Fareeha18
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Please refer to the question down the threadName:  IMG_20200929_130142.jpg
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simonmorrish
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(Original post by Fareeha18)
Please refer to the question down the threadName:  IMG_20200929_130142.jpg
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First define a variable Rq for the resistance of Q. Then write an expression for the resistivity of the wire that P is made of, and an expression for the resistivity of the wire that Q is made of.

Is that enough of a hint? If not, post your attempt.
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Fareeha18
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(Original post by simonmorrish)
First define a variable Rq for the resistance of Q. Then write an expression for the resistivity of the wire that P is made of, and an expression for the resistivity of the wire that Q is made of.

Is that enough of a hint? If not, post your attempt.
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I can't do this furthur I'm sooo confused
Hey wait Rq part is wrong i forgot to sqaure the 2 aswell the final ans would be Resistivity×L/pi d^2
Last edited by Fareeha18; 3 weeks ago
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simonmorrish
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(Original post by Fareeha18)
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I can't do this furthur I'm sooo confused
Hey wait Rq part is wrong i forgot to sqaure the 2 aswell the final ans would be Resistivity×L/pi d^2
Have faith - you're almost there! This kind of question - in fact most maths and physics questions imo - is about writing down mathematical expressions for what you know and what you want to find out, and then eliminating the variables you don't want in your answer.

Take your expression for Rp and rearrange in terms of ρ, then substitute that into your expression for Rq. You'll find you can do a lot of cancelling and all of a sudden you've got your answer! Post your next attempt.
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simonmorrish
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By the way, this is my version of the step you posted above. You can take this forward substituting rho as I mentioned above, but there is a shortcut. Can you spot it?

Name:  16013902280005775253901432312941.jpg
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The other thing I'm gonna say is, before you even start the calculation, you could think about the relative thickness and length of P and Q. Are you expecting Rq to be more or less than Rp? Having this in your mind will help you notice if you make a silly mistake in your calculation.
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Fareeha18
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(Original post by simonmorrish)
By the way, this is my version of the step you posted above. You can take this forward substituting rho as I mentioned above, but there is a shortcut. Can you spot it?

Name:  16013902280005775253901432312941.jpg
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The other thing I'm gonna say is, before you even start the calculation, you could think about the relative thickness and length of P and Q. Are you expecting Rq to be more or less than Rp? Having this in your mind will help you notice if you make a silly mistake in your calculation.
OHH WAIT WE ARE GONNA FACTORIZE IT RIGHT??? resistivity×L/pi d^2 would cancel each other then 12 will be divided by 8.
but I'm only assuming that 12 will be divided by 8 but Idk why would it be divided?
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simonmorrish
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Yay! You got it. Don't assume anything though. Work it through, like this:

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Did you think about what you were expecting the answer to be? As Q is thicker than P, that will make it lower resistance, right? Also, as Q is shorter than P, that also makes it even lower, right? Well, our answer says it's 8 times lower than P, so that's good - at least it's a quick check that we didn't make some huge error.

Are you all clear on this now?
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Fareeha18
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(Original post by simonmorrish)
Yay! You got it. Don't assume anything though. Work it through, like this:

Name:  16013984899984901729459598751325.jpg
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Did you think about what you were expecting the answer to be? As Q is thicker than P, that will make it lower resistance, right? Also, as Q is shorter than P, that also makes it even lower, right? Well, our answer says it's 8 times lower than P, so that's good - at least it's a quick check that we didn't make some huge error.

Are you all clear on this now?
It's all clear now.Thank you so much for helping:thumbsup:^~^
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Fareeha18
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(Original post by simonmorrish)
Yay! You got it. Don't assume anything though. Work it through, like this:

Name:  16013984899984901729459598751325.jpg
Views: 10
Size:  66.2 KB

Did you think about what you were expecting the answer to be? As Q is thicker than P, that will make it lower resistance, right? Also, as Q is shorter than P, that also makes it even lower, right? Well, our answer says it's 8 times lower than P, so that's good - at least it's a quick check that we didn't make some huge error.

Are you all clear on this now?
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can you please check this aswell?Am I solving this correct?
the ratio is 8:1
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simonmorrish
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(Original post by Fareeha18)
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can you please check this aswell?Am I solving this correct?
the ratio is 8:1
Could you post the start of the question, so I can see the circuit diagram?
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Fareeha18
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(Original post by simonmorrish)
Could you post the start of the question, so I can see the circuit diagram?
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simonmorrish
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You nailed it! Here is my working out, which is basically identical to yours:

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simonmorrish
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Did you notice how much cancelling out goes on with these calculations? You can save some writing if you think about the proportionality. In fact, once you get comfortable, you could almost see the answer direct from the table. I've posted an alternative solution below using this method. If you get it, that's great - it's a more advanced understanding and will help you do the questions quickly. If you're not sure, you can always use the full method you used above.

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Fareeha18
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(Original post by simonmorrish)
Did you notice how much cancelling out goes on with these calculations? You can save some writing if you think about the proportionality. In fact, once you get comfortable, you could almost see the answer direct from the table. I've posted an alternative solution below using this method. If you get it, that's great - it's a more advanced understanding and will help you do the questions quickly. If you're not sure, you can always use the full method you used above.

Name:  16014496705972489760667118177982.jpg
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Thank you so much honestly you explained it so well :thumbsup:I tried solving few more questions and I finally learned how to solve these type of questions thank you^-^
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(Original post by Fareeha18)
Thank you so much honestly you explained it so well :thumbsup:I tried solving few more questions and I finally learned how to solve these type of questions thank you^-^
You're welcome ☺️
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