Physics 17: 7-11 Electric Capacitance

Homework

Reading Preparation
Key Equations
WebLecture
Study Activity
Chat Preparation Activities
Chapter Quiz
Lab Work

Reading Preparation

Text Reading: Giancoli, Physics - Principles with Applications, Chapter 17: Sections 17 to 11

Study Points

17.7: Excess charge can be stored and held, creating a capacitor whose capacitance depends on the potential difference or voltage across the charged plates: C = Q/V, with C in units called farads. The capacitance of a parallel plate capacitor depends on the area of the plates and distance apart: C = &epsilon₀ A/d.
17.8: A dielectric is the medium between between the plates of a capacitor. It changes the capacitance by a factor K, which is 1 for a vacuum and greater than 1 for all other materials.
17.9: Stored energy U depends on the charge and can be expressed in different ways: ½ QV = ½CV² = ½Q²/C = ½ (ε₀A/d) (E²d²) = ½(ε₀E²Ad)
17.10 Signal voltages carry information by varying voltage according to specific rules that can be interpreted by the receiving device. Real voltage varies continuously, but modern computer data is stored in digital format, so devices must use analog-to-digital converters to send signals, and digital-to-analog converters for analog-dependent devices like music speakers and CRT displays. Conversion to digital data allows the sending device to compress data, making it possible to transmit large amounts of information in shorter periods of time.
17.11: CRTs and oscilloscopes use beams of charged particles that interact with target screens. The patterns formed on the target can be controlled by changing the electrical field through which the particles travel.
17.12: Because nerves operate by passing electrical charges, we can detect and monitor changes in electric potential of cells and tissues, and effectively "map" healthy and unhealthy patterns of change. Electrocardiogram records to changes in electrical potential in a person's heart.

Key Equations

Charge on capacitor: $Q = CV where C = ϵ_{0} \frac{A}{d}$
Charge on dielectric with permittivity Kε₀: $C = {K ϵ}_{0} \frac{A}{d} = ϵ \frac{A}{d}$
Electrical energy on capacitor: $PE = \frac{1}{2} QV = \frac{1}{2} (CV) V = \frac{1}{2} C V^{2} = \frac{1}{2} Q (\frac{Q}{C}) = \frac{1}{2} \frac{Q^{2}}{C}$
Energy density $\frac{PE}{Volume} = \frac{1}{2} ϵ_{0} E^{2}$

Web Lecture

Read the following weblecture before chat: Electric Fields and Capacitance: Storing Charge

Study Activity

Use the simulation below to construct capacitors with different battery voltages and distances.

How does doubling the voltage change capacitance?
How does changing the direction of the current (using negative voltage) change capacitance?
How does doubling the separation of the plate change the field between them?
How does doubling the area of the plate change the field? How does it change the capacitance?

Physics simulation Java Applets are the product of the PHET Interactive Simulations project at the University of Colorado, Boulder.

Chat Preparation Activities

Forum question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

Chapter Quiz

Required: Complete the Mastery exercise with a passing score of 85% or better.
Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!

Lab Work

If you want lab credit for this course, you must complete at least 18 labs; you may complete more if you are preparing for the AP exam.. One or more lab exercises are posted for each chapter as part of the homework assignment. We will be reviewing lab work at regular intervals, so do not get behind!

Lab Instructions: More Complex Circuits

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Physics

Chapter 17: 7-11 Assignment