How do we know that both plates of a capacitor have the same charge?

In summary, when two conductors of different shapes are connected to a battery, equal amounts of charge will appear on each conductor. This is due to the principle of charge conservation, which states that if the total charge on a system is zero, then the positively charged components must have equal and opposite charge in total to the negatively charged components. The initial net charge in the system can vary, and charge cannot be created or destroyed. However, charge conservation does not necessarily imply equal amounts of charges on the two plates of a capacitor in a particular set-up. Understanding how a battery works is important, as demonstrated by Heisenberg's near-failure on his PhD examination due to his lack of knowledge on the topic.
  • #1
Kashmir
465
74
Suppose we have two conductors ( can be of different shapes) and connect them to battery.
Why would equal amounts of charge appear on the two conductors?
 
Physics news on Phys.org
  • #2
What was the initial net charge in the system? Can charge be created or destroyed?
 
  • #3
Kashmir said:
Suppose we have two conductors ( can be of different shapes) and connect them to battery.
Why would equal amounts of charge appear on the two conductors?
Do you have a particular set-up in mind? In general, if the total charge on a system is zero, then the positively charged components must have equal and opposite charge in total to the negatively charged components.
 
  • #4
Ibix said:
What was the initial net charge in the system? Can charge be created or destroyed?
Maybe the two plates of capacitor have unequal charges. The excess goes to battery somehow.
 
  • #5
PeroK said:
Do you have a particular set-up in mind? In general, if the total charge on a system is zero, then the positively charged components must have equal and opposite charge in total to the negatively charged components.
Yes that should be true for the whole system. Battery, wires and plates of capacitor.

Charge conservation doesn't necessarily imply equal amounts of charges on the two plates of capacitor.
 
  • #6
Kashmir said:
Yes that should be true for the whole system. Battery, wires and plates of capacitor.

Charge conservation doesn't necessarily imply equal amounts of charges on the two plates of capacitor.
IMO, you asked the wrong question. Your question should have been: how does a battery work?

In fact, Heisenberg nearly failed his PhD examination because he didn't know how a storage battery worked!
 
  • Like
  • Wow
  • Informative
Likes vanhees71, Demystifier, Kashmir and 1 other person
  • #8
They don't have to be precisely equal. However if they are not, that means there is an overall charge. In electrical terms, that overall charge is on the self-capacitance of the capacitor. Let's put some numbers on it.
Take a 100uF capacitor. Place 1,000 microcoulombs on one plate and -1,000 on the other. The capacitor is then charged to 10v and there is no left-over charge. But now add a further charge of 1 nanocoulomb to one plate. That's one part in a million imbalance. A typical small capacitor might have a self-capacitance of 1 pF. The nanocoulomb raises its potential by 1000 volts - 100 times the plate-to-plate voltage.
You also have what are usually called "stray" capacitances, such as from the body of the capacitor to adjacent conductors, be they "earth/ground" or something else. These may very well be a bit larger than the self capacitance but if we say 10pF, that still leaves the one part per million imbalance raising the overall potential by 100 volts.

So there is no absolute law saying the charges are the same. However if they are not, there will be huge voltages trying to get the imbalance away. Besides which, you are most unlikely to want to operate a capacitor like that.

No, you can't just get rid of a charge "into the battery". Sometimes garage mechanics don't use scientific terms very accurately. To a mechanic, putting charge into a battery means sending a current through it. So the charge that goes in is equal to the charge that comes out. A battery doesn't just magically absorb charge.
 
  • #9

1. How do we know that both plates of a capacitor have the same charge?

When a capacitor is connected to a voltage source, electrons flow from one plate to the other until the potential difference between the plates matches the potential of the source. This results in both plates having the same charge.

2. Is there any experimental evidence to support the idea that both plates of a capacitor have the same charge?

Yes, there have been numerous experiments conducted to demonstrate that the charges on the plates of a capacitor are equal. One common experiment involves measuring the potential difference between the plates using a voltmeter.

3. Why is it important for both plates of a capacitor to have the same charge?

Having both plates of a capacitor with the same charge allows for the storage of electrical energy in the form of an electric field between the plates. This is essential for the functioning of capacitors in various electronic circuits.

4. Can the charges on the plates of a capacitor ever be different?

In an ideal capacitor, the charges on the plates will always be equal. However, in real-world capacitors, there may be slight differences in charge due to factors like leakage currents or imperfect insulation. These differences are typically very small.

5. How does the concept of charge conservation apply to the equal charges on both plates of a capacitor?

The concept of charge conservation states that charge cannot be created or destroyed, only transferred from one object to another. In the case of a capacitor, the total charge on the system remains constant, with the charges on the plates balancing each other out to maintain equilibrium.

Similar threads

2 charged conductors form a capacitor
    • Electromagnetism
Replies
15
Views
907
I Charging Capacitor with one terminal grounded
    • Electromagnetism
Replies
16
Views
588
2 charged conductors connected by a wire
    • Electromagnetism
Replies
20
Views
2K
Exploring Potential Differences between Conductors of the Same Charge
    • Electromagnetism
Replies
5
Views
1K
I Work done by electric field of an irregularly shaped conductor
    • Electromagnetism
Replies
3
Views
496
B Definition of ground potential in infinite sheet charge distributions
    • Electromagnetism
Replies
11
Views
862
I How is Potential Difference Created across a Resistor?
    • Electromagnetism
Replies
4
Views
1K
B Uniform charge distribution in a conductor
    • Electromagnetism
Replies
5
Views
1K
A An electrolytic capacitor keeps charging by itself
    • Electromagnetism
Replies
14
Views
472
The real reason for a capacitor having the same amounts of + and - charges on the two plates
    • Electromagnetism
3
Replies
103
Views
5K

Hot Threads

Recent Insights

Back
Top