $\endgroup$
    – DKNguyen
    May 11, 2023 at 19:33
  • 3
    $\begingroup$ VTC - You are asking for hard science on a made up, magic material. Those two things are mutually exclusive. $\endgroup$ May 11, 2023 at 23:30
  • 5
    $\begingroup$ I'm voting to keep open. All the guy is asking is, if you had a material that could efficiently convert kinetic energy to electricity, how much electricity could it produce? On the outside, he's asking how much damage that electricity could cause. If that amount of electricity isn't enough to do what he wants, then you can invoke "yet more magic." $\endgroup$ May 12, 2023 at 1:50
  • 2
    $\begingroup$ @MS 'Could base it on Quartz' - did you specify quartz? I love a good frame challenge as much as the next guy - but as written, you want scientific values for a magical material. That is an impossibility. 'Standard efficiency around 6-12%' - is that the properties your material has? Is it above standard due to magic? below? You mentioned it should be like Steel except if it is required not to be - Steel isn't piezoelectric - so again, we cannot answer. This has nothing to do with the critique of another user and everything to do with a bad question. $\endgroup$ May 12, 2023 at 9:24
  • 4
    $\begingroup$ @TheDemonLord the fact that you cannot fathom what the OP is asking does not necessarily mean that the question is objectively bad. The question is about piezoelectric materials. The magic is secondary to the key part of the question. If you read the world "magic" and find yourself unable to think about anything else, then perhaps you should move on to a different question. $\endgroup$ May 12, 2023 at 9:30

5 Answers 5

3
$\begingroup$

You can get piezoelectric devices right now which will noticably shock a person if applied to their skin, though I'd hesitate to call them weapons.

You have two problems here. The first is that piezoelectricity is complex... you can't really boil it down to a simple efficiency value. There's some potentially interesting introductory reading in this sample chapter of Fundamentals of Piezoelectricity. Getting the magic numbers you need (and there are many of them... this page on piezo electric constants lists 14) is surprisingly awkward, though a snippet in this Science Direct Piezoelectric Voltage Coefficient topic gives things like

lead zirconate titanate is the most widely used ceramic for such composite materials after dispersion in polydimethysiloxane. This combination gives excellent piezoelectric properties, such as permittivity up to 40, piezoelectric charge constant up to 25 pC/N, piezoelectric voltage coefficient up to 75 mV m/N

If this marketing blurb on electroshock weapons is to be believed, you want to deliver a charge of a few microcoulombs to make something actually hurt, rather than the tiny pinprick a piezo gas lighter delivers. Using the material above, that means you need to develop forces of the order of meganewtons an honestly at that point you don't actually need to generate an electric shock to hurt someone, and you start having to worry about your weapon chipping, bending and shattering. (aside note: computing the actual force of an impact is awkward especially when non-rigid things are involved like meatbags, and squishy things spread out the impact time and reduce peak forces for the same transfer of momentum. hard armor makes the maths easier and the impact more forceful, but also makes trying to shock people much harder). A single zap is not likely to be enough to deter anyone, which is why stun guns deliver a continuous stream of pulses and that's akward for a weapon that's can only develop power at the moment of impact. Delivering a charge sufficient to kill or seriously injure someone in a single jolt is at least an order of magnitude harder.

And that brings me on to part two of the problem: there ain't no such thing as a free lunch. You can't get more energy out of the piezoelectric material than you put in, and enough energy to meaningfully hurt or injure someone by electrocuting them is only going to be generated by an impact that would already hurt or injure them, and converting that impact into an electrical shock is going to reduce the effectiveness of the weapon. Moreover, glancing blows and hits to soft tissue that could disable or kill were they done with a cutting or stabbing weapon might not be able to trigger the zapping effect at all!

This starts wandering into the territory of flaming swords. Sure, they look impressive, but if I chop off your head or run you through with a regular sword you'll be just as dead, and I don't have to worry about accidentally setting everything on fire.

Stick with regular weapons.

