googly from ATGS 13

last one for today!

suppose we are charging a capacitor of capacitance C across a potential difference(battery) of V.

Energy stored in capacitor = 12CV2

work done by battery=q.V

but q=CV
so work done = CV2.

where did the rest 50% energy vanish??

we can say that this is due to the energy loss due to i2R losses.

but suppose we use super-conductors and ideal batteries, so there is no R,

the equations still remain the same.

how do u now account for the loss in energy????

37 Answers

49
Subhomoy Bakshi ·

i think my question got misinterpretated so i am reframing it!!

rutherford said that electorn will rotate around nucleus.

electrodynamics proved the instability of this structure by saying this...

charge accelerating in electric field, releases electromagnetic radiations and thus lose energy!
thus as the electron is accelerating in the electric field created by the nucleus (since the directon of velocity changes). thus acording to the theory it must radiate energy and as a result, the v decreases and so now the electron falls into nucleus.

but why in the first place the electromagnetic radiations took place???

i feel now the question is clearer and will be provided with an explanation!! :(

66
kaymant ·

Okay subho. As I promised here's a detailed discussion. (However, you need to read a bit of inductance). The point is that we generally tend to compartmentalize a real situation. For example, since we are dealing with capacitors we generally ignore some other characteristic property that all physical situations will have. As in this case, if we try to charge the capacitor via superconductor wires connected to an ideal battery, we still need to take into account the physical property of self-inductance (don't confuse this with self-indulgance [6])which any conductor possesses.
Basically, the inductance is something like an (electrical) inertia. The system does not want to change its state. If a current is flowing in a conductor, it will try its best even if you cut it (that's the reason why you see a spark when you switch something off --- the self inductance tries its best to maintain it even if it means jumping across a gap). Similarly, if you want to establish a current, the self-inductance tries to oppose it by inducing an emf in the opposite direction.

Suppose the wires have an inductance L. Let's look at the transients when the capacitor is being charged up. Suppose at any instant a charge q has been accumulated on the capacitor's plates and at that instant a current I is there in the circuit. In the next dt interval of time, a charge dq = I dt will flow through the battery, which will do a work
δW = I dt V0
on it. (V0 is the emf of the battery). Part of this work will go to increase the magnetic energy (there always a magnetic field around a current, and the magnetic energy is 12 LI2) and the rest of the energy goes to increase the capacitors electrical energy. Using the fact that the increment in the magnetic energy is LI dI and the increment in the capacitor's energy is qC dq, and using the work energy principle, we get
I V_0\,\mathrm dt = LI\,\mathrm dI + \frac{q}{C}\,\mathrm dq
Dividing through out by dt, we get
I V_0 = LI\dfrac{\mathrm dI}{\mathrm dt} + \frac{q}{C}\dfrac{\mathrm dq}{\mathrm dt}
Using the fact that I = dq/dt, we obtain finally the equation
\dfrac{d^2q}{\mathrm dt^2}+\dfrac{1}{LC}\,q=\dfrac{V_0}{L}
Define 1LC =ω02
Then the previous equation becomes
\dfrac{d^2q}{\mathrm dt^2}+\omega_0^2q=\dfrac{V_0}{L}
Take V0/L to the left and pull outω02 common out to get
\dfrac{d^2q}{\mathrm dt^2}+\omega_0^2\left(q-\dfrac{V_0}{\omega_0^2L}\right)=0
Using \xi =q-\dfrac{V_0}{\omega_0^2L}\qquad \Rightarrow \dfrac{\mathrm d^2\xi}{\mathrm dt^2}=\dfrac{\mathrm d^2q}{\mathrm dt^2}
the above equation finally transforms to SHM equation
\dfrac{\mathrm d^2\xi}{\mathrm dt^2}+\omega_0^2\xi=0
So the variable ξ (and hence q) oscillates simple harmonically with an angular frequency ω0. We can write out the solution as
\xi =A\cos(\omega_0t+\varphi)\quad \Rightarrow \ q=\dfrac{V_0}{\omega_0^2L}+A\cos(\omega_0t+\varphi)
However, ω02 L = 1/C, hence we ultimately get
\boxed{q=CV_0+A\cos(\omega_0t+\varphi)}
The term CV0 is easily seen to be the equilibrium charge. So then in this case the charge will actually oscillate about its usual equilibrium value. The same thing remains true for the energy of the capacitor as well. At any moment, the energy will in general not same as CV02/2, however the average value will turn out to be that much. So that means while charging via resistance-less wires, the extra work done by the battery will be in the magnetic field of the current in the wire.

P.S. Though I am pretending no loss in the above discussion, actually while the charge surges to and fro in the capacitor, it will behave like a dipole antenna and electromagnetic radiations will continuously come out taking away with them some energy. Once the battery has ran out of its emf source, the oscillations will start to damp out and finally all the energy will be gone. In this case, it will seem that the capacitor will discharge through a "characteristic" resistance given by √μ0/ε0 ≈ 376.7 Ω. This effective resistance is called the impedance of free space.

