Conductors, Capacitors and Dielectrics

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  • AP Physics Rapid Learning Series - 15

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    Rapid Learning CenterChemistry :: Biology :: Physics :: Math

    Rapid Learning Center Presents Rapid Learning Center Presents

    Teach Yourself AP Physics in 24 Hours

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    Conductors, C it dCapacitors, and Dielectrics

    Rapid Learning CoreTutorial Series

    Rapid Learning Centerwww.RapidLearningCenter.com/ Rapid Learning Inc. All rights reserved.

    Wayne Huang, Ph.D.Keith Duda, M.Ed.

    Peddi Prasad, Ph.D.Gary Zhou, Ph.D.

    Michelle Wedemeyer, Ph.D.Sarah Hedges, Ph.D.

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    Learning Objectives

    Understand the factors that affect conduction

    By completing this tutorial, you will:

    that affect conduction.

    Learn about the construction and use of capacitors.

    Describe the effects of dielectric materials in a

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    dielectric materials in a capacitor.

    Concept MapPhysics

    Studies

    Previous content

    New content

    Resistivity

    Forces

    Electrical Forces

    Conductors

    y

    Length

    Area

    and

    and

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    Electric Charge

    Caused by

    Stored inCapacitors

    store Electrical Energy

    Dielectrics

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    Conductors

    For electrons to move or flow, they must travel through some substance This is a

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    travel through some substance. This is a conductor.

    Conductors

    Conductors: Material where electrons are loosely bound and are able to flow throughout due to the free electrons. Examples include metals, impure water, and human bodies.

    6/40Dont try this at home...

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    Insulators

    Insulators: Materials where electrons are bound and dont flow easily. Examples include glass, rubber, and plastic.

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    Semiconductors

    Semiconductors: Materials in between insulator and conductor. Examples include silicon, and germanium.

    Used in transistors and other electronic components.

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    Superconductors

    Superconductor: a material where electrons flow without any resistance. Generally, superconductivity only occurs at very low temperatures. Magnetically

    f flevitated fast trains are one application of superconductivity.

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    Resistors

    Resistors are used to control the amount of electric charge flowing.

    R i tResistor symbol

    Many types exist. U ll l d i

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    Usually a color code is used to determine the value.

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    Resistivity

    An intrinsic property of a material is its resistivity, . Along with other factors, this is used to determine the resistance to electron flow of a particular piece of wire.

    has units of m. Thus, resistivity is a measure of resistance per length of a material.

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    However, length is not the only factor to determine resistance.

    Factors of Wire Resistance

    The resistance of a wire can depend on several things including:

    thi k ( ) f i

    length of wire

    thickness (area) of wire

    temperature of wire

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    g

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    Resistance Formula

    Resistance,

    Resistivity of material, m

    ALR =

    Cross sectional

    area of wire Length of

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    Notice, units turn out to equal for resistance. This unit will be discussed further in a later tutorial.

    area of wire, m2

    gwire, m

    Temperature and Resistance

    Resistance of a wire does depend on temperature. In general, a higher temperature means a higher resistance.

    Imagine the atoms of a hotter wire moving very rapidly. This could interfere with the conduction of electrons, thus increasing resistance.

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    Cold atoms Warmer atoms

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    Superconducting

    In 1911, it was first observed that mercury at 4.2K has a resistance of zero!

    This state is called superconducting.

    Generally, only very cold materials are superconducting, so it isnt very practical for everyday use Maybe in the future

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    everyday use. Maybe in the future...

    Capacitors

    Sometimes there isnt enough electrical energy available for a particular need

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    energy available for a particular need. Capacitors are used to store energy for future use.

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    Capacitors

    A capacitor ( condenser ) is a device used to store electrical energy.

    They accumulate and store energy for later use. Its like saving your pennies in a piggy bank for a large future purchase.

    S b l f it

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    Symbol for a capacitor:

    Capacitor ConstructionA capacitor consists of two separated conductive plates connected to an +connected to an electric potential difference.

    + -

    Positive charge builds up on one plate, negative

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    Battery or voltage source

    + -charge builds up on the other.

    Overall, the net charge on a capacitor is zero.

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    Charge Stored in a Capacitor

    The amount of charge a particular capacitor can store depends on the voltage, and the rating or capacitance of the particular capacitor.

    CVq =Electric

    potential, VCharge, C

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    Capacitance, Farads, F.

    1 Farad = 1Coulomb/Volt

    One Farad

    One Farad would be a particularly large capacitor. This would be large in physical size and charge storage.and charge storage.

    1 F=10-6 F

    Most are much smaller, usually in units of micro Farads, F.

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    1 F 10 F

    The capacitance unit is named after Michael Faraday. He also invented the notion of field or force lines.

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    Charge Storage Example

    How much charge is stored in a 100 F capacitor that is connected to a 12V battery (electric potential source).

    First change F to F.

    =

    F 1F10 x F 100

    6

    F1x10 4

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    CVq === F)(12V)(1x10q 4 C12x10 4

    1 Farad = 1Coulomb/Volt

    Capacitance Formula

    This rating or capacitance of a capacitor depends on a few factors.

    Area of each

    dAC o=

    Area of each plate, m2o = permittivity of free space

    8.85 x10-12 C2/Nm2

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    dCapacitance,

    Farads, F Distance between plates, m

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    Increasing Area

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    To have a more powerful capacitor, you would want a larger area. However, that may be impractical, so some capacitors are rolled to maximize space. This give the typical round or cylindrical shape of capacitors.

    Discharging

    A capacitor fills up while electrons are being deposited onto one of the plates.

    The capacitor can be discharged if the two plates are connected. (A switch is usually used)

    This causes all the accumulated electrons to

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    suddenly flow back. This movement of charge can be used.

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    Discharging Diagram

    +

    + -

    -The switch is closed to complete the

    + -

    Notice how the h d it

    circuit and discharge the capacitor.

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    charged capacitor can temporarily power the light bulb as it is discharged.

    UsesIn the flash of a camera, charge is stored by a capacitor until needed (flash).

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    A short time is needed for that charge to re-accumulate, then the flash can be used again.

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    Analogy

    Imagine a stream gradually running into, and filling a dam (capacitor). The water (charge) accumulates until it is released (discharged).

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    Dielectrics

    A dielectric is a material that enhances the capacity of a capacitor

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    the capacity of a capacitor.

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    Dielectrics - Illustrated

    Often an insulating material is inserted between the plates of a capacitor. This substance is called a dielectric.

    +++++++

    ______

    Dielectric

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    Dielectrics resist charge flow more than air. Thus higher volta

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