# PHYSICS Concept Questions

Jul 22, 2023

PHYSICS

[400 words minimum for each question]

Concept Questions (Select One)

1. When finding thermal expansion and heat values from temperature differences, leaving temperature values in Celsius degrees is okay. Why are we not required to convert temperatures into Kelvins to get these equations to work? Explain a real-life application where thermal expansion needs to be taken into account when building/making something.
2. An ice cube at sub-zero temperature is dropped into a room-temperature glass of water. Describe the heat terms we must use for the heat loss of the water and the heat gain of the ice in terms of phase and temperature changes. Then give an example of where we would want to keep close track of heat loss and gain in real life.
3. Relative humidity isn’t everything. Why is it that outdoor air of 80 Foat 55% humidity will feel more “muggy” than a 50 Fo cave at 75% humidity? What other parameter is a better measure of the actual amount of moisture in the air, and how do we define it? Then give an example of when keeping close track of moisture in the air is important.

Concept Questions (Select One)

1. Explain why the hottest rooms in a house in the summertime are usually on the top floor. What thermal process is involved here? Is it better to have air ducts near the ceiling or floor when you are trying to cool a room? Give one other example where this process comes into play and is important.
2. In terms of conduction and conductivity, why is copper-bottomed cookware looked at as having an advantage over stainless steel or iron? Give another example where either conduction or conductivity is important in the design or structure of an object.
3. How does the result of doubling the Kelvin temperature of a body impact the rate of radiative heat loss? What does this tell us about the temperature dependence on radiation rates? Discuss one other example that takes into account this concept.

Concept Questions (Select One)

1. A mole is defined as a collection of an Avogadro’s number of particles or objects. How is the mole related to the atomic or molecular mass of a substance? Say we have two 1-inch cubes each comprised of a different element. You pick the two different elements and talk about any major differences between the two.
2. What is Brownian motion, and relate how the kinetic theory applies to it. Give one real-life example where the kinetic theory is used to our advantage or when it might be a disadvantage.
3. Diffusion is an important process in nature that helps regulate molecular distributions. There are four principal factors that influence the rate of diffusion. Pick one of these factors, explain it in detail, and give an example of what happens when this factor is changed. Make sure to claim in your Subject heading what you are going to discuss

#### When finding thermal expansion and heat values from temperature differences, leaving temperature values in Celsius degrees is okay. Why are we not required to convert temperatures into Kelvins to get these equations to work? Explain a real-life application where thermal expansion needs to be taken into account when building/making something.

Ans: While finding thermal expansion and heat values from temperature differences, leaving temperature values in Celsius degrees is perfectly all right. To understand why, first, we have to understand the concept of thermal expansion and heat values. Thermal expansion of a material can be defined as the change in area, density, or volume, as well as a shape in response to changes in temperature, only with this assumption that there will be no phase transition or chemical reaction due to heating. So, here, the total change is associated with a change in temperature. The change in temperature measures the same in both the Celsius and Kelvin scale, as the size of the difference in temperature is the same for both of the scales. Elaborately, the temperature in both Celsius and Kelvin scales differ by a constant value. Let that value be α. If we denote temperature in Celsius scale by T’ and in kelvin scale by T, then the relation will be

T=T’+α……………………………………….(1)

Or, delta T= (T2-T1) = (T’2+α) – (T’1+ α)=T’2-T’1=delta T’;

So, the change in temperature in both scales measures the same. Hence, it is not necessary to change the temperature values to the kelvin scale for the concerned cases. While making railway bridges and tracks thermal expansion needs to be taken into account. During summer days, or while the train runs through the railway tracks, frictional energy gets converted to heat energy. This generated heat promotes the expansion of the railway track. Keeping this in mind, while making railway tracks there is always a gap between the two joints of the tracks. So that the thermal expansion can be accommodated. So, to make railway tracks practically the case of thermal expansion needs to be taken into account.

#### In terms of conduction and conductivity, why is copper-bottomed cookware looked at as having an advantage over stainless steel or iron? Give another example where either conduction or conductivity is important in the design or structure of an object.

Ans: In terms of conductivity or conduction copper-bottomed cookware is advantageous over stainless steel or iron. As the conductivity of copper is greater than iron or steel, it transfers the generated heat from the flame to the food more rapidly than the iron or steel container would do. If the iron or steel container is used, the rate is much slower, so the generated heat might be lost to the environment.

In chemical reactors or heating chambers of industries, sometimes a flow of cold water pipe is attached. During the chemical reaction, the chemical reaction transfers chemical energy to heat energy, and the chamber gets heated up. In the heating chambers also the reactor or the chamber gets heated up after a few hours of working. Now if within due time, the chamber is not cooled off, the reactor may damage or the chamber may get disturbed. As the cold water pipeline is attached to the chamber by the process of conduction the extra heat is conducted to the water flow and with due time the chamber gets cooled off. So, the process remains undisturbed. This is a practical example of the conduction of heat in real life where the process of conduction is advantageous.

#### What is Brownian motion, and relate how the kinetic theory applies to it? Give one real-life example where the kinetic theory is used to our advantage or when it might be a disadvantage.

Ans: Constituent particles in fluids (both gas and liquid) move randomly. This movement is termed after botanist Robert Brown. They do this because they are bombarded by the other moving particles in the fluid. Larger particles can be moved by light, fast-moving molecules. If a certain number of particles are undergoing Brownian motion in a certain medium, there is no particular direction for this Brownian motion. This motion is a random phenomenon. It is largely a result of random motion originating due to thermal agitation. The properties of Brownian motion are:

1. The size of the particle is inversely proportional to the speed of motion, which signifies the smaller or the lighter particles will move faster.
2. The transferred momentum during the motion is inversely proportional to the mass of the particle. This means that the smaller particles will gain more energy and will move much faster.

Kinetic theory of the gases deals with the molecular or particle motion or phenomena of the gas. Brownian motion is also a random phenomenon and it is generated due to the collision of slower particles with the faster molecules within the gas. Brownian motion is basically an experimental confirmation of the kinetic theory of gas. Einstein’s theory mainly relates to the diffusion constant, which is the mean square displacement of any particle for a given time interval. Avogadro’s number and size of the molecule, these two factors can be experimentally determined from the concept of kinetic theory approach to Brownian motion.

The movement of any particle from an area of higher concentration to a lower concentration is called diffusion. This diffusion mechanism relates to the kinetic theory and the Brownian motion. The diffusion of calcium through bones is an excellent real-life example of kinetic theory which is advantageous for our health. The bones are the building block of living organisms and the absorption of calcium by the bones in living organisms involves the process of diffusion only.