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PHYS 241 Recitation

PHYS 241 Recitation. Kevin Ralphs 19 Sep 2012. Overview. HW Questions Capacitance Exam Review/Tips. HW Questions. Ask Away…. Capacitance. What does it tell me? The charge that accumulates on two conductors is proportional to the voltage between them Why do I care?

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PHYS 241 Recitation

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  1. PHYS 241 Recitation Kevin Ralphs 19 Sep 2012

  2. Overview • HW Questions • Capacitance • Exam Review/Tips

  3. HW Questions Ask Away….

  4. Capacitance • What does it tell me? • The charge that accumulates on two conductors is proportional to the voltage between them • Why do I care? • Capacitors are vital components in electronics • They can be used to temporarily store charge and energy, and play an even more important role when we move to alternating current systems • Camera flashes, touch screen devices, modern keyboards all exploit capacitance

  5. Capacitance • What does capacitance depend on? • Geometry of the plates • Material between the plates • Unit of capacitance is the Farad • To demystify this, units are (Length *permittivity)

  6. Capacitance • Dielectric • Put simply, a dielectric is a material (an insulator) that weakens the electric field around it • This allows more charge to be placed on the plates for the same voltage (i.e. capacitance is increased) • The permittivity of a dielectric tells you how it affects the capacitance • The ratio of the permittivity of a dielectric and the permittivity of free space is the dielectric constant

  7. Capacitance • The permittivity of free space has no physical meaning • It merely changes physical quantities into their appropriate SI units

  8. Capacitance • A charged capacitor has potential energy • Note: This means that inserting a dielectric into a capacitor while it is disconnected from a voltage source will lower the potential energy (in fact, it will be sucked in)

  9. Capacitance • In circuits • In well-behaved configurations, capacitors may be combined into a single equivalent capacitor • Parallel * This is like increasing the area of the plates * • Series * This is like increasing the separation distance *

  10. Capacitance • Capacitors are in equilibrium • Series: when they have the same charge • Parallel: when they have the same voltage

  11. Exam Review/Tips • Exam covers through chapter 24 • Old exams are online: review them to see what you might want to put on your crib sheet • When building your crib sheet, be sure to include any underlying assumptions related to a formula • Most of the problems are structured so that conceptual knowledge will permit a quick solution while brute force calculation will bog you down

  12. Exam Review/Tips • Exam review will be conducted on Sunday February 10 from 2:00-5:00pm in PHYS 112 • First hour is content review, followed by an hour of practice problems and an additional hour where I will answer questions and do more practice problems • iClickers are recommended, but not required

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