PhysicsLAB Resource Lesson
Capacitors and Dielectrics

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If the electric field between the plates becomes too strong, the air between them can no longer insulate the charges from sparking, discharging, between the plates. In order to keep this from happening, a dielectric is often inserted between the plates to reduce the strength of the electric field, without having to reduce the voltage being placed across the plates.
 
A dielectric is a polar material whose electric field aligns to oppose the original electric field already established between the plates. The dielectric is measured in terms of a dimensionless constant, K ≥ 1, whose value is usually referenced from a table. If this insulating material is insufficient the capacitor can leak allowing current to flow between the plates. If this occurs, the electric device "smells as if something is burning."
 


 
K = Eoriginal/Edielectric
K = Eo / Ed

K = Cdielectric/Coriginal
K = Cd / Co
 
When the battery is removed, the dielectric will decrease the electric field strength and the voltage between the plates while it increases their capacitance.
 
E = V/d
 
 
 
Refer to the following information for the next question.

A parallel-plate air capacitor is charged by placing a 90-V battery across it. The battery is then removed, and an insulating, dielectric fluid is inserted between the plates. The voltage across the capacitor is now 28 volts.
 
Since the battery is removed, the voltage is permitted to change but the charge on the plates must remain constant. The presence of the dielectric also allows the capacitance of the plates to be changed.
 
 
What is the dielectric constant of the fluid?

Refer to the following information for the next question.

A parallel-plate air capacitor holds a charge of 30 nC when a voltage of Vo is placed across its plates. If the battery is not removed and a dielectric fluid is inserted between the plates, the charge on the plates increases to 87 nC.
 
Since the battery is NOT removed, the voltage must remain constant since it is regulated by the battery's presence in the circuit but the charge on the plates is permitted to change. The presence of the dielectric also allows the capacitance of the plates to be changed.
 
 
What is the dielectric constant of the fluid?




 
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