$\endgroup$
7
  • $\begingroup$ Jeez meganewtons, so basically a 1kg weapon accelerated to 1,000,000 m/s? That basically throws the concept out the window. Regarding your fourth paragraph, would you actually experience less force being hit with a blunt piezoelectric weapon as opposed to a regular weapon? I thought it would just take the 'waste energy' that goes into deforming the weapon and convert it into something useful (although apparently of far too little magnitude). $\endgroup$
    – M S
    May 12, 2023 at 11:14
  • 1
    $\begingroup$ @MS a meganewton applied to a 1kg object accelerates it at 100000 gravities, but it doesn't have to do it for very long... if it were travelling at 50m/s, braking it to a halt in 50 microseconds would generate that acceleration (this is still tricky, but it doesn't require a railgun!). You can generate tens of kilonewtons of force just by falling over... weapons hitting solid bits of your body can do that and more. $\endgroup$ May 12, 2023 at 12:16
  • $\begingroup$ @MS as for "waste energy", the piezoelectric material has to deform to produce the shock, and the deformation absorbs energy. For an exaggerated comparison, imagine fitting a spring to the face of a warhammer: you still wouldn't want to be hit in the face with it, but it'd be better than the unsprung version. Steel weapons are pretty rigid, so its the thing you hit that generally does the deformation, rather than the weapon itself. $\endgroup$ May 12, 2023 at 12:19
  • $\begingroup$ True, but thats assuming the piezoelectric material to be more elastic than steel. If the two weapons are equivalent except one exhibits piezoelectricity, surely they hit with the exact same force, with the piezoelectric one additionally converting wasted energy that goes into elastic deformation into electricity? $\endgroup$
    – M S
    May 12, 2023 at 12:55
  • 2
    $\begingroup$ @MS very little energy will be lost in the deformation of a steel weapon wielded with human-like strength. Its hard to dent the head of a hammer! This either means no-one will notice the shocks because they're too small, or you have to fire your piezoelectrical projectiles out of a railgun so that the deformation generates enough power to shock the remains of the target as it is being splattered ;-) $\endgroup$ May 12, 2023 at 12:58
3
$\begingroup$

Based on the comments and some Back-of-the-envelope maths:

Assuming 30% efficiency and an above average sword slash of 200 joules (average is about 140), that's 60 joules of energy for a Melee weapon.

The limit here is the human body, I've seen other data (Baseball throws etc.) that put the output of around 140 joules - but I'm being generous.

60 joules is 60 watts in 1 second. So a small lightbulb, for 1 second.

To put that in context, unless you are able to get into the bloodstream and across the heart, 60 joules of electricity isn't likely to do much.

A Defib unit runs at around 400.

When put into the context of weaponary, you'd be better off just sharpening a blade some more as a bladed weapon can require as little as 10 joules to pierce a major blood vessel (which would be a potential fatal injury).

$\endgroup$
1
  • 1
    $\begingroup$ Wile unlikely to to cause total paralysis or coronary issues. It seems possible a blow from such a weapon could cause an involuntary spasm or temporary loss of motor function at least at the point of impact. Obviously a surprise contraction of a hamstring could be deadly. Of Course only if the right balance of durability and effectiveness can be combined into a single device. $\endgroup$
    – Gillgamesh
    May 12, 2023 at 15:06
2
$\begingroup$

Standard efficiency for piezoelectric materials is 6-12%. Your typical bullet has about 2000 joules, so you'd generate about 2 kilowatt-seconds at 100% efficiency. Enough to fully power an outlet for about 15 seconds. You will want to decide how quickly it was discharged.

For reference, a taser does 50,000 volts, at 3.6mA, for 180 watts. The force of this bullet could power that discharge for about eleven seconds.

There are problems. Piezoelectric materials create juice when you stress them, but then suck that back in when they snap back. This means that, if the person's grounded, you won't be able to use it as a weapon. If they aren't grounded, then the same electrical zap that they hit the other person with will be returned upon him the next time anyone hits him.

