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hokhani
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If we directly connect the two plates of an ideal charged parallel-plate capacitor, a spark will generate. What is the exact reason of spark?
hokhani said:If we directly connect the two plates of an ideal charged parallel-plate capacitor, a spark will generate. What is the exact reason of spark?
In an ideal parallel-plate capacitor, the electric field between the plates is always constant no matter in which distance they are. Why the dielectric breakdown occurs when we make them closer while it doesn't occur when they are far?ZapperZ said:If by "connect", you mean that you let the two plates touch each other, then I have 2 comments:
1. Shouldn't you create a short by touching them together? If this is the case, why would there be "sparks"? You may get sparks as they both get closer, but not after they touch!
2. A spark can happen even when they are not touching. That's how you can destroy a capacitor.
This "spark" is a dielectric breakdown in the capacitor, similar to lightning in air. The exact mechanism is complicated, and one may even argue that this is similar to a vacuum breakdown.
Zz.
hokhani said:In an ideal parallel-plate capacitor, the electric field between the plates is always constant no matter in which distance they are. Why the dielectric breakdown occurs when we make them closer while it doesn't occur when they are far?
Please consider the case where the plates are disconnected from the battery. In this case, the electric field is always constant.ZapperZ said:This doesn't make any sense. If your capacitor is connected to a constant voltage source (i.e. batteries), then when you make them closer, the electric field in between the plates increases (E = V/d), since the separation distance "d" gets smaller. So your claim that the electric field is always a constant when the distance between them changes is wrong.
Zz.
By this, do you mean that short circuiting the plates never cause spark and spark is only due to dielectric breakdown?ZapperZ said:1. Shouldn't you create a short by touching them together? If this is the case, why would there be "sparks"?
Zz.
hokhani said:By this, do you mean that short circuiting the plates never cause spark and spark is only due to dielectric breakdown?
I think that by shorting the plates, the electrons gain high acceleration and so there will be great radiation that we observe as spark. Is this right?
hokhani said:Please consider the case where the plates are disconnected from the battery. In this case, the electric field is always constant.
Andrew Mason said:Perhaps hokhani is thinking of something other than an ideal parallel plate capacitor - i.e. a real one. In an ideal parallel plate capacitor with a fixed charge distributed evenly across the plates right to the edges, the field is constant. V = Ed, so as the plates approach, V⇒0 (potential energy of charge ⇒ 0) and no sparks. But in a real capacitor surfaces will not be completely smooth (say, ± δ). So that as the plates approach and the separation d → δ, the charges will move around and accumulate at the closest points, which could cause a spark before they touch.
AM
ZapperZ said:But the problem here is that in the very first post, there IS a spark! That appears to be the central question, i.e. why is there a spark? If there is no spark, this question does not exist, does it?
Indeed, agreed, at the voltages involved, 5000V and much more, the gap is easily seen as is the spark jumping across those several mm.ZapperZ said:During winter or when the air is very dry, we sometime get a jolt when we are about to touch a door handle or a metallic object. This happens when we get very close to it, but NOT when we actually touched it.
Bringing a conductor across the capacitor leads is a very different scenario than the OP's scenario of bringing the plates physically close together.davenn said:well ... shorting out a capacitor does produce a spark ... have done it many, many times over the years in the workshop
But I haven't looked at it on the microscopic level to see when the spark occurs
A 100V 10uF electrolytic cap will produce a healthy spark and "crack" sound when the terminals are shorted out by a screwdriver
and will leave burn marks on the cap terminal and on the screwdriver tip.
Assumption is that there must be a gap for a spark to occur ?
if so, the gap must be really small in that it is impossible to tell doing a quick experiment whether the spark occurred
with micrometres between the 2 conductors or at the time of contact ?
=
davenn said:well ... shorting out a capacitor does produce a spark ... have done it many, many times over the years in the workshop
But I haven't looked at it on the microscopic level to see when the spark occurs
A 100V 10uF electrolytic cap will produce a healthy spark and "crack" sound when the terminals are shorted out by a screwdriver
and will leave burn marks on the cap terminal and on the screwdriver tip.
Assumption is that there must be a gap for a spark to occur ?
if so, the gap must be really small in that it is impossible to tell doing a quick experiment whether the spark occurred
with micrometres between the 2 conductors or at the time of contact ?Dave
davenn said:Indeed, agreed, at the voltages involved, 5000V and much more, the gap is easily seen as is the spark jumping across those several mm.
But with a screwdriver etc across a capacitor, the gap isn't obvious and it is easy to understand
that people possibly mistakenly think it happens at the point of physical contactDave
ZapperZ said:1. If there is no potential difference, there will be no spark. So when something is shorted, after the potential difference has dropped, breakdown can't happen.
great response and it goes towards explaining my own ponderings that I was trying to put into words in my previous postsZapperZ said:This is where careful consideration has to be done on what exactly what happened. Did the spark occurred right before contact? Or did it continue a millisecond second or two after contact? Here's the issue, and I've sketched a particular scenario here:
When you first make contact with the plate with another grounded object, the capacitor since won't be fully discharged immediately. The contact point often has a high resistance, and so the time constant for discharge isn't 0. There is still a momentary potential difference between the plate and the rest of the object during discharge. I illustrate 2 points where the potential difference can easily cause further arcing, especially if the object has other sharp points or corners.
So the arcing is in between the two due to the large electric field developed over a small distance.
Spark generation in capacitors refers to the phenomenon where a high voltage discharge or spark occurs between the plates of a capacitor. This discharge can happen when the capacitor is charged to a high voltage and the electric field between the plates becomes strong enough to ionize the air or dielectric material between them, allowing current to flow and resulting in a spark.
Spark generation in capacitors is caused by the breakdown of the dielectric material between the plates. This can happen due to a high voltage charge, physical damage to the capacitor, or a decrease in the dielectric strength of the material over time.
Yes, spark generation in capacitors can be dangerous as it can result in a high voltage discharge that can cause damage to the capacitor and surrounding components. In some cases, it can also lead to electric shocks or fires. It is important to handle and use capacitors properly to prevent spark generation.
Spark generation in capacitors can be prevented by following proper handling and usage guidelines, such as not exceeding the maximum voltage rating and avoiding physical damage to the capacitor. Using higher quality capacitors with better dielectric materials can also reduce the likelihood of spark generation.
Spark generation in capacitors is commonly used in spark gap circuits, which are used in applications such as high voltage power supplies, laser systems, and lightning protection. It can also be used in some types of ignition systems, such as in internal combustion engines.