Physics listening notes ocr b

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  • Created by: Rhys
  • Created on: 22-05-12 16:35

Charge and current

  • Electric current is a basic quantity defined in terms of the force between two long, parallel, current-carrying conductors.
  • Current is the rate of flow. charge (q)= current (I) times time (t). Current is measured using an ammeter connected in series.
  • 1 coulomb is the quantity of charge which passes when a current of 1 ampere passes for a time of 1 second.
  • The elementary charge is e=1.60 times 10 to the-19. This is the size of the negative charge of an electron and the positive charge of a proton.

Charge Carriers

  • In a metal charge is carried by movement of free electrons; in an electrolyte, charge is carried by positive and negative ions.
  • The conventional current is the direction in which positive charges would move. The electron current is in the opposite direcction.
  • Conductors, semiconductors and insulators have different numbers (n) of charge carriers per unit volume.
  • In a metal wire of cross-sectional area (A) with electrons moving with drift velocity (V), the current is I=charge carriers (n) times Cross sectional area (a) times voltage (v) times charge on a electron (e).

Potential difference and e.m.f.

  • The potential difference between two points is defined as the work done in moving unit charge between those points. Voltage (v) =Work done (w) divided by  charge (q) .
  • The p.d. or voltage across a component is measured using a voltmeter (of very high resistance) connected across it, i.e. in parallel with it.
  • There is a p.d. of 1 volt between two points if 1 joule of work is done when 1 coulomb of charge moves between the points. 1Volt = 1Joule per Columb.
  • The electromotive force (e.m.f.) of a sourse is defined as the work done in

    Resistance

    ·The functions of a resistor in a circuit are to control the current, and to dissipate energy in heating.

    ·The resistance (R) = p.d across the conductor (V) divided by current though it (I).

    ·A conductor has a resistance of 1 ohm (Ω) if the current though it is 1 ampere when the p.d. across it is 1 volt.

    ·Ohm's law: for a metallic conductor at constant temperature, the current through it is proportional to the p.d. across it.

    moving unit charge round a circuit.

Current-Voltage characteristics

· The I/V characteristic (graph) for a fixed resistor at constant temperature is a straight line through the origin.

· The I/V characteristic for a filament lamp curves away from the current axis, because the resistance of the lamp increases with temperature.

· The I/V characteristic for a diode shows zero current for negative p.d.s, and for positive p.d.s up to about 0.5  V. The current then rises steeply, showing that the diode has low resistance in the forward direction.

Resistivity

· The resistivity of a conductor of length L, cross-sectional area A and resistance R is resistivity = resistance times cross section area  divided by length. Thus R = ρ.L divided

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