66
kaymant ·

To help you visualize the situation I will give you a mechanical analog. Imagine that you have a vertical spring (which follows the Hook's law) whose one end is fixed from the ceiling. Let the spring constant be k. If you now hook a mass m from the free end and let go, the system will oscillate. If there is some internal resistance present in the spring, the oscillations will ultimately die out and system will come to rest with the spring extending by an amount x = mg/k.
If you look at the decrease in the potential energy of the mass, its mgx = (mg)2/k. On the other hand, what is the increase in the spring potential energy? That is, simply
12kx2 = (mg)2/2k
which is only half the loss in p.e. of the mass. What about the other half? That's gone as heat (the spring will be a little warmer).

Now think of the situation when the spring has no internal resistance. What will happen in this case?

39
Pritish Chakraborty ·

Wow sir....why didn't any of us think of inductance in the first place??? Subho is alright, he has yet to do AC/EMI....but in my case it's unforgivable lol.

Interestingly, W = 1LC, which is the angular frequency at which resonance occurs and is also the frequency at which LC oscillations occur(in LC-only circuits). So this supports the fact that the charge will oscillate about its equilibrium value in the system(ideally).

If the spring has resistance, it performs damped oscillations which eventually die out, and the mechanical energy is spent as heat, which sir has mentioned. With no internal resistance, it performs SHM without any hindrance, and no energy loss occurs(highly ideal). So if we pull the resistance-less spring "x" distance(which is the maximum it can be pulled to while retaining its elastic property), and draw a force body diagram for this equilibrium, we obtain x = mg/k. Now we release the spring and it performs SHM with amplitude A = x because x is the stretching limit(maximum distance). Usual norm for some SHM problems..

49
Subhomoy Bakshi ·

i had promised myself not to even look at the solution until i am officially done with all of electromagnetism theory.... so i am back in the thread now...

sir suppose we are taking most ideal of ideal cases where the circuit is PURELY CAPACITIVE.

i know the thing is not possible practically but if we have idealised the problem to the extent of R=0 let us extent it to L=0

now still the equations are valid that energy stored is 12CV2 and energy supplied is CV2.

(as Anant sir pointed out to clear my misconception that the potential difference across the ends of an ideal battery is always constant V thus the equations are valid).

so basically i am asking to account for energy loss in all the 4 following cases::

(i) L-C-R circuits.
(ii) R-C circuits.
(iii) L-C circuits.
(iv) purely C circuit.

now that i have got the answers myself i will post them. :)

49
Subhomoy Bakshi ·

i will TRY to give a COMPLETELY MATHEMATICAL solution so that no one can oppose it by logic :P

let us try to derive ohm's law...given in many books i suppose

let vd is the drift velcity and Γ be the average relaxation time(time between two consequetive collision of the electron) for the conductor V be the potential difference and i the current flowing through the conductor(suppose a cylinder of length L and area of cross section A).

so we get, E=VL [E=electric field.]

so, Fe=qE=eVL [Fe=force experienced by electron; e=charge on electron; m=mass of an electron.]

so, ae=eVmL

we get, vd=aΓ=eVΓmL

also i=neA(vd) [i am too lazy to prove this now :(]

so, i=ne2VΓmL
so, Vi=mLne2Γ

now, R=0 implies, Γ=∞
this implies that nothing will be present to stop the electron from accelerating.
thus the electron accelerates through an electric field.

now this enlightened on me when i was revising atomic structures all over again.

why did we reject rutherford's atomic model?
it is because according to that model electron was getting accelerated in presence of an electric field and according to laws of electrodynamics a charge accelerating in presence of an electric field loses electromagetic radiations and thus the electron spirals into the nucleus.

thus similarly in our case of sperconductor (and L=0) i.e purely capacitive circuits, the energy is lost by electromagnetic radiations.

i know Sonne had already pointed this out but at that time i was not as such convinced because of the reason he gave i.e. he wrote in my chatbox that the electromagnetic radiation come as the electrons are promoted to higher energy levels and rebounded back!!

donno if it is correct!

now the rest 12CV2 is indeed lost as electromagnetic radiation but that is not due to change in energy levels but due to acceleration of electron in an existing electric field.

now i do not know the reason why the electron loses the energy when it is accelerated in an existing electric field. only what i know is that it is according to a law of electrodynamics and that it got historically applied to reject rutherford's atomic model and so i am using it again!

SO THE THREAD IS STILL ALIVE WITH A NEW QUESTION!!

why does an electron lose energy when t is accelerated in an electric field??

49
Subhomoy Bakshi ·

L-C circuit case explained by anant (kaymant) sir

L-R circuit the energy lost as heat...i2Rt

49
Subhomoy Bakshi ·

is #25 correct? anant sir plz tell!

1
dpc chakraborty ·

if a focal chord passes through <x1,y1> and <x2,y2> to a parabola y2=4ax then→
a.x1y1=a2
b.x1x2=a2
c. none
d.y1y2=a2

62
Lokesh Verma ·

x1x2=at12at22 = a2 (t1t2)2

t1t2=-1 (for a focal chord)

So x1x2=a2

49
Subhomoy Bakshi ·

yeah.....no one other than anant sir!!

he promised me to give a detailed discussion on this!!

