finding the current with parallel and series

Discussion in 'Homework Help' started by xoxkittkatt10xox, Jan 28, 2009.

  1. xoxkittkatt10xox

    Thread Starter New Member

    Jan 28, 2009
    for this problem i have to find the current that goes through the 10 ohm resister but im not sure how to calculate the correct amount of voltage to use in the equation I = V/R please help :]
  2. KL7AJ

    Senior Member

    Nov 4, 2008

    There are a lot of ways to approach this, but for now, let's just look at the static currents...the ones with the switch open. Let's substitute the 10 and 2 ohm series resistance with a single resistor of 12 ohms. Now, this 12 ohm resistor is in PARALLEL with the 4 ohm resistor. For two parallel resistors, we can use the product over the sums (48/16=3) ohms. Now by substituting the 3 ohms, we have that in seriesl with our 6 ohm resistor. So the total load on the source is 9 ohms.

    The TOTAL current (I=e/r)=8A. What is the voltage drop across the 6 ohm resistor? Ohms Law, E=IR, tells us we have 8x6, or 48 volts. Subtract that from 72, and we have 24 volts at the "hot" end of everything else. This means we also have 24 volts across the combination of our 2 and 10 ohm resistor. Again I=E/R, so I=24/12=2A!

    Nothing to it!

    Next time, we'll close the switch and see what happens.

  3. xoxkittkatt10xox

    Thread Starter New Member

    Jan 28, 2009
    so 2A would be the current across the 10 ohm resistor?
  4. Cabwood


    Feb 8, 2009
    Don't confuse yourself. Current is never "across" anything. It flows through.

    Voltage exists across, current flows through.

    Terminology is everything in this game. If you get into the right habits from the start, you'll avoid a lot of confusion later. Interestingly, if one understands what current is, and what voltage is (which I would imagine is a prerequisite for this kind of study), then this error should not happen, unless English is not the first language.