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    Chapter 2:

    Electricity

    Form 5

    1

    Physics

    Next >

    The study of matter

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    Objectives:(what you will learn)

    1) electric fields & charge flow

    2) electric current & potential difference

    3) series & parallel circuits

    4) electromotive force & internal resistance

    5) electrical energy & power

    Physics: Chapter 2

    2

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    Electric Fields

    Electric field: region where a charged bodyexperiences a force

    It is shown by a field pattern that are lines of forces.

    line of force = path of a test charge in the field

    direction = motion of a free positive charge

    +

    Positive point charge

    Negative point charge

    electric field pattern

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    Electric Fields

    Between a positive

    and a negative

    point charge

    Between two

    positive point

    charges

    Electric lines of force

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    Electric Fields

    Electric field between twoparallel metal plates that

    are oppositely charged.

    Electric field between

    two opposite charges.

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    Electric Fields

    +

    +

    + F F

    Ball coated with conductor

    hangs vertically in the centre

    because it is neutral.

    Ball oscillating between 2

    plates, after it touches one side

    causing a force,Fto repel the

    ball due to like charges.

    +

    Negative ions

    Positive ions

    Candle flame spreading

    sideways between 2 plates due

    to attraction between oppositelycharged ions and metal plates.

    Experiments to show existence of electric fields.

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    Electric Fields

    Electric fields cause charges to move.Net movement of charges = electric current

    In the late 1700s scientists chose the direction of electric

    current to be the direction in which positive charges move in

    an electric field. They did not know that electrons andprotons were the negative and positive charge particles, and

    that the electron moved much more easily.

    In a copper wire, the outer electrons of the copper

    atom move relative to the nucleus of the atom.

    + -Current,I electrons

    So, the charge carriers (electrons) move in the

    opposite direction to the current.

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    Electric Charge

    8

    Electric current = Rate of flow of electric charge

    < Back

    Next >Electric charge, Q =It

    units Q in Coulomb,Iin Ampere, tin second

    I=

    Q

    t , t=time

    C = A s

    Basic unit of electric charge = Coulomb (C)Charge of a proton or electron = 1.60 10-19 C

    A Coulomb of charge is a lot, at 6.25 x 1018 electrons

    most objects have charges in the C (10-6 C) range.

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    Potential Difference

    V=

    W

    Q

    Work done

    Charge=

    Potential difference (V) between 2 points inan electric field = work done (W) in moving 1

    coulomb of charge (Q) between the 2 points.

    Unit of potential difference:

    Volt (V) = = J C-1

    J

    C

    A BMoving 1 coulomb of charge

    Potential difference between 2 points

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    Electric CurrentOhms Law

    The current (I) in a conductor is directly

    proportional to the potential difference (V) across

    the conductor if the temperature is constant.

    V

    I = constant

    Ohmic conductorA conductor that obeys Ohms Law.

    I

    V0

    A

    V

    I

    Conductor

    Switch

    Rheostat Circuit used to find the

    relationship between current

    Iand potential difference V

    for a conductor.

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    Electric Current

    Non-ohmic conductorA conductor that does not obey Ohms Law.

    I

    V0

    I

    V0

    I

    V0Dilute sulphuric acid Filament lamp Junction diode

    Examples

    A circuit element is non-ohmic if the graph of

    current versus voltage is nonlinear.

    A filament lamp is a non-ohmic conductor since its

    resistivity, like most materials, varies with

    temperature. As the filament gets hot, theresistance increases quickly.

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    Resistance

    The resistance,R of a conductor is defined as theratio of the potential difference Vacross the

    conductor to the currentIin the conductor.

    V

    IResistance,R=

    The unit of resistance is the ohm ().

    conductor

    V

    I I

    Potential difference, V=IR

    H li ti T iti

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    Resistance

    Factors that affect the resistance of a conductor:a. length of wire, l

    b. cross-sectional area,A

    c. type of material with resistivity,p

    d. temperature, T

    pl

    AResistance,R=

    Based on a constant temperature:

    R

    T/o

    C0 Metal Semi-conductor

    R

    0 T/o

    C

    H li ti T iti

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    Series Circuit

    I

    R1 R2 R3

    V1 V2 V3V

    V1 =IR1

    V2 =IR2

    V3 =IR3

    When resistors are connected in series:a. Same currentIis in all the resistors

    b. Potential difference,

    c. V= V1 + V2 + V3

    d. Effective resistance,

    R =R1 +R2 +R3

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    Parallel Circuit

    I R1 I1

    R2 I2

    R3 I3

    V

    When resistors are connected in parallel:a. Same potential differences across all resistors, V