Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: More on electromagnetism

  1. Jan 20, 2004 #1
    OK first off I'd like to thank Warr for helping me with one problem
    again thanks a lot

    Okay now to business I have an ISU (Independent Study Unit) due this thursday and I'm pretty much all done, but it would be great if you guys could just look over it for any mistakes and or advices. ISU is 6 quetions, one I already posted so here we go:

    1.A charge of 2.730 C is placed at each corner of an equiolateral triangle 1.520 m on a side. Direction of force and magnitude on each charge?

    Solution: ok in all cases the magnitude of the force will be the same - because its an equiolateral triangle. to find it I used

    F=k x q1q2/r^2= 9.00x10^9 x 7.453^2/2.310^2=2.89x10^10 I got directions as well 2 for each charge -- compass ones -- N30W and NW, N30E and NE, and S30W and S30W

    This one was easy

    2.Two 1.5 V batteries are connected in series to a bulb with resistance of 10 Ohm . How many electrons leave each battery per minute?

    R=V/I 10=3V/I I=o.3A
    Q=It Q=0.3A x 60s=18C
    Q=Ne N=Q/e=18C / 1.6x10^-19 = 1.125x10^20 -- for 2 batteries

    But here is where I get lost/confused -- if the batteries are in series then say 2 electrons leave first battery, go into second one and then second has it's own 2 electrons leaving plus 2 that came into it, right or wrong???

    4. A transformer has 78 turns in primary and 158 in secondary. What kind of transformer is this and if it is 100% efficient, by what factor does it change the current.

    Okya this is obviously a step up transformer andits factor is Vs/Vp=Ns/Np Vs/Vp=158/78 Vs/Vp=2.03

    So by the factor of 2

    5. This is the one I'm having major trouble with -- Over long distances high-power transition lines split into four parallel cables. Given that these four cables are parallel and carry current in the same direction, do they repel ir attract one another? Explain your reasoning clearly.

    Okay this is the scoop -- my textbook just says that 2 parallel wire with current in same direction attract. They just do. Thats what book says. I think that they would actually repel but I really can't explain so any sort of help on this bugger will be much appreciated.

    6. What's the mass of 100mC of electrons ( m being the symbol for micro)

    100x10^-6C= 10^-4C

    10^-4C= 6.24x10^15e

    total mass= (6.24x10^15e) x mass of 1 e
    = (6.24x10^15e) x (9.11x10^-31 kg)
    = 5.68x10^-15 kg

    Okay so that was it and please please please can you look into it and pooint out mistakes. I would've done this earlier but I only came over the site yesterday.

    Thank you
  2. jcsd
  3. Jan 20, 2004 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You found the force between any two charges. What you need to find is the net force on each charge. Each charge is repelled by the other two: add up the force vectors from each charge to find the total force.
    The current is the same at all points in a series circuit. So the same number of electrons flow through each battery.
    Why do you have an opinion on this? The book is correct. To understand it, realize that each wire creates a magnetic field, which exerts a force on any nearby current carrying wire. Assuming you've covered magnetic forces, you should be able to find the direction of the force that the magnetic field exerts on the wire.
    Check the exponents.
  4. Jan 20, 2004 #3
    Thanks for looking into it, Doc Al but i have a few quetions:

    1. it's triangle so each charge is repelled by other two? if that's the case then How could I find the repelling force?

    2. ok I guess, then that it's just a concept I'm having trouble understanding. Was I correct though? -- 1.125x10^20 ?

    5. Me and my classmates brainstormed this one today -- this is roughly the answer -- current generates magnetic field which is same for all wires - say N on teh right and Son the left hence:
    o - wires N- north S - south

    N o S N o S N o S N o S

    Hence wires would attract

    6. I don't know what about the exponents -- it's either me or my calculator

    PS. This unit -- electromagnetism -- was not taught in the class except for a few of general concepts that were reviewed ( the ones that re usually covered in grade 9 science)

    These questions are a method of making sure that we read on the book and study the material required. As well the teacher doesn't really help us - since it's an ISU (Independent Study Unit) just you know gives us a hint here and there if he thinks that you won't get anywhere.

    Me being top student (I'm not kidding or praising myself) it's really hard since teacher thinks that I have potential to do all these problems myself so I don't get any help at all.
  5. Jan 20, 2004 #4

    Doc Al

    User Avatar

    Staff: Mentor

    All the charges repel each other, right? (Like charges repel.) To find the force on any charge, add up the force due to each of the other charges. (You found the force between each pair already, now use it.) Remember that these forces are vectors. Draw a diagram. You should be able to guess (use symmetry) the direction of the net force. Components along that direction add up; components perpendicular to it cancel.
    Looks good to me.
    Close enough.
    Don't blame the calculator. You multiplied 10-4 by 1018 and got 1015. No excuses! :smile:
  6. Jan 20, 2004 #5
    Allrighty I think I'm ready to hand the damn thing on thursday. If you want I'll tell you my mark

    Other than that -- thanks a lot for your help guys
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook