MIT Physics Demo -- Adjustable Capacitor with Dielectric

First, an adjustable parallel-plate capacitor is held at a constant voltage. As the separation between the plates is widened, the electrometer shows charge (or current) flowing off of the plates, while the electroscope shows no change in voltage. Notice (by the deflection of the needle) that more charge flows off of the plates when they are closer together, and less when further apart. When the plates are brought back together, we see charge flowing back onto of the plates.
Next, we deposit a fixed amount of charge onto the left plate. Now, when the separation is widened, the electroscope shows a rising voltage, even as the amount of charge stays constant. When a block of plexiglass (a dielectric) is inserted between the plates, the voltage drops. However, when the plexi is removed, the voltage rises back up again, showing that the charge is still there.
This is one of the main reasons capacitors are built with dielectrics between their plates; more charge can be stored at a lower voltage.
See the original video and more on MIT TechTV here - techtv.mit.edu/...

[@ericsu4667]

The first half of the experiment proves that there is no charge on the capacitor. As the gap between the capacitor plates is increased, the electroscope remains the same. C descreases but Q is constant. Therefore, QCV since V is also constant.

[@Madkite]

This is basic dielectric phenomena. Unless it is leaked away the energy stored must remain the same so the voltage increases in the plaits as the capacitance decreases so the energy stored remains equal. Increase the capacity and the voltage decreases. It will work with any good dielectric. As long as it does not leak the voltage away it will read like this.

[@hrmofutopia9024]

But charge/Current stays the same, yet Voltage rises... c*v*v*.5...

[@madebyoneman5006]

So in another video you guys show the charge being held in the pyrex dielectric glass. isn't this slightly misleading? or is it kind of different because the voltage represents the drop not the charge? I want to believe the charge can be stored in glass as light (or even other?).

[@totoff92]

explanations during the video would be great! (even when you know whats happenning)

[@hrmofutopia9024]

Thanks for verifying my theory :)

[@Vsor]

same, its 2am and I can't stop thinking about charge lol. I came to the same conclusion and was pleased to see that it is true. Idk why having a dielectric changes the voltage though. I'll save that for some other day at 2am

[@IsaacDLovesChrist]

That was wonderful! Thank you!

[@PashaSiraja]

coool !!! :P !!! nice !!
I am just learning about capacitors and their discharge cruves in A2 Physics... its awesome...
thanx !! =D

[@jeanvaljean4135]

Nice video. It would also have been instructive to see what happens when one puts metal objects between the plates without touching the plates.

[@ericsu4667]

Is the voltage still constant in the second half of the experiment after a fixed amount of charge is deposited to the capacitor?

[@GreenLearning]

This is very cool! Can any one tell me if this still works with say a glass dielectric between the plates?

[@PashaSiraja]

@insightllc ok... right. thanx :S

[@shotgun3628]

An adjustable capasitor? A capasitor that can be in constant change. One might say a flux capasitor?

[@fahmylina123]

آخر خمس ثوانى هما المهمين لبيان انخفاض الفولت والتيار عند إدخال عازل بلاستك بين لوحى المكثف مما يؤدى إلى زيادة العزل الكهربى أى زيادة سعة المكثف فينخفض الفولت (لأن الشحنة ثابتة) - وهو ذات ما يحدث عند قياس رطوبة الأراضى بطريقة غير مباشرة تعتمد على وضع مجس تردد منعكس فى التربة (فعند زيادة رطوبة الأراضى تزداد سعة مكثف المجس لأن العزل الكهربى للماء مرتفع بالمقارنى بمعادن التربة والمادة العضوية وهواء التربة فينخفض الفولت الخارج من المجس عند وجود رطوبة أرضية مرتفعة - أما فى الأراض الجافة فلن تزداد سعة المكثف كثيراً فلا ينخفض الفولت كثيراً). ثم أهم شىء هو معايرة الفولت الخارج مع المحتوى الرطوبى المقدر بالطريقة المباشرة (الوزنية).