In response to comments:

The laws of thermodynamics insist that the energy of deformation can be no greater than the energy introduced to the system by the bullet. This models the bullet as a blunt pushing object, since you can't tell us other properties of the material. Most energy would be lost to heat, unless magic.

Yes, you'd have a net zero electrical balance. They aren't used for electrical generation. Piezoelectric lighters deliver a high-voltage spark by throwing the spark before deformation reverts. I'm not sure where it draws the charge from afterwards. It's probably taking advantage of greater surface area on a non-conductor to allow the charge to revert relatively slowly.

The jolt will actually equalize the charge between the two people. If the other person wears armor, then there will be little or no damage at these levels of current. Incrementally half-strength discharges would occur until the charge is dissipated across enough surface area.

You WANT them to be grounded, so that they deliver the jolt to an opponent instead of to the ground. Yes, shoes would be enough, but high voltage leaks out into the atmosphere pretty quickly. Play with a Van de Graaff generator some time to get a better feel for it.

$\endgroup$
8
  • $\begingroup$ Admittedly my understanding of electrical physics and mechanics is poor, but if piezoelectric materials suck the charge back when their shape reverts (I assume this means they become positively charged) how are they used for energy generation? Doesn't that mean they produce zero net energy? $\endgroup$
    – M S
    May 12, 2023 at 8:28
  • $\begingroup$ And would a person simply wearing shoes prevent them from being grounded? Or is it more complex than that? And am I correct in understanding that the opponent who is initially zapped becomes negatively charged, and if anyone touches him they themselves get zapped? $\endgroup$
    – M S
    May 12, 2023 at 8:30
  • $\begingroup$ Last comment here, but I also thought you'd need to calculate the amount of energy that goes into elastically deforming the weapon, rather than taking the total energy of the collision, but is it actually that simple to calculate, since all the energy of the collision is being transferred through the weapon? $\endgroup$
    – M S
    May 12, 2023 at 8:32
  • 1
    $\begingroup$ Grounding, and electricity in general, doesn't work that way. The piezo material will generate a voltage difference across the weapon, and drive a current through conductive materials in contact with those charged parts of its surface. The current will flow one way as the piezo material is compressed and the other as it relaxes. The only way contact with the ground could matter is if the weapon is intended to close the circuit through its handle, user, and the ground between them and their target. This would be an...*unusual* design decision. $\endgroup$ May 12, 2023 at 18:52
  • 1
    $\begingroup$ @MS, I didn't say that you can't use it for that, I said that they aren't used for that. You can recoup energy lost to stress (like from our footsteps), but the equipment to do so is complex, expensive, and not terribly durable, and the energy recouped is small. You can power LED's off of steps on a pavement, at a cost of $75 per square foot, not counting maintenance costs. $\endgroup$ May 17, 2023 at 19:06
2
$\begingroup$

Frame challenge: if you have piezo materials that could even add a noticeable effect to a hammer beyond the impact of being hit, you could put them to far more effective use in a weapon by using them to charge a high voltage capacitor. A simple Leyden jar can store enough of a charge to cause muscle spasms that would throw one across a room, and could be paired with more conventional materials better suited for weapons.

Additionally, to generate electrical power, piezoelectric devices must flexing and absorb mechanical power, which is counterproductive when you're trying to smash your enemy. However, the combination of a piezoelectric transducer and charged capacitors could be used to enhance the impact rather than counterproductively absorbing it to convert some fraction of it to an electrical discharge.

$\endgroup$
2
$\begingroup$

Yes - build an electro-whip

As specified in the previous answers, the amount of energy you will get from a single sword slash would not be enough for a target to even notice the electric effect.

So you would need to charge up.

Imagine you embed multiple units of piezo elements in the structure of a whip and pack some batteries in the handle.

As you start crackle the whip, energy flows to the batteries. Until you can release it on a deadly strike on your hapless opponent.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .