Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
Hi, i have drawn up 2 resistor examples, both have 15ohm totals! i was wanting to know what one would be the correct one to use or can i use anyone of them? "my" calculations tell me i can use anyone of them...
thanks...
Thanks eric! the main reason i asked was because at tafe the teacher told use to make a series/parallel combination to form 15ohm 20W! he told me that the two parallel resistors in series (bottom picture) was incorrect? i just dont want to mention the topic to him until i am sure its right!
isnt it the same thing? the top circuit produces the same current and wattage as the bottom circuit, the top two series (in the top picture) resistors have 500mA running through them and the same for the two under them, so 1Amp total draw...Ah, both of your drawings have the correct R Total, however, your initial post failed to mention what the teacher said about the power dissipation of the circuit. The teacher is correct in that the lower circuit will not dissipate 20 watts (only 10 watts) but the upper is correct for 20 watts. Look closely at your circuits.
Ron
Ah, both of your drawings have the correct R Total, however, your initial post failed to mention what the teacher said about the power dissipation of the circuit. The teacher is correct in that the lower circuit will not dissipate 20 watts (only 10 watts) but the upper is correct for 20 watts. Look closely at your circuits.
Ron
Actually, both circuits are identical save the useless jumper in the lower version presented. In the lower circuit, there is no current through the jumper given the potential difference at that point in both legs is 0.000...0V considering/assuming all resistors are absolutely 15 ohms out to many 0's. Therefore, the current through all resistors is equal in both cases. If the current and resistance are equal in both cases, the power dissipation must be equal in both cases for all resistors. The student's instructor appears to be in error.
But the bottom circuit becomes basically two 7.5ohm resistors in series at a 5watt rating each resistor, so that would mean that two 15ohm resistors in parallel would effectivly have a power rating of 10watts 7.5ohm?
i think i am starting to understand what the teacher means.... because the two 15ohm parallel resistors basically = one 10watt 7.5ohm resistor, and two of them in series (the bottom circuit) cant get any higher than 10 watts because they are in series?
now that i have had everyones help i understand it better now... the teacher didnt say "why" i was wrong! so i started to question what i thought i already new... So now i know why i was wrong,
One circuit "allows" up to 20watts and the other "allows" up to 10watts? am i right in saying that the second circuit (the bottom one) is drawing 5watts more that its "rated" to draw?
because the two 15ohm parallel resistors basically = one 10watt 7.5ohm resistor, and two of them in series (the bottom circuit) cant get any higher than 10 watts because they are in series?
That is where your error is.
Two resistors in series have the same current. If each resistor is rated at 10 W, 7.5 ohms then they have a rated current of 1.15 A. Put the resistors in series, and that current doesn't change, you can only get 1.15 A though the pair without risk of damage.
However, the two resistors in series have twice the voltage drop, so with the same current, there is twice the power dissipated.
I think that there is a simpler way of looking at this. If one (of the original 5W) resistors can convert 5W of electrical power into heat without overheating, then four of them can convert 20W. The electrical circuit has to be symmetrical so that all four are in equivalent positions, and both of your example circuits are.
(It is also important that the electrical circuit is stable, but with simple resistors it will always be stable. However, a flip-flop circuit is symmetrical but not stable when the current in both load resistors is the same. It will end up with all the current in one load resistor).
Hello,
Another way of looking at this is how much power does the resistor set draw from the power supply of 15v?
Since we know the source is 15v, if we know the current draw we can calculate the total power used by the resistors.
In the top picture, the total resistance is 15 ohms, and that means the top set draws 1 amp which means the source supplies 15 watts.
In the bottom picture, the total resistance is 15 ohms and that means the bottom set also draws 1 amp which means the source still supplies 15 watts.
Both circuits draw the same amount of current from a constant source voltage so they both dissipate the same power.
BTW, the short does nothing to change the circuit because it effectively jumpers two potentials that are EXACTLY the same for ALL time.
The only other way to look at this is from a statistical standpoint, where each resistor can be off by a certain tolerance like 10 percent. Given a random difference in each resistor, which set comes up statistically closer to 15 ohms total, or do they both come up the same anyway? See if you can figure this out
(Note that if each resistor can be off by a random amount up to 10 percent off, applying the short might then have an effect. If this were a real life circuit we might think about this too).
Thanks... so if all resistors in the bottom circuit are for theory sake the EXACT same ohms (15 exactly) then the "jumper" will do absolutley nothing? meaning that i am not wrong? in theory!