Hey!
I was taking a shot at the VCAA 2010 paper and I had trouble understanding their solution to question 16 on electricity.
I understand how to work it out, just using the value of 2V to find the current rather than 10V, when the question says operating at 4W, when at 2V it will not operate at 4W, but 10V? Lol, if you had trouble trying to fathom what I was saying, let me know and I will try to explain it further.
Can someone please clarify this?
Thanks!
The 10V you are talking about is from Q15 right? That 10V was NOT the voltage across the globe. The 10V is the voltage at the supply, to ensure that the globe operates properly (taking into account voltage loss in the lines) The globe at 4W (and when it does, it has a potential difference of 2V). The 10V accounted for the voltage loss in the lines. The voltage loss to get to the globe is 4V. The voltage loss going back is 4V. So this is why we need 10V at the supply - 8V of that will be lost, leaving 2V for the globe to operate.
So now we can approach Q16. We want to find out the power loss in the lines.
One approach is to use
Total resistance in the lines is 4 Ohms.
So we get a power loss of 16W.
This is what I would say the best option. I^2R is the preferred power loss equation (V isn't recommended because it refers to the potential difference, which we don't always get) and also it doesn't rely on answers from a previous question. The safest option is to always use the information given and avoid using answers you've figured out yourself from previous questions (as that may very well be incorrect).
In this case, we do have the potential difference across the lines - which is NOT the 10V supplied, it is the 8V lost across the lines.
So
Do note that the 'V' in the power equations is potential difference, NOT voltage supply.
edit: fixed my incorrect latex.
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