ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Physics => Topic started by: asterio on July 30, 2017, 01:20:09 am
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For anyone who comes across this and is curious what the issue was, the original text (from my email notification) was:
The text of the topic is shown below:
Hey guys I happen to be reading the examiner report for last year's (2016) physics exam.
I noticed a typical short-circuit problem at Q7c which clearly the amp-meter should give a reading of ZERO as all the current will flow through the diode instead.
However, surprisingly I found out the examiner report shows the answer was not ZERO, and as far as I can see, the examiner made a typical mistake when he/she was deriving the answer. Believing just because there is a potential difference, there will be a current. WHICH IS CLEARLY WRONG!!!
I have noticed the chaotic new study design and heard rumors from different teachers saying the new team doesn't know what they are doing... BUT SERIOUSLY?! A WRONG ANSWER ON THE VCAA EXAM?!!
WOW!!! either my master of engineering degree has been a awarded to me wrongly or this examiner need to gets his or her job fired.
I'd just like to try clear up a couple issues in this post:
- The new study design for Physics 3/4 only starts this year, Q7c is from the old study design which hasn't changed since 2009.
- There are very rarely any errors in VCAA solutions and if there are then they make amendments quite quickly.
- An ideal diode has a zero switch on voltage, i.e. it actually does act like a short-circuit and all of the current will flow through the diode.
- A real diode such as the one in question will have a non-zero switch-on voltage which means that it can be treated as having 'resistance' (albeit non-constant).
- Since the diode will only consume a fixed amount of voltage (5V) in this case it is possible to imagine the branches as two resistors in parallel with a fixed 5V potential difference which is how it is sometimes taught in year 12.
I will double check with my professor, and conduct my own experiment to confirm.
Let us know how this goes. You may struggle to find an LED of exactly 5V but the resistors and multi-meters should be readily available :) All the best with your masters and physics tutoring :)