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Weekend Quiz. Back to Basics.

Zoomup
Q.1. There are two resistors in series in a circuit, of 18 Ohms and 22 Ohms resistance. The current through the 18 Ohm resistor is 6 Amps. Calculate the supply Voltage to the circuit.


Q.2 There are three resistors of resistance 20 Ohms, 10 Ohms and 8 Ohms connected in parallel to a 6 Volt D.C. supply. Calculate:


a, The total resistance.


b, The current taken from the supply.


Q.3. A coil of 30 Ohms reactance is connected to a 200 Volt A.C. supply circuit, in series with a of 50 Ohms resistance. A capacitor of 30 Ohms reactance is connected in series with the coil. Calculate,


a, The circuit impedance.


b, The current drawn from the supply.


c, The power used.


Q.4. An electric heater uses 5.4MJ in 45 minutes. What is the rating of the element?


a, 1kW.


b, 1.5kW.


c, 2kW,


d, 3kW.


Q.5 The input of an electric motor is 5.8kW. The output is 4.75kW. The efficiency is,


a, 1.05%


b, 10.5%


c, 82%


d, 122%


Z.


  • 0
    1: 180v


    2: a) 3.636 approx ohms

       b) 1.65 amps


    3 a)110 ohms

    b) 1.81818 amps

    c) 90.9 watts?


    4. c,2kw


    edit, missed 5: it's c, 82%


    No3 I am really really rusty on so probably entirely wrong
  • 0
    Q1 = 240 volt


    Q2a = 3.6363636 recurring


    Q2b = 1.65 amp


    Q3a = 50 ohms resistive


    Q3b = 4 amp


    Q3c = 800 watts


    Q4 = c = 2 kW


    Q5 = 82%


    Clive


  • 0

    AncientMariner:

    Q1 = 240 volt

     



    D'oh, how did come up with what I did?
  • 0
    The 18 ohm and the 22 ohm resistors are in series, thus the circuit resistance of the two resistors in series is 40 ohms. If 6 Amps flows through one resistor, then there must be the same current flowing through the other. So from V = I x R   6 amps x 40 ohms = 240 volts.


    Clive
  • 0
    What AncientMariner said:



    Q1   240V


    Q2a    3.63Ohms 


    Q2b    1.65A


    Q3a    50Ohms


    Q3b   4A


    Q3c   800W


    Q4    2kW


    Q5    81.9%


    mrf
  • 0

    AncientMariner:

    The 18 ohm and the 22 ohm resistors are in series, thus the circuit resistance of the two resistors in series is 40 ohms. If 6 Amps flows through one resistor, then there must be the same current flowing through the other. So from V = I x R   6 amps x 40 ohms = 240 volts.


    Clive 




    I did the 18 ohm 6amp calc then applied 22ohm 6amp and added them together to get the answer, same answer but a bit clumsy. I did a couple more but all in mee heed. No calculator with me to speak about, then i gave up being away from home . A few days by the lake, nice countryside even in this weather

  • 0
    Thanks all,

                         Ancient Mariners are good at physics and maths it seems. All correct.


    Ref. Q.3 (a)  The 30 Ohm reactance of the capacitor and the 30 Ohm reactance of the coil cancel out to zero effect. So, x = 30-30=0. Therefore Z=R.


    Q.4. Watts=Joules (energy) divided by time (seconds). One Watt=One Joule per Second.


    Q.5. Efficiency = OUTPUT divided by INPUT and is always less than 100 percent as all machines have losses.


    Z.




  • 0

    Zoomup:

    Ancient Mariners are good at physics and maths it seems. All correct.




    Agreed. I got exactly the same answers but didn't see much point in posting a duplicate set. As another ex-seafarer I suspect it is a result of the long hours on a ship with little to do which leaves a lot of time for thinking (after all you can only drink so much duty free booze.....)

    Alasdair

  • 0
    A

    Hello Alasdair,

                           that just proves that time well spent in learning is a benefit in later life, rather than wasting time engaged in trivial pursuits. Good school and college education should be focused and concentrated, as that will provide good results. I think that we studied well because we enjoyed the subjects and did well as we applied ourselves, even to homework after school. (Please don't tell anyone, but I am not very clever. I did not make up the questions, I "borrowed" then from an I.E.T. book.....shhhhhh).


    Z.


    Z.
  • 0
    AC Theory was drummed into me long before I went to sea. Q3 being a nice series resonant circuit.


    After leaving school in 1965, instead of going for ONC/OND etc as I had considered, I was talked into applying for a MoD position which included an intensive training course. So I must thank one of the Instructors there, a Mr Brocklebank, whose rendition of, “One Over Two Pi Root LC Gives You the Resonant Frequency” ensured that to this day I still remember this particular formulae. His other ability, being ambidextrous, was to write to the bottom of a section on the wall length blackboard with his left hand and then immediately start from the top of the next section using his right hand...
    The RAF Maintenance Command Civilian Technical Training School at RAF Sealand Dan Archer, occupied the buildings of the US Air Force prison which was on the RAF Sealand site during the American occupation of RAF Sealand between 1951 to 1957 when it was home to the 7558th Air Depot Group, 3rd Motor Transport and others. I can remember seeing into one of the cells, not very appealing. The cells then being used for storage of what looked like junk, although thinking back I can remember seeing what appeared to be the control stick of a World War 2 fighter aircraft.
    The bonus for being in an American designed building over the winter months was that there was central heating, excessive central heating in fact.
    I wonder why I receive strange looks when I say that I once spent 20 weeks in an American prison? Well, at least I came home every night!


    Clive




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