Some basic questions

Discussion in 'General Electronics Chat' started by aamirali, Mar 26, 2012.

  1. aamirali

    Thread Starter Member

    Feb 2, 2012
    415
    1
    1. Why electric trains don't feel any current shock. There are high voltage wires dirrectly above train, it can induces charges, doesn't it.

    2. In ac plug , 200W bilb can glow, 100W can also work, a fan can work or anything which we run in our houses. Load may be very small or larger also.

    I want 2 ask, if I have a small load lets ay 100W bulb, it draws less current, while a 200W bulb can draw larger currents. Similarly a computer tales larger lod.
    It means 100W bulb have larger resistance while 200W bulb have less. But in actual I don't any resistive element. How to explain this. How does any load draws current, according to its need.


    3. In CE transistor application, when we say its input & output impdence. What are we actually pointing at.
    Its input resistance frrom what???????????
     
  2. buzaiandras

    Member

    Jul 18, 2011
    54
    0
    I will try to answer your questions but please note that my information may not be accurate (I am no expert in electricity, in fact I am a beginner). So maybe someone with more experience can give you better answers.

    1. I am not sure if this answers your question but if I understand it correctly a train acts as a Faraday cage so there won't be any electric shock on the inside of the train.

    2. As far as I know the fact that the "load draws" current is just an expression.
    In fact it opposes the "flow of current". The more it opposes the lower is the
    current. In a light bulb for example the opposition is done by the tungsten
    filament
     
  3. buzaiandras

    Member

    Jul 18, 2011
    54
    0
    Hi,

    Could a more experienced/knowledgeable user shed light on this for us?

    Thank you,

    Buzai :)
     
  4. Stuntman

    Active Member

    Mar 28, 2011
    189
    50
    1. Why electric trains don't feel any current shock. There are high voltage wires dirrectly above train, it can induces charges, doesn't it.

    Just because the power is overhead doesn't mean you will get shocked. The power lines for your home are (most likely) located overhead, but they are safely routed and transformed down to your home to for use... without using your home's structure or YOU as a conductor.

    Consider if the train's over-head line was just a huge extension cord with little receptacles every inch. Why couldn't the train run by moving and inch, unplugging, plugging into the next receptacle, running another inch, etc. Silly, yes, but in essence, this is what they are doing only with and exposed continuous contact.


    2. In ac plug , 200W bilb can glow, 100W can also work, a fan can work or anything which we run in our houses. Load may be very small or larger also.

    I want 2 ask, if I have a small load lets ay 100W bulb, it draws less current, while a 200W bulb can draw larger currents. Similarly a computer tales larger lod.
    It means 100W bulb have larger resistance while 200W bulb have less. But in actual I don't any resistive element. How to explain this. How does any load draws current, according to its need.

    Both your 100 and 200 watt bulbs have resistance to limit their power usage. So does the house fan, and your computer. The issue is, not all resistances are in the classical form. Some devices, such as an electric stove element, really are a pretty classical resistor. Others, such as a flourescent light have special circuitry to create this resistance (otherwise the apparatus would self destruct from the current).

    In short, each device has it's own method of limiting current. Some methods, such as the electric stove element, are very straight forward. Others, such as the fluorescent light, can be more complex, but still achieve the same basic task of limiting current.



    3. In CE transistor application, when we say its input & output impdence. What are we actually pointing at.
    Its input resistance frrom what???????????

    This can be a little abstract, so I will leave this in your hands to research further, but consider this. Lets say you have a 1.5V AA battery. First, you probe the battery with the voltmeter to find it indeed supplies 1.5V with no load attached. Next, you hook the battery to a 0.25W light bulb. As the bulb is lit, you connect the voltmeter to the AA battery terminals to find that the voltage is 1.48V. Now, this is only a small amount of voltage drop because the light bulb has a high input impedance, and the battery has a fairly low output impedance.

    Now, two scenarios to better describe input and output impedance:

    First, you replace this 1.5V AA battery with a tiny 1.5V watch battery. The bulb still lights, but much dimmer than with the AA. When you check the terminals on the battery, you find it is 1.28V! This is because the watch battery has a higher output impedance than the AA battery. So in essense, the output impedance is how "well" a electrical source can supply power.

    Now, scenario two. You keep the AA battery in the circuit, but replace the 0.25W light bulb with a 1W (less resistance) bulb . You use your voltmeter to probe the battery and find a voltage of 1.15V. The voltage is lower because the input impedance (resistance) of the 1W light bulb is lower than that of the .25W (1/4 to be exact).

    These same rules apply in microelectronics but get much harder to measure and comprehend. Input impedance is the easier of the two as you simply are measuring the resistance looking into the load (whether that be a light bulb in the example above, or the base of of BJT in common emitter mode).

    The output impedance, however, is more difficult, as you are really measuring how effectively the output (such as the Vout from the collector in CE mode) can DRIVE a load. A low output impedance (AA battery) means it can drive a low resistance load. The higher the output impedance, the less it can drive. Although it seems odd we would use a term such as impedance to measure such a quantity, but realize that no battery or voltage source (even mains power) is perfect, instead, is can be modeled as a voltage source AND a source impedance (in series).... this source impedance is your output impedance...
     
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