25.7
誘電体を使用したコンデンサ
平行板コンデンサは、一定の距離を隔てた 2 つの導電板で構成されます。 しかし、実際に接触せずに大きなプレートを互いに平行に保持することは機械的に困難です。 したがって、一般にプレート間に誘電体層が配置され、小さなギャップでプレートを保持するための簡単な解決策が提供され、コンデンサの静電容量が増加し…
平行平板コンデンサは、わずかな距離で隔てられた2つの導電板で構成されています。しかし、プレートを接触せずに近づけることは機械的に困難です。これは、コンデンサプレートの間に非導電性媒体を挟むことで解決できます。
最も一般的なのは、コンデンサは、誘電体媒体で区切られた長い金属箔のストリップでできています。誘電体は、緩く結合した電子や自由電子を持たない非導電性材料です。
プレート間のギャップを埋める誘電体スラブを備えた完全に充電された平行プレートコンデンサを考えてみましょう。誘電体内の双極子は、電界の方向に沿って整列します。
正極板に向かう負電荷と負極板に向かう正電荷の位置合わせにより、プレート間の電圧は低下しますが、総電荷は同じままです。電圧が低下すると、静電容量が増加します。
静電容量が増加する係数は、材料の誘電率です。誘電体の存在は、常に静電容量を増加させます。したがって、誘電率は常に1よりも大きくなります。
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Q1: Why is a dielectric material placed between capacitor plates?
A dielectric is placed between capacitor plates to solve the mechanical difficulty of holding large parallel plates without contact. The dielectric layer maintains a small gap between the plates while increasing capacitance. Dielectrics are non-conducting materials with no free or loosely bound electrons, making them ideal for this purpose.
Q2: How does a dielectric affect the voltage and charge in a capacitor?
When a dielectric fills the gap between capacitor plates, its dipoles align along the electric field direction. This alignment reduces the voltage across the plates by orienting negative charges toward the positive plate and positive charges toward the negative plate. However, the total charge on the plates remains conserved despite the voltage decrease.
Q3: What is the dielectric constant and why is it always greater than one?
The dielectric constant is the factor by which capacitance increases when a dielectric is introduced between the plates. Since the presence of a dielectric always increases capacitance, the dielectric constant is always greater than unity. It quantifies how much a specific material enhances the capacitance compared to an empty capacitor.
Q4: How does a dielectric increase capacitance in a parallel plate capacitor?
A dielectric increases capacitance by reducing the voltage across the plates through dipole polarization. When dipoles align with the electric field, they create an opposing field that lowers the overall voltage. Since capacitance is inversely related to voltage for a fixed charge, the voltage reduction directly increases the capacitance value.
Q5: What happens to the electrical energy stored when a dielectric is added to a capacitor?
The electrical energy stored in a capacitor is reduced when a dielectric is introduced. If the initial energy stored is U0, the energy U with a dielectric is reduced by a factor of the dielectric constant. This reduction occurs because the dielectric lowers the voltage across the plates while charge remains constant.
Q6: What are the structural characteristics of dielectric materials?
Dielectric materials are non-conducting substances with no free or loosely bound electrons. In practical capacitor construction, long strips of metal foils are separated by a dielectric medium to create the capacitor structure. These materials allow dipoles to align with applied electric fields without conducting current.
Q7: How does dipole alignment in a dielectric reduce the electric field between capacitor plates?
When a dielectric is placed in an electric field, its dipoles align along the field direction. The negative charges orient toward the positive plate and positive charges toward the negative plate. This alignment creates an internal opposing field that reduces the net electric field and voltage between the plates.