These 2 questions are actually the same. Q23 is from 2010 Pure Physics P1 while Q11 is from 2014 Sci Physics P1. Take a look at these 2 questions. If you are not sure, view the video below for the explanation.

Answer to Q23: Option A

Answer to Q11: Option D

If you do not know how to answer these 2 questions, view this video and also refer to the lens summary below.

To find the unknown resistor R, the following apparatus are setup.

Refer to the video below for the setting up of the apparatus.

Why do you need a variable resistor (rheostat)?

Without the variable resistor, you will have only one set of current I and potential difference V readings. Using the formula R = V/I, you are able to find the unknown resistor. But this method is not so accurate.

Hence, to make it more accurate, we include a variable resistor to control the size of the current through the circuit. Thus having different readings of the potential difference V across the unknown resistor.

Instead of just one set of readings of I and V, we now have about 5 sets.

This allows us to plot a graph of V against I.

By finding the gradient of the best fit line, we are able to find the resistance more accurately. [gradient = V / I = R, hence the gradient of V-I graph represents resistance R]

For pure metallic conductor, like the fixed resistor R, it obeys the Ohm’s Law, hence it is an ohmic conductor.

From the graph, the current I flowing the conductor is directly proportional to potential difference V across the conductor, provided physical conditions like temperature remains constant. [the graph is a straight line with constant gradient, and passes through the origin]

Drawing ray diagrams for converging lens come in many forms. But the basic concepts needed are the 3 rays (which have rules to follow) to locate the image. The following ray diagrams for the 4 scenarios must be learned well, together with the respective image characteristics and applications.

With the basic concepts learned, when questions are asked in different ways, you should be able to draw the ray diagrams. Refer to some different ray diagram questions below and their video tutorials.

Electromagnet is like a ‘temporary magnet’. When a current flows through the solenoid, it will be magnetised. When the current is switched off, the eletromagnet loses its magnetism.

To make the electromagnet stronger,

1) Increase the number of turns in the solenoid.
2) Increase the size of the current.
3) Insert a soft iron core in the solenoid.
– iron is a soft magnetic material
– it can be magnetised easily
– it can be demagnetised easily