CHP 9 - Energy, Power and Resistance Pt1

CHP 9 - Energy, Power and Resistance Pt1

1. Circuit symbols

   • A battery is two or more cells put together. The longer end represents the positive terminal, the short represents negative

2. P.d and E.M.F

   • One Volt is the p.d across a component when 1J of energy is transferred per unit charge
     • 1V=1JC^-1
     • 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.
   • E.m.f is work done on the charge carriers per unit charge, by a cell or battery. The charge carriers gain electrical energy
     • The formula is the same as the one above

3. The electron gun

   • 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.
     • This is placed in a vacuum, with a high, accelerating p.d between the cathode and anode
       • The freed electrons are accelerated towards the anode, gaining
         • If there is a small hole in the anode, a fine beam of electrons is created.
   • As electrons accelerate, they gain kinetic energy. P.d=eV, V being the accelerating p.d.
     • Work done on electron = gain in kinetic energy
       • eV=1/2mv^2

4. Resistance

   • Resistance is the ratio of p.d Across a component/current in a component
     • The unit is the ohm, were 1Ω=1VA⁻¹
   • Ohms law states: For a conductor kept at a constant temperature, current in the wire is proportional to the p.d across it
     • 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.

5. I-V characteristics

   • The circuit shown left is used to collect data for an I/V graph, but the fixed resistor can be any component
     • A filament lamp is non ohmic, since p/d isnt proportional to Current. The resistance of the lamp is also not constant
       • Resistance increases as the p.d across it increases, since the wire increases with temperature until it glows.
   • Fixed resistors have constant resistance, regardless of temperature. Voltage is proportional to temperature, therefore they follow ohms law
     • Shallower line = higher resistance, since R=Inverse of gradient (R=V/I, not I/V)

6. Diodes

   • A diode only allows current to flow in one particular direction (shown by the way the 'arrow' faces)
     • Some diodes are light emitting diodes, shown by two arrows. They function the same way as normal diodes, but light up when current flows
       • A diode is non ohmic, and it's behaviour depends on the polarity
   • 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
     • 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)
       • From this point, for every small increase in p.d, resistance drops sharply.

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