Ch 24: Capacitors & DielectricsSee all chapters

Sections | |||
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Capacitors & Capacitance | 9 mins | 0 completed | Learn |

Parallel Plate Capacitors | 15 mins | 0 completed | Learn Summary |

Energy Stored by Capacitor | 13 mins | 0 completed | Learn |

Capacitance Using Calculus | 8 mins | 0 completed | Learn |

Combining Capacitors in Series & Parallel | 14 mins | 0 completed | Learn |

Solving Capacitor Circuits | 28 mins | 0 completed | Learn |

Intro To Dielectrics | 15 mins | 0 completed | Learn Summary |

How Dielectrics Work | 3 mins | 0 completed | Learn |

Dielectric Breakdown | 5 mins | 0 completed | Learn |

Concept #1: Dielectric Breakdown

**Transcript**

Hey guys. In this video we're going to be talking about a process known as dielectric breakdown, let's get to it. We know that dielectrics are insulators and the insulators charges can't move, right? Well, ideally charges can't move, in reality, if you motivate them enough, if you provide, for instance, a large enough voltage they will move across an insulator, okay? It's very difficult and it doesn't occur commonly but it can occur, one such instance of this is a process known as dielectric breakdown, okay? Just like the dielectric constant Kappa is a fundamental quality of a dielectric, we have a second fundamental quality called the dielectric strength, and what the dielectric strength is, is it's the maximum electric field supported within a dielectric before breakdown occurs. So, if you go past that maximum electric field past that dielectric strength breakdown occurs, in dielectric breakdown what we get, is we get electrons jumping from atom to atom, electrons can exist freely within an insulator like they can in a conductor. So, all they can do is move from one atom to another, when they reach the other atom they sort of knock off the electron that goes to another atom that knocks off an electron goes to another atom etcetera and eventually the electrons cross the insulator like so. They sort of just jump from atom down until they cross the insulator, okay?

A very common example of dielectric breakdown is lightning, in a thunderstorm, we have some sort of thundercloud, that through a process, that we don't really know about, charges separate within this thundercloud, we're going to get an accumulation of positive charges near the top of the thundercloud and negative charges near the bottom of the thundercloud, on the ground this is going to pull positive charges near the surface of the ground, and this separation of charges right here is going to act as a capacitor where the air filling the space between is going to be our dielectric, and eventually if the charge separation becomes large enough, the electric field becomes strong enough, to pass the dielectric strength breakdown occurs and we get lightning, okay?

Let's do a quick example of this. A parallel plate capacitor spills with air and connected to a power source of 100 volts, what is it closest you can put the plates together if dielectric breakdown of air occurs at an electric field of 3 times 10 to the 6 volts per meter. So, 3 times 10 to the 6 volts per meter is the dielectric strength of air. Remember, for air, we're always going to treat the dielectric constant as 1, okay? So, the electric field, right? Max, is 3 times 10 to the 6 volts per meter, this is the dielectric strength, well, within a parallel plate capacitor the electric field is always going to be the voltage over the distance, okay? So, if we want to know the closest distance all we have to do is multiply this distance up and divide this electric field over, okay? So, the distance is going to be V over E max, right? And a larger the electric field is the smaller the distance, that's why, we have the closest distance, right? The smallest distance, for the largest electric field, this is going to be 100 volts over 3 times 10 to the 6, which is going to be about 0.33 times 10 to the negative 4 meters, okay? 0.33 times 10 to negative 4 meters that is our answer. Alright, this wraps up our discussion on dielectric breakdown. Thanks for watching.

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Concept #1: Dielectric Breakdown

The square plates of a 9000-pF capacitor measure 50 mm by 50 mm and are separated by a dielectric that is 0.13 mm thick. The voltage rating of the capacitor is 600 V. The maximum energy that can be stored in the capacitor, in mJ, is closest to:
A) 2.4
B) 1.6
C) 2.8
D) 3.2
E) 2.0

A parallel plate capacitor is constructed with conducting plates of area of 0.0500 m2 and a plate separation of 0.500 mm. The space between the plates is filled with paper with a dielectric constant of 3.7.
Determine the capacitance of this capacitor.
If the dielectric strength of paper is 16 x 10 6 V/m, determine the maximum charge that can be stored in this capacitor.

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