68
votes
Why do we use capacitors and not batteries in defibrillator?
Batteries usually use electro-chemical reactions to store energy. These reactions have a limit to how fast they can transfer that energy. For example, a typical lead acid car battery can only draw so ...
- 1,228
48
votes
Accepted
Why doesn't Kirchhoff's Law work when a battery is shorted with an ideal wire?
Just to complement the other answers: This isn't really about Kirchhoff's law. Rather, it is about an idealised situation that does not have a solution at all.
When you draw such a diagram, you can ...
- 4,497
46
votes
Why doesn't the voltage increase when batteries are connected in parallel?
tl;dr Batteries do not create electric fields to move charges. They move charges, which creates electric fields.
a battery [...] gives out some electric field that moves through the circuit and gives ...
- 561
44
votes
Why do we use capacitors and not batteries in defibrillator?
The defibrillator requires a high voltage to do its job. ordinarily this would require a very large battery stack (hundreds of individual cells) to achieve the voltage requirement. Instead, ...
- 96.7k
31
votes
Increase in mass of battery of my smartphone
There will be an increase of the mass of the battery when you charge it, though that increase is going to be undetectably small.
I would do the calculation in reverse i.e. start with a fully charged ...
- 359k
30
votes
Why doesn't the voltage increase when batteries are connected in parallel?
Think of a battery as an escalator (potential gravitational energy and potential electric energy/voltage are analogous here).
If you have two escalators side by side, taking either one will get you to ...
- 7,687
29
votes
Accepted
How do batteries lose capacity in winter?
It is true that battery performance is reduced at colder temperatures.
This is because temperature has an effect on chemical processes within the batteries, especially lithium-ion batteries, used most ...
- 30.1k
25
votes
Accepted
Does current run forever in water? (assuming the supply voltage is there forever)
It is energetically unfavourable to split a water molecule into the two ions $\text{H}^+$ and $\text{OH}^-$ i.e. you need to put in energy to do it. However at room temperature water molecules have a ...
- 359k
25
votes
Why do we use capacitors and not batteries in defibrillator?
The short answer is that although capacitors do not hold as much total energy as a battery the same size, they can release energy faster than batteries can.
In a portable defibrillator (or a taser!) ...
- 371
24
votes
Why do we use capacitors and not batteries in defibrillator?
The ability to deliver energy relatively quickly is basically the distinction between a "capacitor" and a "rechargeable battery". This isn't a physics factoid so much as just what the words mean.
...
- 4,670
24
votes
How does a wireless charger work if there is no transfer of electrons?
The time-varying magnetic field created by the wireless charger induces an electromotive force in the wireless charging coil of the device. This electromotive force (after being rectified) drives ...
- 12.2k
21
votes
Accepted
How does grounding work?
It will normally just reach the negative terminal. Generally, current only flows in closed circuits. Hence, it could only flow into the ground if the positive terminal was also connected to ground, ...
- 7,534
21
votes
How do batteries lose capacity in winter?
A deeper, physics/chemistry approach to the question:
A battery (whatever chemistry it uses) invariably contains solid electrodes and liquid electrolyte (there are batteries that have it the other way ...
- 7,914
19
votes
Why doesn't Kirchhoff's Law work when a battery is shorted with an ideal wire?
The law does hold up perfectly here. There's a battery, with v volts. Let's use 5v.
Then, there's a wire. In the circuit above, there will be some (high) current ...
- 1,003
18
votes
Gravity train with the help of batteries?
Re: charging a battery on the trip to overcome the friction.
No.
Friction will always oppose the direction of travel. So it slows the train on the way down, losing energy. And it slows the train on ...
- 502
18
votes
Accepted
How could one measure the capacity of a battery with one instantaneous meter reading?
Here is how.
That special meter contains a load resistor, which places a load on the battery roughly equal to that of a starter motor (big load!). When the meter is clipped onto the battery terminals, ...
- 96.7k
17
votes
Accepted
I Do Not Understand a Textbook Example: Calculating Voltage Across a Resistor
The book is indeed wrong and the quoted answer $V=0.5\ \text{volts}$ does not appear to make any sense.
Using conservation of energy, one can compare the power going into the source and the power ...
- 30.1k
17
votes
Accepted
Is every finite circuit "solvable" using Ohm's law and Kirchhoff's loop rules?
The other answers have noted that the total number of equations is correct, but that's not actually enough to guarantee a unique solution. There are at least two ways things can fail:
The equations ...
- 104k
16
votes
Why doesn't the voltage increase when batteries are connected in parallel?
Since you seem to be eager for an analogy-free answer, I'll try to give it a stab. I think this question is actually a bit deeper than some of the answers are giving it credit for, and so far I think ...
- 935
16
votes
Why is there an electric field around this closed electric circuit?
Isn't the electric field guided entirely through the wire?
The wires are (very nearly) perfect conductors, and therefore the electric field within the wires is (very nearly) zero.
why is there an ...
- 28.8k
15
votes
Understanding the zero current in a simple circuit
Consider the potential. Let’s call the left upper corner A and right upper corner B. First see upper side. There are two batteries and they’re symmetric. It is easy to imagine that the potential of ...
- 356
15
votes
How does a wireless charger work if there is no transfer of electrons?
Does the charged up battery have more electrons than the depleted one ?
No. A charged battery has exactly the same amount of electrons as a depleted one. It is just that the electrons in the charged ...
- 57.6k
14
votes
Accepted
Why does a drop in resistance lead to a drop in voltage?
There are three voltages in this circuit: the voltage, $V_{batt}$ across the battery terminals, the voltage, $V_r$ across the fixed resistor, $r$, and the voltage, $V_R$, across the variable resistor. ...
- 36.3k
14
votes
Why does voltage increase in a series circuit?
Think of a water pump which can lift water to a height between inlet and outlet of $1.5\,\rm m$.
Putting two of these pumps in series results in the total lift height of $1.5\,\rm m$ (pump $1$ ground ...
- 98.6k
13
votes
Why is there no current between two capacitors connected in series?
Short answer: Potential is defined depending on the choice of a origin (e.g. ground). The positive plate of a capacitor has potential $Q/C$ greater than negative plate of the same capacitor. We do not ...
- 3,200
13
votes
Does current pass through an ideal battery?
A battery, ideal or not, does not follow Ohm’s law. Ohm’s law is an observed behavior of a specific class of materials/devices, sometimes called a constitutive equation. It is not a universal law of ...
- 105k
13
votes
Proof of internal resistance acting like it does
Internal resistance is just a concept for approximating the non ideal behaviour of a battery. This latter is ideally defined as a device that is capable of provinding a constant voltage regardless of ...
- 231
13
votes
Accepted
Why are potential drops concentrated at resistive elements?
That the potential drop across an ideal (resistivity $\rho = 0$) wire is zero is a consequence of Ohm's Law:
$$\mathbf{E} = \rho\,\mathbf{J} = \mathbf{0} \Rightarrow \Delta V = -\int{\mathbf{E}\cdot\,...
- 561
12
votes
Why do we use capacitors and not batteries in defibrillator?
One aspect that hasn't been covered in the other answer is what is really needed to make a defibrillator work reliably and safely.
Defibrillating a heart isn't simply "OK, let's electrocute the ...
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