39
Pritish Chakraborty ·

http://www.physics.tyrannosaur.eu/Accelerating_electrons_don_t_r/accelerating_electrons_don_t_r.html
You might find this interesting!
It's quite thoroughly covered in this. And you'll be a little shocked to see the conclusion, because we're fed textbook crap which is too outdated. Especially this "theory" which says that electrons accelerating in an electromagnetic field lose energy by radiation. It was part of Planck's initial quantum theory. It's just used as a buffer of continuity from Rutherford's model to Bohr's model in textbooks, with no explanation on the validity of the statement.

66
kaymant ·

@Pritish,
I would rather be careful in passing any verdict. The realm which you are pointing to are deep waters and one should tread carefully. The question whether an accelerating charge radiates or no is an open one. You may take a look at this view as well:
http://www.mathpages.com/home/kmath528/kmath528.htm

66
kaymant ·

@Soumyabakshi,
Your derivation of Ohm's law is definitely okay in #25. However, the case R=0 implying tau =0 is not exactly true. You should note that this derivation is based on a (classical) model where you picture electrons as balls colliding with each other and the lattice. But this picture won't be enough if you try explaining superconductivity with it. Superconductivity, in fact, can't be explained by any classical picture. Accordingly, your inference that in this case the electron won't collide with any other electron or the lattice points and would accelerate is not correct.

39
Pritish Chakraborty ·

kaymant sir, I am fully aware of the depth of the topic...it's just that subho wanted a proper answer to this question, and it baffled me too. In the link I gave, the person just disproves of the fact that charged particles lose energy by radiation when accelerating in fields. Rather, he says it is a different mechanism by which they lose their energy and no one knows of it yet. So the question is indeed an open one.
Meanwhile I'll go through your link!
Most of it made sense till here : "The Equivalence Principle is nothing but the spacetime version of the proposition from Riemannian geometry that a differentiable manifold is locally Euclidean" <- what does this mean?

66
kaymant ·

"....differentiable manifold is locally Euclidean." intuitively means that a small part of a smooth curved surface seems almost flat --- like a differentiable function in the vicinity of any point could be approximated by the tangent at that point. Equivalence principle being a spacetime version of that fact means that the only in flat space time the equivalence principle holds.

66
kaymant ·

@Subho,
In #33 I mistakenly typed your name as Soumyabakshi :( sorry for that..

49
Subhomoy Bakshi ·

eh ! :P

soumya bakshi is ,y bhaiya! :D (how u know him? :-o)

49
Subhomoy Bakshi ·

my most favourite till now!

49
Subhomoy Bakshi ·

look this is the case!!!!

this case is absolutely not possible...
this is not because ideal battery and ideal superconductors are not possible but because, this thing violated the KVL...

at full charge attained, the KVL is , V-q/C=0
so, V=q/C

but at an momment, when charge on conductor, q' is less than q,

q'/C-V'=0
i.e, V'=q'/C

so the potential difference across the battery also varies as same way as the potential difference across the plates of capacitor...

so, the energy supplied is not CV2 but it is 12CV2 only

thus there is no loss in energy!!!

[1][1][1]

49
Subhomoy Bakshi ·

arre thats it!!

how is heat formed!!

sry no pinks for this anand LOL!!

1
Sonne ·

its radiated as electromagnetic radiation

1
Sonne ·

i have a mega googly if anyone is interested on the same topic

1
hacker ·

yaar i wrote how it is fromed in my post #2 only if u could read it :P :P ....waise i don't have nay more time to waste on this question now anyways!

abey tht pink thing is a comedy...i don't even care if i get a 1000 or 0 pinks here!

49
Subhomoy Bakshi ·

actually this was not a googly ... this was a doubt

and no anand that is absolutely not the answer...

nishant bhaiya gave me the answer today...

i know it now :D

i ll post it now after i have finished my homework :P
(if nishant bhaiya doesn't already)

1
aposlil ·

@ The Indian, wats ur mega googly?

1
Sonne ·

so is my reasoning incorect ?
its not i hope !
source:NCERT

49
Subhomoy Bakshi ·

arre no..

the answer would be heat(electromagnetic radiation) if there was any resistance
there E=i2Rt

but here R=0

this is the case i wanted u to give the answer about!!!

1
hacker ·

this energy is lost during transient period in the form of HEAT and ELECTROMAGNETIC RADIATIONS!

66
kaymant ·

@subhomoy,
What you have said above is not correct. If you take the potential difference across the terminals of a battery to be same as that across the capacitor plates, why should the capacitor charge at all. And probably, you forgot that the battery has the task of maintaining a constant potential difference across its terminals. Furthermore, the total work done by the battery is definitely CV2 and not half of it as you have written in the last line.

The reason for this apparent loss of the energy if R=0 is a bit different. Try to see if you can find the actual reason.

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