Oxford PAT 2015, Question 21

2015_paper__page21

Below, there is a sequence of images showing how the original diagram gets transformed into the diagram in section (c), by applying these simple principles:

The key point to remember here is that any two points that are directly connected by a wire are electrically the same point.

So first ignore any wires that just ‘stick out’ from the points A, B, C and D. They don’t add any information.

Then notice that B and C are connected by a wire and so in electrical terms B and C are the same point. So erase the point C and redraw C at the same position as B.

Then imagine that you can pick up the point D and move it to the left of the point A without breaking any of the connections.

2011_1_page1

16 thoughts on “Oxford PAT 2015, Question 21

  1. Hi, I understand part a and c – but with part b I don’t understand why R is divided by 3/2 R. I understand the use of R= V/I and P = V^2/R but i would have thought you added the total resistance between the points.

    Thank you for your solutions and hope you can help me!

    1. When there is a voltage V between A and B, there is some current I that flows through the point D, and this is the current in the resistor between B and D.

      We can work out the value of the current I: the resistance between A and D is R/2 (two resistors R connected in parallel), and the resistance between D and B is R, so the resistance along the path through D between A and B is 3R/2; therefore the current I is equal to V/(3R/2).

      Given the value for I we can work out that the voltage across the resistor between B and D is RV/(3R/2), which equals 2V/3.

  2. do you have any way to practice redrawing circuits like in the last part here im not sure how you really go about it?
    how can i learn this?

    1. One way could be to try simplifying the examples discussed in a web page like this one: http://www.allaboutcircuits.com/textbook/direct-current/chpt-7/re-drawing-complex-schematics/. I found this page with a web search for “complex resistor circuits”, and this turns up quite a few other links. Failing that, just start drawing messed up combinations of resistors yourself and then try and simplify them. After a bit of practice you should find it easy.

      1. Can you explain to me your thought process as you resketched the circuit?
        I don’t get the two resistors and the other one parallel between a and c on the redrawn graph

        1. OK, here’s a way of redrawing the circuit.

          The key point to remember here is that any two points that are directly connected by a wire are electrically the same point.

          So first ignore any wires that just ‘stick out’ from the points A, B, C and D. They don’t add any information.

          Then notice that B and C are connected by a wire and so in electrical terms B and C are the same point. So erase the point C and redraw C at the same position as B.

          Then imagine that you can pick up the point D and move it to the left of the point A without breaking any of the connections.

          After doing those steps, you should have a diagram that is the same as the one I have drawn.

          1. when i redraw it like that what happens is the resistor between B and D goes in the same line that A and b are and the parallel compenent in your diagram theyre parallel to AB but when i move d to the left i move that whole bottom like to the left of A and that happens

            Thank you for your time

            1. I cant seem to reply to your comment so i hope replying here will be ok
              if i transfer the bottom circuit beside A i get this, maybe the key word you used was not breaking any of the connections but im having a hard time visualizing that

                    1. No problem, glad it helped. I’m afraid I don’t do any paid-for classes. But I’m happy to answer any questions that you want to ask here for no charge.

Add your comments

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s