The Definition of Resistance Pt 2

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Can see Physics in a PET bottle used for carbonated drinks? Two days ago some of the students in my school are busy designing water rockets from used PET bottles. Yesterday was the Co-curriculum Day in my school. One of the highlights is the Water Rocket Competition organised by the panel of Physics teachers.
Back to the definition of resistance. Can resistance be also defined as the gradient of the V-I graph?
Consider Fig. 2, which shows the V-I relationship for two conductors X and Y. If resistance can be defined as the gradient of the V-I graph, then at current i, the resistance of Y is greater than X. If resistance is the ratio V/I, then at current i, both X and Y have the same resistance. So what does this show?
You cannot in general say that resistance is equal to the gradient of the V-I graph. This is only true for ohmic conductors.
In the Malaysian SPM 2006 Physics Paper 3, there is a question which gives a V-I graph as shown in Fig. 3.
The question goes on to say, "The gradient of the graph represents resistance. State how resistance varies with current."
In the light of what has been discussed, what do you have to say? If we leave out the first sentence in the question, "The gradient ..... ", can the question still be answered?
Bye for now.

Anonymous said...

Hi Koay, proud to see you joining the blogger group! Titles and topics of discussion are very relevant! Will recommend strongly to my circles of friends!

April 30, 2008 3:36 PM

Thinking Allowed said...

Hei! Koay. Glad that you are sharing your expertise with everyone. Physics is fun. But not everybody sees it that way. Hope you can show them how.

April 30, 2008 11:29 PM

The definition of resistance Pt 1

Look into any Physics book and you will see the definition for electrical resistance as shown above. In many books, voltage is stated as potential difference. For ohmic conductors, the voltage and current are directly proportional to each other provided the temperature and physical dimensions of the conductor remain constant. A graph of V against I will be as shown Fig 1. The gradient of the graph is constant and is equal to the ratio V/I for any value of the current. Now the gradient of the line is (change in V)/(change in I). In this particular case, the ratio V/I for different values of current is always equal to the gradient . Does this mean that resistance can be defined as V/I and also (change in V)/(change in I) ???

By the way, can anyone tell me whether it is possible to type formulae with ratios etc when posting a blog? Right now I have to insert it as an image. Quite troublesome.

My students and friends sometimes laugh at me and say "I see Physics in everything". I think they are quite right about that. I guess it's because as a physics teacher, I have always tried to make Physics as simple as possible (but not simpler, I think that was Einstein's quote!!!). Seeing Physics in everything helps me make Physics relevant and contextual to my students. Over the years I have seen some concepts in Physics being gradually "blurred" by what was written in books, teacher's notes, exam questions and marking schemes etc. Correct concepts have become misconceptions. Just a quick example. Is electrical resistance equal to voltage/current or is it (change in voltage)/(change in current)??? Will discuss this later. c-ya

Thinking Allowed said...

Well, Koay, we used to discuss and argue on how to present a scientific concept in the best way to be understood by students while partnering each other in various physics books we wrote. We used to share the exciting moments when we told each other stories of how we contextualise abstract contents to make physics more palatable. Since the end of our collaboration and my transfer to an administrative post, such mental sparing has been rare and difficult. Further more we are at the opposing end of the peninsular Malaysia. I really miss the fun thought it was during the high-pressured time when we have to beat the deadline to submitting our manuscripts. However, through the new medium--blogging, physics distance is no more a factor. Let's hope that our other partners will also join us and perhaps in become sort of an 'e-IFM' one day. As for now, I am glad that there is one more blog to help physics teacher, especially in Malaysia.

May 1, 2008 11:16 AM