Discussion in 'Homework Help' started by bigtomoh, Mar 4, 2016.

1. bigtomohNew Member

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hi big,
As this is homework I will just give you a hint.
Lets do it step by step.

What is the circuit resistance, if the current is 40Amps at 200Vdc.???

E

3. Les JonesWell-Known Member

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Google "ohms law" Show us how you have attempted to solve the the 4 parts of the question.

Les.

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5. bigtomohNew Member

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200/40 =5 ohms

6. Les JonesWell-Known Member

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That is correct for the value of the 3 resistors in parallel. I can see two ways to proceed. 1/ You now know the value of the 3 resistors in parallel. You know the value of two of the resistors so you can work out the value of the resistance of those 2 in parallel. You then have two resistors in parallel that give a resistance of 5 ohms and you have just calculated the value of one of them (R1 and R2 in parallel.) so you now can calculate R3. 2/ Calculate the current in R1. Calculate the current in R2. You can then see what the current in R3 must be. (As you have been given the total current.) From that you can calculate the value of R3.
(I used the second method.) You now have all the information to calculate the power dissipated in each resistor.

Les.

7. bigtomohNew Member

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r3 = 29.8 ohms

thank you so much it was doing my head in

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9. Les JonesWell-Known Member

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Your value for R3 is not quite right. I suspect that you have not been working to enough decimal places. (This is a situation were fractions give a more accurate answer than decimals.)

Les.

10. bigtomohNew Member

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in australia we have been told to round up
200/10=20 200/15=13.3 r3=6.7=29.85 or 30 ohms ?

11. bigtomohNew Member

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thanks for your help have been studying all day my brain is a bit fried

12. JoeJesterActive Member

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If you were to round to zero decimal places, you would get:

If you were to round to three decimal places, you would get:

In some circles, in another time, we rounded to two decimal places using scientific notation.

The calculated cells are marked in Grey. The yellow highlighted cell was calculated using the parallel resistance formula, taking the reciprocal of the sum of the conductance of the resistors.

Last edited: Mar 5, 2016