Units
Before we begin, let's get our units straight.
- Energy - an absolute amount of energy measured in Joules, Watthours, and calories.
- Power - energy applied over time, measured in Watts or Joules per second.
1 Watthour is 1 Watt of power delivered for 1 hour. A Watt is 1 Joule/second. 1 hour is 3600 seconds. So 1 Watthour is 3600 Joules.
This gets important because while some biological organisms can deliver a lot of power, like an electric eel shock, they do it for a very, very, very short period of time resulting in very little energy transfer.
"Draining" an organism's electricity
No, you can't make an organism that drains the electrical energy of another organism. Organisms aren't batteries that can be drained and nerves aren't wires.
Instead, each neuron acts like a member of a bucket brigade: An electrical potential travels along one of its axons, which is somewhat like a tentacle on an octopus, all the way to the tip of the axon through the movement of sodium and potassium cations. These cations move in and out of small “gates” in the membrane surrounding the partitioned compartments which are strung along inside the axon: The ions do not move down the line, but they cause the gates in the adjacent compartment to open and close. And, so on. Nowhere there does electricity flow like it does through metallic wires.
At the tip of each axon is a synaptic gap between it and the tip of another neuron's axon. The signal is transmitted across the synapse by the release of special uncharged — electrically neutral — chemicals called neurotransmitters.
All grossly simplified, of course.
I said organisms aren't batteries... but they are. Nerve cells are "charged" by chemical energy from things like ATP. They use this chemical energy to transport ions against the magnetic gradient creating potential energy in the form of a charge between the two cells. Nerves are, in effect, little capacitors. But you can see there's normally nothing to draw energy from as nerve cells only have a charge relative to each other, and it's all insulated from the outside world anyway.
Were you to somehow magically overcome all that insulation and absorb the potential energy in the nervous system, the nerve cells would use the chemical energy from ATP to charge up again. I don't know what effect this would have on the organism, but nerve cells normally charge and discharge thousands of times a second.
Basically "absorbing" the organism's electricity means eating the organism. Which is what many organisms already do.
Electric Eels and Static Shock
Though you can build up a charge in an organism by insulating it from the outside world, what we call "static electricity", and can harvest that little bit of electricity, it's inconsequential in most organisms.
Because it can damage sensitive electronics, Fundamentals of Electrostatic Discharge Part Five--Device Sensitivity and Testing provides a delightful simplification of the human body.
The HBM testing model represents the discharge from the fingertip of
a standing individual delivered to the device. It is modeled by a 100 pF capacitor discharged through a switching component and a 1.5kΩ series resistor into the component. This model, which dates from the nineteenth century, was developed for investigating explosions of gas mixtures in mines.
The human being, according to an Electrostatic Discharge tester.
Can we get any useful power out of this?
We can calculate the energy of such a jolt. $\text{Energy} = \frac{\text{Capacitance} \times \text{Voltage}^2}{2}$ Numbers vary for the human body, but the highest I've seen is $C = 400 pF$ and $V = 50 \text{kV}$. Make the voltage much higher and it will ionize the surrounding air and discharge. Plugging those in we get 500 mJ which is roughly the energy to lift an apple 50 cm, or more poetically, the acoustic energy of 50 whispers.
Electric eels produce their shock in a similar way, but they have modified muscle and nerve tissue to create a voltaic pile, a simple battery. They're charged in parallel, then switched to series to release a jolt. There's thousands each producing 0.15V which leads to quite a high voltage. Discharged at 1 amp, it can produce 860 watts of power which can literally quite a shock. But it only happens for 2 ms so it only delivers 1 or 2 J.
To put this in perspective, 1 gram of meat contains 9000 J of energy. If you're looking to get energy from an electric eel, eat it.
Absorbing from an outlet
In theory, you could run the process backwards. Instead of using ATP to create a charge you could use a charge to create ATP. The problem is getting this energy to the cells without frying the organism. You can't just absorb it through the skin, skin and fat are very good insulators so you'd fry them overcoming their resistance. You'd need specialized organs to act as "wires".
Handwaving exactly how this would work, how much electricity do you need?
To get a rough estimate, a typical human weighing about 75 kg needs about 2000 kcalories, or 8 MJ, per day. That's about 100kJ/kg. Let's say you "charge" for 2 hours a day, that's $50 \frac{kJ}{kg \times hour}$ or about 14 Watts/kg. So our typical 75 kg human charging 2 hours a day needs 1050 Watts.
This is roughly the power draw of a microwave oven or kettle or toaster, which isn't unreasonable. The problem is 1050 Watts at 110 Volts is 9.5 Amps ($Power = Voltage \times Current$) which will definitely kill you.
But maybe your organism can handle this somehow. Point is, it's a lot of energy coming in fast. It's enough to toast bread and boil water. Your organism would need a way to dissipate and distribute it fast.
Magnetic field detection
Numerous creatures can detect magnetic fields, and electricity generally creates a magnetic field. The Magnetic Sense of Animals (really organisms) breaks down the detection mechanisms for us.
- Mechanical - tiny magnetic particles that act like little compasses and orient themselves with a magnetic field
- Induction - moving through a magnetic field induces a current in an organ in the organism
- Chemical - magnetic fields can change spin states which can be noticed by an organism
Normally there are two reasons to detect magnetic fields.
- Navigation and orientation along the Earth's geomagnetic field
- Prey/predator detection - since organisms put out a weak magnetic field
If you wanted to try to get energy from this, the best option would probably be induction. But as we showed above, there just isn't much energy to be had from other animals. Meat is very efficient at energy storage.
Conclusion
Chemicals are much better than electric charge at storing energy. That's why we evolved to use it to store energy, technologically and biologically. Organisms don't store electricity, except in special cases where there's some advantage to be had, and then very small amounts, and very inefficiently. We store chemical energy and convert that to electricity on demand.
That's also why we evolved to eat each other. There isn't much electrical energy in an organism at any given time, even an electric eel's jolt is only 1 or 2 J. An organism's energy is locked up in chemicals. It's much easier to eat your prey's chemicals than to get your prey to convert their chemicals to electricity for you, absorb that, and turn it back into chemicals for efficient storage.
Organisms which do feed directly off energy, photosynthesis, are quite inefficient and don't have the energy to spare for movement or complex actions.
