Other examples in our daily lives: In some supermarket, the seafood are placed outside of air-conditioned place. The seafood is kept cold by putting crushed ice covering the seafood to keep the them cold and fresh. Refer to this Sci Physics question N2008P2Q6(b) Solutions:  For the solid that does not melt, when thermal energy is absorbed from the surrounding  food, its temperature starts to rise. So it is not so effective at keeping the food cool. For ice-pack, when thermal energy is absorbed from the surrounding food, it starts to melt. During melting process, a much larger quantity of thermal energy is absorbed from the food to melt per unit mass of ice, the temperature remains constant at 1oC, and the melting process is long. Hence ice-pack is more effective at keeping the food cool.

This simple circuit involves a resistance wire on the meter rule and the use of jockey tapping at different length L on the resistance wire. Note that this is not a potential divider. Rather this set up works like a variable resistor (rheostat) in the circuit. When the switch is closed (jockey NOT tapping on resistance wire), there is no current flowing as voltmeter (infinity resistance) is connected in series with the ammeter and the battery. Hence the voltmeter is showing the battery’s electromotive force (emf) of 3.0 V. When the switch is closed and the jockey is tapped on the resistance wire on the ruler, the longer the L (length of resistance wire) , the higher the resistance of the circuit , the higher the potential difference (voltmeter) across the resistance wire and the smaller the current through the ammeter . Hence the tapping of the jockey on the resistance wire is similar to adjusting the resistance on the variable resistor. (If you are wondering why the voltmeter is not showing the emf of the battery when the jockey touches the resistance wire L, it is because in reality there is internal resistance in the battery or even in the connecting copper wire. For olevel theory we assume no internal battery resistance or resistance in copper wire or ammeter). To learn how to set up the experiment, refer to the video below. Both ammeter and voltmeter have two terminals , ( positive and negative ). The conventional current must flow into the positive terminal (+) of the meters and out of the negative terminal (-) .   If the connection is the opposite, the needle will deflect below the zero marking. Refer to the video below. Sometimes, the connection terminals on the voltmeter and ammeter are different. Likewise, different types of wire with different connection heads have to be used. Refer to the video to see how are they connected in general.

2019PPP1Q33 Ans: Option B 2007PPP1Q28 Ans: Option B Torch lamp or bulb is a non-ohmic conductor . The V-I graph has an increasing gradient indicating resistance increases. As the current through the filament of bulb increases (due to p.d across increases) , temperature increases and resistance increases. For an ohmic conductor , the graph of V against I is a straight line with constant gradient and passes through the origin . Something to thing about: What if there is an option E whereV increases from 0, 1, 2, 3I increases from 0, 6, 10, 12 respectively. The graph will be a typical ohmic graph, similar shape to option B, but will you choose E? Refer to the last section of the video from 4:37 s.

A thin convex lens is placed on a plane mirror and an object pin is then moved along the axis of the lens until an image is seen to coincide with the object pin when viewed from above. What is then the distance between the pin and the lens? (Take f as the focal length of the lens) A      0.5 f            B      1.0 f          C       1.5 f          D       2.0 f Answer: Option B A common mistake is to assume this is the scenario (2nd scenario) where the object is at 2F and the image formed is at 2F, hence the image is the same size as the object, inverted and real. But this is not the case. Refer to the video tutorial for the explanation. This set up of the lens with the mirror is using the concept that when parallel light rays (parallel to principal axis) enter the lens, the rays will converge to a point after passing through the lens. This point is called the focal point, F . The distance between F and the optical center of lens is the focal length f . Refer to the diagram below. When you adjust the object (pin) until both the object and the image coincide even when you move your eye forward or backward perpendicular to the axis, the distance between the optical center and the object (pin) is the focal length f. At this position, as the rays from the object pass through the lens, due to refraction, the rays converge and becomes parallel to the axis. Due to the mirror, the parallel rays will be reflected back to the lens and then converge to a point that coincides with the object. Refer to the video on how to get that position. Before you start the experiment to find the focal length f, there is a fast and easy way to estimate the f. Refer to the video below. Below is another image of another set-up but with the same concept using pin and mirror. Another similar question below. The answer is Option B

The video below shows a typical lens experiment. (reference to O-Level SciPhy 2015). I will briefly go through the set-up, main steps and how to get the 1st set of readings. In the next video, it highlights the various types of lens practical which you might have in the school lab. e.g. different kind of crossed-wire, a beaker of water as a converging lens and different kind of images formed. Key points : 1) Make sure the object (illuminated crossed-wire), lens and screen are aligned properly. 2) Source of Error: identifying the sharpest image Improvement: (i) Repeat the experiment a few times for the same independent variable to identify the sharpest image. (ii) Move the lens (or screen) forward and backward about the sharp image, until the sharpest image is determined. 3) In general, the focal lens of the lens used in the lab is usually 10 cm or 15 cm . Most lens experiment requires you to find the focal length. There is also a easy way to quickly determine the focal length before starting the experiment.

Vernier caliper and micrometer screw gauge are used to measure length. Vernier caliper is used in general for measuring length between  0 to 15 cm . Micrometer screw gauge is used in general for measuring length between  0 to 2.5 mm . The accuracy (precision) of – ruler (0.1 cm) – vernier caliper (0.01 cm) – micrometer screw gauge (0.01 mm)