Class 46 Current and Electricity - Mr. Gopie Class ?· θωερτψυιοπασδφγηϕκλζξχϖβνµθωερτψ…

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  • Physics Current and Electricity

    Mr Rishi Gopie

  • Physics by Mr R Gopie

    2

    Current and Electricity Current and Charge

    Current is the rate of a directed flow of charge carriers. In a metallic

    conductor. Current is due to a directed flow of mobile (i.e. Free) electrons. The

    direction of conventional current is that in which positive charge carriers would

    move (if they could) and this is opposite to the direction in which the negative

    charge carriers (such as electrons) would move.

    The S.I. unit of electric current is the ampere and this is defined in terms

    of forces exerted between two straight, parallel, current-carrying conductors-in

    fact , this is the current flowing in each such conductor if they are 1 meter apart

    and exert equal and opposite forces of magnitude 1N on one another. The unit of

    electric charge is the coloumb (C) and this si defined as one ampere second

    (since quantity of charge Q = current, I x t) or as the quantity of charge flowing

    past a given point in one second when a steady current of one ampere is flowing.

    D.C. and A.C. Current exists as direct current, d.c. and as alternating current a.c. D.C represents

    a flow of current in one direction or sense only over time while a.c. represents a

    flow of current in two opposite directions or senses over time, i.e. flow and

    reversal of flow continuously.

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    Consider examples of current (I) / Voltage (V) time (t) graphs representing d.c. and a.c. d.c. On one side of the time axis

    a.c. on both side of the time axis

    Square wave or pulse d.c

  • Physics by Mr R Gopie

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  • Physics by Mr R Gopie

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    AC current

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    Rectified waveforms

    Once the variation is either completely above or completely below the time axis , it is d.c. Once the variation is both above and below the time axis , it is a.c. Consider sinusoidal a.c.

    The period T is the time taken to complete one cycle.

    The frequency, f, is the number of cycles per second.

    Note: T = 1/f and f = 1/T The peak value or amplitude is the maximum value (of V or I) in either direction.

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    Typical effects of an electric current include:

    1) Heating effects

    2) Magnetic effects

    3) Chemical effects

    Quantities, units, symbols and instruments of measurements. Quantity Typical

    Symbol Unit & Typical Symbol

    Instrument of Measurement

    Comments

    1) Unit Charge

    q or c Coulomb - C

    2) Number of Charge carriers

    N

    3) Total Quantity of charge

    Q Coulomb C

    4) Time t Second - s Watch or clock

    5) Current I Ampere-A Ammeter (or galvanometer)

    6) Voltage or Potential Difference

    V Volt - V Voltmeter

    7) Electromotive force (e.m.f)

    E or Volt-V Voltmeter

    8) Resistance R Ohm- Ohmmeter 9) Energy E Joule- J (kWh) Joule meter 10) Work Joule-J Joule meter 11) Power P Watt- W

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    Equations 1) Q = nq (where q = e and e = 1.6 x 10-19 C) 2) Q = IT 3) R = V/I 4) V = IR 5) I = V/R 6) W = QV (where W is work is also electrical energy) 7) P = IV 8) P = I2R 9) P = V2/R 10) E = Pt 11) E = V2t/R

    1 kWh = 1000W x 3600s = 3,600,000 J Circuits and Circuit components and their symbols

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    Another common component is a potentiometer or potential divider it is an

    arrangement for tapping off a variable or fixed fraction of a fixed applied voltage.

    Certain rheostats can be arranged to operate as potential dividers;

    Potential Difference (p.d.) /Voltage

    In order for current to flow through a component there must exist a

    potential difference (i.e. p.d) or voltage across the component. The p.d. / voltage

    between or across the ends of a conductor or component is the electrical energy

    per unit charge converted to other forms of energy, i.e.

    V = E/Q => E = VQ The unit of p.d. is the volt and it is defined as one joule per coulomb. The

    maximum voltage that can be obtained between the terminals of an electrical

    power supply, such as a cell, is called the electromotive force (i.e. supply) of the

    power supply.

