basics for electrical engineers

Discussion in 'General Electronics Chat' started by whale, Oct 9, 2009.

  1. whale

    Thread Starter Active Member

    Dec 21, 2008
    Hello genius,

    I always use to think about electric circuits in my free time, and iam doing my engineering degree in electrical & electronics. Although I know working of big booms in electrical, I often use to ask basic questions to me and iam still not clear with that. Since these questions are very basic I don't know ,from where to get answers for these questions.
    I tried a lot but no use , those articles are not clear.

    Ok, let I come to my question.

    let us consider a 10 watt generator,with a output voltage of
    2 volt ,connected to a resistive load of rating 2 volt, 3 watt.
    When the circuit is closed there will be a current of 1.5 amps through the circuit.
    Thus the load resistance is 1.3333 ohms.

    Thus the power produced is 10 watts,power used is 3 watts. Where does the remaining 8 watts goes?how the power equation get balanced?

    will 5 amps of current flow through the circuit to balance the power equation?

    If you say, 3 watt load cannot be connected with 10 watts source, then how come different loads of different power ratings are connected to road side transformer with unique power rating?

    My another question is,

    if a 230 volt ,460 watt ac line is stepped down to 2 volt with a transformer,then the short circuit current will be 230 amps at the output of transformer.
    If we connect a 5 ohms resistance across the output terminal of transformer , accourding to ohms law input voltage of 2 volt connected across a resistance of 5 ohms should produce current of 0.4 amps.

    But in our circuit the current flow will be 230 amps to balance the power equation.
    How much current will exactly flow through the circuit?

    I hear that resistance of human body is 2 mega ohms,and only a voltage level above 40 volt will produce enough amps to burn our tissue. Since in our circuit the output voltage is 2 volt ,can I touch the output terminal with bare hands?will it shock me?

    I have touched the terminals of very high voltage in electric welding machine, how come such a high voltage don't shock me?
  2. steinar96

    Active Member

    Apr 18, 2009
    Generally it takes around 40V before you risk getting electrocuted because at that voltage the insulation of the skin breaks down. At lower voltages (assuming your skin is in normal dry condition) the insulation of the skin holds and present big resistance. Once it breaks down the resistance falls down rapidly making it easier for larger currents to pass trough your body.
  3. JDT

    Well-Known Member

    Feb 12, 2009
    When you say a "10 Watt generator with an output of 2 Volts", the 10W is the maximum power it can supply, it doesn't need to supply this all the time. You correctly state that when you connect it to your 1.33333 ohm load, 1.5A flows (ohms law) and 3W is dissipated. The other 8W does not go anywhere - it was never generated.

    Watts (power) is a consequence of of the voltage and current in a circuit. When we say that a resistor is 3W (say) it means that it will be damaged if we try to dissipate more power this. It is not a fundamental characteristic of the device.

    So, when you plug in your 3000W electrical kettle to make a cup of tea, all of a sudden in some power station somewhere the generator has to work a little harder to make that power especially for you. When you switch off your kettle that power stops being generated.

    Regarding electric shock, it is the current that gives the shock, not the voltage. It is thought that a current of 10mA can be felt and 30mA can stop the heart in some people. The human body is basically a salt-water solution covered in skin so the resistance of the human body depends on how moist the skin is. Wet or sweaty and the resistance is much reduced. A typical figure is about 10k ohm. Try using a digital multimeter on the resistance range and hold a probe in each hand. Wet your fingers and try again.

    Generally, for health and safety, voltages less than 50V are thought to be safe. Welders give out around 50V. Nothing to do with power!
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    Wattage is not term attached to transforming. Transformers are typically rated in VA, and convert those values. A shorted transformer can deliver a lot of current on short. Your secondary currents are determined by dividing the rated current by it's %Impedance.