Charging capacitor - no resistor

AI Thread Summary
Charging a capacitor without a resistor results in the potential difference (p.d.) across the capacitor immediately reaching the maximum equal to the electromotive force (emf) of the source, as per Kirchhoff's Law. The discussion highlights that if wire resistance is assumed to be zero, the p.d. across the wires remains negligible, allowing for this instantaneous charging. However, in practical scenarios, even minimal wire resistance and internal resistance of the power source would lead to a very short charge time rather than an instantaneous one. The conversation also touches on the implications of infinite current and the limitations of traditional circuit theory, suggesting a need to consider advanced concepts from Maxwell's equations and distribution theory. Overall, the idealized scenario presented serves more as a theoretical exercise than a reflection of real-world circuit behavior.
jsmith613
Messages
609
Reaction score
0

Homework Statement



When charging a capacitor with NO resistor in the circuit, the p.d of the capacitor IMMIDIATELY reaches a maximum...

CapacitorQ.png


Homework Equations


The Attempt at a Solution



...is the following reasoning correct:

As stated by Kirchoff's Law, the sum of the potential differences across all components in a series circuit is equivalent to the the emf of the source...as there are no other compoenents in the circuit the p.d across the capacitor = emf of source immediatley.

(Note: when a resistor is present the p.d across the resistor decreases intil the p.d across capacitor = p.d source)

Is this correct?
 
Physics news on Phys.org
jsmith613 said:

Homework Statement



When charging a capacitor with NO resistor in the circuit, the p.d of the capacitor IMMIDIATELY reaches a maximum...

CapacitorQ.png


Homework Equations





The Attempt at a Solution



...is the following reasoning correct:

As stated by Kirchoff's Law, the sum of the potential differences across all components in a series circuit is equivalent to the the emf of the source...as there are no other compoenents in the circuit the p.d across the capacitor = emf of source immediatley.

(Note: when a resistor is present the p.d across the resistor decreases intil the p.d across capacitor = p.d source)

Is this correct?

I like your reasoning - but would like to see some reference to the idea that you have been instructed to assume the resistance of the wires is zero, and thus regardless of current flow, the p.d across them will be zero, and then an explanation that if real wires were used, their tiny resistance would mean a very short charge time, rather than an instantaneous charging.

Peter
 
PeterO said:
I like your reasoning - but would like to see some reference to the idea that you have been instructed to assume the resistance of the wires is zero, and thus regardless of current flow, the p.d across them will be zero, and then an explanation that if real wires were used, their tiny resistance would mean a very short charge time, rather than an instantaneous charging.

Peter

this was a question I made up myself based on a diagram in my book
thanks so much, through, for confirming my reasoning :)
 
You are also leaving out the internal resistance of the power source. There is no such thing as a totally ideal power source.
 
Also, any wire, no matter the resistance, has some inductance. Between that and the results of attempting to drive an infinite current (requiring infinite speed for finite charge carriers) you've left the realm of normal circuit theory and entered Maxwell's and Einstein's domain. Happy adventuring :smile:
 
The circuit as posed is not a physics problem but a circuit theory problem. The battery has zero internal resistance as do the wires. Assuming you switch the discharged capacitor onto the battery you will have an infinite current for an infinitesimal time as the capacitor charges. The math can be handled by the theory of distributions, aka delta functions.
 
I multiplied the values first without the error limit. Got 19.38. rounded it off to 2 significant figures since the given data has 2 significant figures. So = 19. For error I used the above formula. It comes out about 1.48. Now my question is. Should I write the answer as 19±1.5 (rounding 1.48 to 2 significant figures) OR should I write it as 19±1. So in short, should the error have same number of significant figures as the mean value or should it have the same number of decimal places as...
Thread 'A cylinder connected to a hanging mass'
Let's declare that for the cylinder, mass = M = 10 kg Radius = R = 4 m For the wall and the floor, Friction coeff = ##\mu## = 0.5 For the hanging mass, mass = m = 11 kg First, we divide the force according to their respective plane (x and y thing, correct me if I'm wrong) and according to which, cylinder or the hanging mass, they're working on. Force on the hanging mass $$mg - T = ma$$ Force(Cylinder) on y $$N_f + f_w - Mg = 0$$ Force(Cylinder) on x $$T + f_f - N_w = Ma$$ There's also...
Back
Top