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    Resistance All components in a circuit offer electrical resistance to the flow of

    current- some more than others. Certain components offer very low resistances

    and examples of these are connecting wires, switches, power supplies and

    ammeters. Other components offer much higher resistances and examples of

    these are voltmeters and resistors (fixed and variable).

    The resistance of a component such as a resistor in the form of a wire depends

    directly on its length and inversely on its area of cross-section. Also , the

    resistance depends on the nature of the material of which it is made- for

    instance, materials such as silver, gold , copper and aluminum have low

    resistances. So the longer the specimen of a given material the greater its

    resistance and the thinner the specimen, the greater its resistance. The reverse

    of both of these ideas is also true. The resistance R , of a component can be

    determined from the equation R = V/I, where v is the p.d./voltage applied across

    the component and I is the current flowing through the component. The unit of

    resistance is the ohm ().

    Resistors in Series and Parallel

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    Note the following

    1) I = I1 = I2 = I3, i.e. the same current flows through components in series.

    2) V = V1 + V2 + V3 i.e. the total individual p.d. across components in series is the sum of the individual p.d.s.

    3) R = R1 + R2 + R3, i.e. the total resistance of components in series is the sum of the individual resistances.

    4) I = I1 + I2 + I3, i.e. the total current through components in parallel is the sum of the individual currents.

    5) V = V1 = V2 = V3 , i.e. the total p.d. across components in parallel is the same as that across individual components.

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    Ammeters and Voltmeters

    An ammeter is an instrument for measuring the current through a

    component and so it must be connected in series with the component. In fact, an

    ammeter measures and indicates the current flowing through itself and it is

    assumed that the same current flows through the component since it is in series

    with the ammeter.

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    It is essential that the resistance of the ammeter itself be very small

    compared with their resistance in the circuit-otherwise inserting it into the

    circuit will change the very current it is to measure. An ideal ammeter has an

    extremely low (close to zero) resistance and hence an extremely low (close to

    zero) p.d. across itself.

    A voltmeter is an instrument used to measure p.d. (i.e. voltage) across a

    component and so it is connected in parallel with the component. in fact , a

    voltmeter measures and indicated the p.d. across itself and it is assumed that this

    is the same p.d. across the component since it is in parallel with the voltmeter.

    It is essential that the resistance of the voltmeter be very large compared with

    any other resistance in the circuit ( especially the resistance of the component

    across which it is connected) otherwise it will itself alter the very p.d. it is to

    measure by drawing a significant current away from the component. So an ideal

    voltmeter has an infinite (i.e. extremely high) resistance and hence draws a

    negligible (almost zero) current.

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    Ohm`s Law This law states that the p.d. applied across a metallic conductor is directly

    proportional to the current through the conductor, provided that physical

    conditions such as strain, temperature and illumination, remain constant, so VI

    and V/I = a constant, i.e. the resistance, R, of the metallic conductor.

    For a metallic conductor at constant temperature there is a linear relationship

    between V and I and a graph of V against I, or I against V, (known as the V-I or I-V

    characteristic), is a straight line through the origin (0,0). Conductors with such

    V-I or I-V, graphs are known as ohmic conductors.

    The slope of a V-I graph gives the resistance, R, of the conductor and that of a I-V

    graph give the reciprocal of the resistance, 1/R, of the conductor.

    Conductors, which do not have V-I or I-V graphs that are straight lines through

    the origin are called non-ohmic conductors. Consider typical I-V characteristics

    for both ohmic and non-ohmic conductors;

    Ohmic Conductors

    1) Metallic conductors (such as pure metals and alloys at constant temperature)

    2) An aqueous solution of copper sulphate with copper electrodes

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    Non-Ohmic conductors 1) Filament lamp/bulb

    2) Carbon resistors

    3) Semiconductor Diode

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    Consider a typical circuit for investigating Ohm`s law and deriving a conductor V-I or I-V characteristic:

    House Circuits

    Within the house, the connecting cables (themselves insulat

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