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]

The variable resistor (rheostat) is a device to control the size of the current by adjusting the resistance on the variable resistor . Hence just device can be used to adjust the brightness of the bulb or the fan speed.

The 3 states of matter –  solid, liquid and gas. In general, when a body is heated, it expands and volume increases. The mass remains the same. Since density = mass/volume, its density decreases (less dense). For instance, warm air rises as it is less dense. In terms of kinetic theory, the particles will increase in kinetic energy. The average spacing between the particles increases (assuming not in a closed container). Likewise, when a body is cooled, the opposite occurs. The body contracts and volume decreases. It becomes denser. Due to the differences in particles arrangement of solid, liquid and gas, each expands by different amount when heated and vice versa. Which expands the most when heated and contracts the most when cooled? The following demonstration of the ‘Pee Boy’ is a good video to show the concepts. Explanation: The tiny hole at the penis is too small for any water to enter on its own. So using thermal transfer in the different states, the following steps are taken: Put the hollow empty boy into the hot water. [air inside the boy expands more than the solid ceramic, hence bubbles are seen coming out of the hole] Put the hollow empty boy now into the cold water. [The air inside contracts and volume decreases. This creates a low pressure and water is then sucked into the boy through the tiny hole] Place the boy on a platform. [The boy is only partially filled with water. The head portion is filled air while the bottom portion is filled with water] Pour hot water over the head. [As the whole boy is heated by the running hot water, the air in the head portion expands much more than the water at the bottom and the solid ceramic of the boy. Hence the air pressure increases and it pushes the water out of the boy] And he pees!!! Quite powerful indeed!

Convection is a process in which thermal energy is transferred within a fluid (liquid or gas) due to the difference in density which creates a current. The video below shows the convection current in a heated tube filled with water. Colour dye is added to enable us to see the convection current through our naked eyes.

In the three states of matter, in general, solid is the best conductor as the particles are closely packed in an orderly manner, hence thermal energy can be passed down by the collision of the particles in the solid faster. On the other hand, liquid and gas are relatively considered poor conductor . Gas is the worst conductor as the particles are far apart. The following experiment demonstrates that water is indeed a poor conductor of thermal energy. An ice cube is kept at the bottom of the boiling tube by the net. The water at the top of the boiling tube is heated and started to boil. But the ice is not fully melted. This shows that thermal energy transferred through the water from heated water at the top to the ice is weak, hence indicating that water is a poor conductor . How about thermal transfer through convection current? In this experiment, the heated water at the top expands, the volume of the heated water increases, becomes less dense and remains at the top. The cooler water, which is denser, remains at the bottom. Hence there is no convection current formed throughout the entire water in the tube . So thermal energy transfer to the ice cube through convection is not present here.

he paper over metal or over plastic/wood will get burned faster? Everyone knows that metal is a good conductor of thermal energy. But for experiments like this, many would have guessed it wrongly. A paper is wrapped over metal and insulator (plastic, wood etc) and is exposed to the flame, the paper over the insulator becomes charred faster and burned faster . The paper over the metal takes a longer time to be charred and burned. This is because metal is a good conductor of thermal energy. When the spot (paper over metal) is exposed to the flame, the metal conducts the thermal energy away from that spot to other parts of the metal . So the temperature increase at that spot is slower , hence the ignition temperature of the paper (approx. 230oC) will be reached much slower , compared to the spot where the paper is over an insulator.

Resistors may be connected in series or parallel. When asked which resistor has higher power (hence higher energy), in general we will think of using P = IV. But when using P = IV, it cannot relate to resistance R of the resistors directly. Hence it will be more appropriate for us to P = I^2R  or P = V^2/R. For resistors in series , the current is the same. Hence it will be more direct to compare P and R using P = I^2R , where P is directly proportional to R . For resistors in parallel , the p.d. is the same. Hence it will be more direct to compare P and R using P = V^2/R , where P is inversely proportional to R .

Ans: Option A View the video tutorial for the explanation:

Refer to the notes below together with the video to learn more about the CRO Videos