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# Resistance of a power cable. watch

1. I guessed that multiplying 10 by 6 and adding 100 will give the answer, i.e 160 ohm, but the answer in the mark scheme is B, i.e 1.6 ohm.
Aren't all of them in series? Please explain.
2. (Original post by Zishi)
I guessed that multiplying 10 by 6 and adding 100 will give the answer, i.e 160 ohm, [...] Aren't all of them in series? Please explain.
The wires would be in series if they were behind one another so that they made up a wire 7km long.

but the answer in the mark scheme is B, i.e 1.6 ohm.
According to the examiner's report, we've to consider copper wires in series with each other and sum of their resistances as parallel to steel core - I don't understand why we have to do this to find the overall resistance.
If you did what they suggested, you would see that it does not give the right answer - as a matter of fact none of the answers provided.

In fact though all the wires are in parallel. They touch each other over the whole length so that electrons have more "space" - cross sectional area is greater than if it were just one wire - so they can choose any wire to go through - just like in the case of resistors connected in parallel.
3. (Original post by jaroc)
The wires would be in series if they were behind one another so that they made up a wire 7km long.

If you did what they suggested, you would see that it does not give the right answer - as a matter of fact none of the answers provided.

In fact though all the wires are in parallel. They touch each other over the whole length so that electrons have more &quot;space&quot; - cross sectional area is greater than if it were just one wire - so they can choose any wire to go through - just like in the case of resistors connected in parallel.
Oops, sorry. I might be confused at that time. It was a typo, the examiner report didn't say anything like that. Anyways, thanks for that. I wonder why I didn't think like that...
4. first step is recognising that it's a question about resistances in parallel.

as there are several 10 ohm wires in parallel you can be sure the overall resistance in the cable cannot be greater than the resistance of a single wire of the lowest resistance - it must be <10 ohm.

There's a well known way of calculating the overall value of resistances in parallel which will come up with one of the multi choice answers, you should have already learned it if you've done circuits.

I don't agree with jaroc - 1.6 is the answer.
5. (Original post by Joinedup)
first step is recognising that it's a question about resistances in parallel.

as there are several 10 ohm wires in parallel you can be sure the overall resistance in the cable cannot be greater than the resistance of a single wire of the lowest resistance - it must be &lt;10 ohm.

There's a well known way of calculating the overall value of resistances in parallel which will come up with one of the multi choice answers, you should have already learned it if you've done circuits.

I don't agree with jaroc - 1.6 is the answer.
Maybe you misunderstood jaroc's statement. He said what you're saying. Both of you're right. 1.6 ohm is the answer.
6. I thought he said none of the answers were right - but as long as you're happy you can recognise questions about parallel resistances and work them out for your self then I'm happy.
7. (Original post by Joinedup)
I thought he said none of the answers were right - but as long as you're happy you can recognise questions about parallel resistances and work them out for your self then I'm happy.
I was a bit confused while solving it, and now I believe that my concept was kinda wrong. I'm really much happy now as I think I'll have no problem with questions like this in future!
8. To make it clear Zishi is right about my intentions, and I'm glad he's understood them well.

Zishi had previously said that the advice given in the mark scheme was:

we've to consider copper wires in series with each other and sum of their resistances as parallel to steel core
I said that if you used this advice, you wouldn't get either of the answers provided, and that to obtain the right value of 1.6ohms you need to treat all the wires as in parallel.

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