Integrated SequencePhysics Chemistry Organic Biology

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HyperPhysics - Polarization of Dielectric

PY106 Notes - Capacitors and batteries
Clear, concise introduction to the concept of capacitance, capacitors, and dielectric materials.



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Special points of emphasis

Electricity

The Chemical Bond

If a dielectric material is inserted between the plates of a capacitor the capacitance increases. In other words, the capacitor can hold more charge before reaching a given voltage if a dialectric is present.

Polar materials make good dielectrics. Why is that? The polar substance acts to weaken the field between the charged plates.

How does a dielectric substance weaken the field associated with a given amount of charge on the plates? This occurs because the electrostatic force from the external field causes polar molecules to rotate and orient themselves opposite to the externally applied field. This produces a weaker overall net field with the individual dipole moments of the molecules in opposition to the orientation of the field.

Because the field is weaker with the dielectric present, the voltage between the plates is lower than it would otherwise be. In other words, more charge is now going to be required to produce the same voltage as before. The dielectric between the plates has increased the capacitance of the capacitor.




Work, Energy, and Power

Electricity

The Chemical Bond

The First Law of Thermodynamics

Water

Solutions

We discussed earlier how the general idea of capacitance was a useful conceptual tool for interpreting the energy of charge distributions at the molecular level. In fact, dielectrics behave during chemical processes in the same way that they behave between capacitor plates.

Similarly to how the dielectric between the plates of a capacitor reduces the voltage between the plates, the dielectric effect of water decreases the work required to separate the anions and cations of an electrolyte solute as it dissolves.

Because the field between them is weaker with the interposition of a dielectric solvent, such as water, the separation of anions and cations of a dissolving electrolyte now occur at lower energy. The aqueous solvent weakens the electric field between the separating charges.




Work, Energy, and Power

Electricity

Functional Groups in Organic Chemistry

Chemical Kinetics

Proteins

It is useful to develop the ability to picture the functional groups present on the amino acid side-chains within the active site of an enzyme. A motif is for certain side chains to be positioned vis-a-vis the substrate to act as dielectrics to reduce the energy associated with the charge densities of transition states of chemical reactions. Reducing the energy of the activated complex is the mechanism in such cases for increasing the reaction rate.

Let us discuss for a moment the complicated position of the MCAT with regard to biochemistry and how to prepare. Although the full biochemistry course is not technically a prerequisite for the exam, MCAT passages often present biochemistry content. Don't panic. If you have not had biochemistry you will still be able to answer the questions correctly if you understand fundamental principles. Often the MCAT writers will not assume that students have prior familiarity with the specific content of the passage. Biochemistry is not a prerequisite of the MCAT, but biochemistry is on almost every exam.

The goal of such passages is to assess your ability to interpret unfamiliar phenomena in the light of fundamental principles. Bring your physical sciences and biological sciences to bear on 'unfamiliar' phenomena and view it as just another instance of fundamental concepts.








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