PHYS 142 Lecture Notes - Lecture 13: Dielectric, Heat Capacity, Electrical Energy

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PHYS 142: Electromagnetism and Optics - Lecture 13: Capacitance and Capacitors Part 3
Storing Energy in a Capacitor:
A capacitor can be charged slowly but then can release the energy very quickly.
An important medical application of capacitors is the defibrillator.
A heart attack or a serious injury can cause the heart to enter a state known as fibrillation in
which the heart muscles twitch randomly and cannot pump blood.
A strong electric shock through the chest completely stops the heart, giving the cells that
control the heart’s rhythm a chance to restore the proper heartbeat.
Electric Energy Density:
Electric energy stored in a capacitor is associated with the electric field in the capacitor.
This relation can be shown through the energy density of the capacitor:
This is a general result. If we know the E-field in a region, we can easily calculate the electric
energy density in that region.
Dielectrics:
A normal parallel-plate capacitor has the plates separated by a vacuum.
When the capacitor is fully charged to voltage (ΔVC)0, the charge on the plates will be ±Q0,
where Q0 = C0VC)0.
The subscript 0 refers to a vacuum-filled capacitor
Now, consider that an insulating material is slipped between the capacitor plates.
An insulator in an electric field is called a dielectric.
The charge on the capacitor plates does not change (Q = Q0).
However, the voltage has decreased: ΔVC < (ΔVC)0
We define the dielectric constant:
The dielectric constant, like density or specific heat, is a property of a material.
Easily polarized materials have larger dielectric constants than materials not easily polarized.
Vacuum has κ = 1 exactly.
Filling a capacitor with a dielectric increases the capacitance by a factor equal to the dielectric
constant:
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Document Summary

Phys 142: electromagnetism and optics - lecture 13: capacitance and capacitors part 3. A capacitor can be charged slowly but then can release the energy very quickly. An important medical application of capacitors is the defibrillator. A heart attack or a serious injury can cause the heart to enter a state known as fibrillation in which the heart muscles twitch randomly and cannot pump blood. A strong electric shock through the chest completely stops the heart, giving the cells that control the heart"s rhythm a chance to restore the proper heartbeat. Electric energy stored in a capacitor is associated with the electric field in the capacitor. This relation can be shown through the energy density of the capacitor: If we know the e-field in a region, we can easily calculate the electric energy density in that region. A normal parallel-plate capacitor has the plates separated by a vacuum.

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