CHP 9 - Energy, Power and Resistance Pt1

Circuit symbols
1. A battery is two or more cells put together. The longer end represents the positive terminal, the short represents negative
P.d and E.M.F
1. One Volt is the p.d across a component when 1J of energy is transferred per unit charge
  1. 1V=1JC^-1
  2. P.d is work done by the charge carriers per unit of charge. The difference in energy before and after a point. Electrical energy transferred into other forms.
2. E.m.f is work done on the charge carriers per unit charge, by a cell or battery. The charge carriers gain electrical energy
  1. The formula is the same as the one above
The electron gun
1. A small metal filament is heated until some elctrons get enough kinetic energy to escape the surface of the metal, in a process called thermionic emission.
  1. This is placed in a vacuum, with a high, accelerating p.d between the cathode and anode
    1. The freed electrons are accelerated towards the anode, gaining
      1. If there is a small hole in the anode, a fine beam of electrons is created.
2. As electrons accelerate, they gain kinetic energy. P.d=eV, V being the accelerating p.d.
  1. Work done on electron = gain in kinetic energy
    1. eV=1/2mv^2
Resistance
1. Resistance is the ratio of p.d Across a component/current in a component
  1. The unit is the ohm, were 1Ω=1VA⁻¹
2. Ohms law states: For a conductor kept at a constant temperature, current in the wire is proportional to the p.d across it
  1. When a wire gets hotter, resistance increases and therefore current decreases. This is because positive ions vibrate with a greater amplitude, increasing the frequency of the collisions. The electrons transfer more energy, which is given off as heat.
I-V characteristics
1. The circuit shown left is used to collect data for an I/V graph, but the fixed resistor can be any component
  1. A filament lamp is non ohmic, since p/d isnt proportional to Current. The resistance of the lamp is also not constant
    1. Resistance increases as the p.d across it increases, since the wire increases with temperature until it glows.
2. Fixed resistors have constant resistance, regardless of temperature. Voltage is proportional to temperature, therefore they follow ohms law
  1. Shallower line = higher resistance, since R=Inverse of gradient (R=V/I, not I/V)
Diodes
1. A diode only allows current to flow in one particular direction (shown by the way the 'arrow' faces)
  1. Some diodes are light emitting diodes, shown by two arrows. They function the same way as normal diodes, but light up when current flows
    1. A diode is non ohmic, and it's behaviour depends on the polarity
2. The I/V graph for a diode is shown left. When p.d is negative, resistance is almost infinite. No current can flow the wrong way through a diode
  1. When positive, current doesn't flow initially. A threshold p.d has to be reached (this is the point and L.E.D starts emitting light, the threshold p.d determines the colour/frequency of light)
    1. From this point, for every small increase in p.d, resistance drops sharply.

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CHP 9 - Energy, Power and Resistance Pt1

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	Creado por Kieran Lancaster hace más de 6 años