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*current and resistance*

  1. Feb 23, 2005 #1
    Alrighty i have some questions, as well as my possible answers. We have a worksheet that i hope to do correctly since the test will be here within a week or so. Anyways ill get to it

    1) a resistance irreversibly converts electrical energy into
    a) mechanical work
    b) chemical energy
    c) heat energy
    d) potential energy

    a and b are out, leaving heat and potential. I am going with D here because energy is LOST as heat, i think to be converted it would have to be conserved. My answer is D)

    2. A current of 100 uA flows in a circuit. How many electrons go past a point each second
    a) 8.1 x 10^6
    b) 4.8 x 10^12
    c) 6.2 x 10^14
    d) 1.5 x 10^8

    I think i could use I = Delta Q/ Delta T.. i have no clue with this one

    3) What potential difference is required across a 6.5ohm resistor for a 370 C of charge to flow through it in 2.5 minutes
    a) 22v
    b) 16v
    c) 55v
    d) .02v

    For this, since we know resistance (6.5 ohm) and Capacitance... im a little confused, but ide say 6.5 Ohm x 370 C = 2405.. change minutes to seconds, 150seconds, 2405\ 150seconds = 16v... so my answer is B) but thats guesswork.

    4)What is the equivant resistance of a 22 ohm, 40 ohm, and a 32 ohm resister connected in series.
    a) 85 ohms
    b) 94 " "
    c) 62 " "
    d) 14 " "

    Rtotal = R1+R2+R3..... so 22+40+32= 94 ohms, my answer is B)

    5) Capacitors of 2.5 uF, 6.6 uF, and 10 uF are connected in series, what is their equivalent capacitance.
    a) 6.4 uF
    b) 1.5 uF
    c) 19.1 uF
    d) 4.5 uF

    capacitors are the same in series.. Ctotal= c1+c2+c3... 2.5+6.6+10 = 19.1 uF my answer is C)

    6) Capicitances of 20 uF, 1.8 uF, 5.5 uF, and 10.6 uF are connected in parallel. What is the equivanent capicitance

    for these...
    1/Ctotal = 1/c1+1/c2+1/c3 (then take the inverse) 1/20+1/1.8+1/5.5+1/10.6 = .88 inverse = 1.13..

    7) Appliances in the us are designed to work on 110-120v, where as appliances in europe operate on 220-240v. what would happen if you plugged a european device into u.s outlets without a converter?

    a) it would work just not as well as in europe. Its the power rating, not the voltage that matters
    b) it would work just not as well as in europe. Its the current rating, not the voltage that matters
    c) it would work but overheat and burn up
    d) It would barely work since the voltage here is too low to push enough current through the device.

    I would say D. If you take a US device to europe it would blow a fuse, so i think it would work just not well at all.

    I have a few more, ill post in a couple minutes
  2. jcsd
  3. Feb 23, 2005 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    No. Since you have eliminated all the other answers, the answer is c. A resistance converts electrical energy into heat.

    One ampere is 1 coulomb per second. One electron = 1.6e-19 C. or 1 C = 6.2e18 e. How many electrons in 100e-6 C.?

    Capacitance?? The C is coulombs. I is the current in coulombs/sec. What is the expression for I in terms of V and R? What is the current if 370 C. flow in 150 seconds?

    Capacitors in series are added like resistors in parallel.

    .. this was the approach needed for ques. 5. and your approach in 5 is needed here.

    Since I = V/R, if you have half the voltage you will have half the current.

    Last edited: Feb 23, 2005
  4. Feb 24, 2005 #3
    new Q...

    A fridge uses 600w when running and operates 8.4 hours each day

    a) how much energy in joules, does it use in 30 days?
    b) what will it cost to operate for 30 days if it costs 8.3 cents/KwH
  5. Feb 24, 2005 #4


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    Homework Helper

    New questions should preferably go into new threads so that they can be referenced more easily.

    Define power. After that everything should become clear.
  6. Feb 24, 2005 #5
    ok well P = IV, P = V2 / r..

    power is equal to a watt which equals a Joule/Second

    600w equals 600 j/s if it goes 8.4 hours it would be 8 hours (60 minutes)+ 24 minutes (for the .4 hours) so thats 504 minutes every day, x 30 days equals 15120 minutes in 30 days. Now convert minutes to seconds, theres 60 seconds in one minute so 15120x60 = 907200 seconds in 30 days @ 8.4 hours a day. Now we have to get seconds to cancel,

    600 j/s x 907200 seconds = 544320000 Joules in 30 days @ 8.4 hours a day

    i think i did that right for part A
  7. Feb 24, 2005 #6


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    Homework Helper

    Yup, your first part is correct. The only definition you need is power = energy over time (or, the rate of energy gain or loss).

    For the second part, you should know that kWh (kilowatt-hour) is actually a unit of energy that electric companies often use in order to work out your utilities bill. You're charged by number of kWh you use every month.

    1 kWh = 1kW * 1 hour. Recall that energy = power * time, so this form is expected.

    Now work out how much energy that fridge consumes in kWh (instead of joules), and you should get the answer. Keep track of your units and let them cancel one another out.
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