Thermal Physics

Chapter 8 - Temperature

1.

*State the thermometric property that defines the temperature scale in a liquid-in-glass thermometer.

  • Volume of a fixed mass of liquid.

2.

*Describe how you would calibrate an unmarked mercury-in-glass thermometer.

  • Place the bulb of the unmarked thermometer in pure melting ice at a pressure of 1 atmosphere.
  • When the mercury level is steady, mark this position as 00C.
  • Next, place the bulb of the unmarked thermometer in steam above boiling water at pressure of 1 atmosphere.
  • When the mercury level is steady, mark this position as 1000C.
  • Divide the interval between 00C and 1000C into 100 divisions.
  • Each division represents 10C.

3.

*Explain what it meant by responsiveness of the thermometer.

  • A responsive thermometer reacts quickly to changes in temperature.

4.

 *Explain what is meant by sensitivity of the thermometer.

  • A sensitive thermometer gives a more noticeable expansion (large increase in the length of the mercury thread) for small changes in temperature.

5.

*State the set up of a thermocouple and describe how works.

  • A thermocouple consists of two types of wires made of different metals.
  • The ends of the wires are joined to form two junctions.
  • When the two junctions are at different temperature (one hot and one cold), a small voltage (e.m.f) is produced.
  • The greater the difference, the greater the voltage produced across the two junctions. That is, the voltage produced is proportional to the temperature difference.
  • By knowing one reference temperature and measuring the voltage, the unknown temperature can be found.

Chapter 9 - Kinetic Model of Matter

1.

Describe how you would demonstrate Brownian motion of smoke particles in the air. State and explain the observations.

  • Direct light into the side of an enclosed smoke cell/chamber.
  • View the smoke particles from the top, under a microscope.
  • Bright specks of smokes will be seen as light reflects from the smoke particles
  • The bright specks are observed to be in continuous random motion which proves that the smoke particles are in random motion.
  • The smoke particles are in random motion because they continually bombarded unevenly on different sides by the air molecules.

 

2.

*Using the kinetic model of gases, explain how gases exerts a pressure on the walls of its container.

  • When a gas particle collides onto the wall of the container, a force is exerted on it.
  • Numerous such collisions by the many molecules results in an average force exerted on the wall.
  • This force acting per unit area give rise to pressure exerted by the gas molecules on the walls of the container.

 3.

*Using the kinetic model of gases, explain why the pressure exerted by a fixed mass of gas increases when its volume is reduced at constant temperature.

  • When the volume is reduced, the number of particles per unit volume increases.
  • Therefore, the gas particles collides more frequency with the walls, resulting in greater force exerted on the container wall.
  • Since pressure P = F/A, a greater force F results in greater pressure.

4.

*Using the kinetic model of gases, explain why the pressure exerted by a fixed mass of gas increases when its temperature is raised. Assume that the volume and mass of the gas remains constant.

  • When the temperature of the gas is raised, the particles have higher KE and moves faster.
  • They collide with the walls of the container more vigorously and at higher frequency, resulting in greater force exerted on the container wall.
  • Since pressure P = F/A, a greater force F results in greater pressure.

5.

*A gas syringe is being heated and the piston begins to be move outwards and eventually stops. Explain.

  • Upon heating, the gas pressure increases to more than that of the atmospheric pressure.
  • As a result, a resultant force acts outwards which pushes the piston outwards.
  • As the piston moves outwards, the volume of the cylinder increases, causing the gas pressure to decrease.
  • When the gas pressure drops back to a value equal to that of atmospheric pressure, there will be no more resultant outward force and the piston will stop being pushed out.

Chapter 10 - Transfer of Thermal Energy

1.

Describe, in molecular terms, how energy transfer occurs in solids.

  • When a substance (object) is heated at one end, the particles at the region gains thermal energy and vibrate faster.
  • These molecules collide with the less energetic neighbouring particles and transfer KE to them.
  • In this way, thermal energy is transferred along the entire substance (object) by molecular collisions from the hot end of the substance (object) to the cooler end.

2.

Why is metal a better conductor of heat than non-metals?

  • In metals, apart from transferring heat energy by the relatively slow process of molecular collision, another faster process of free electron diffusion takes place.
  • Metals contain free electrons that are mobile.
  • When heated, these free electrons gains KE and will diffuse to the cooler end at high speeds.
  • In the process, they will collide with atoms at the cooler end and transfer kinetic energy to them.

3. 

Explain why materials such as fiberglass, wool, polystyrene, fur are commonly used as heat insulators.

  • Fibreglass, wool, polystyrene and furs are poor conductors of heat because they are non-metals.
  • In addition, they contain trapped, still air, which is a poor thermal conductor as well.

4.

Describe, in terms of density changes, heat convection in fluids.

  • When a fluid is heated at the base, it will expand, become less dense and rise.
  • The cooler fluid at the top being denser, will sink.
  • This continuous, cyclic movement of the fluid known as convection current will eventually heat up the entire whole fluid evenly.

5. 

Explain why the heating element of a kettle should be positioned at the base.

  • Heated liquid rises and cooler liquid will sink.
  • Placing the heating element at the base allows convection current to set up, eventually heating up the entire liquid.

6.

In some water heating system, the heating element is placed in the middle instead of the base. State one advantage of this arrangement.

  • Less energy will be used as only the upper half of the water in the container will be heated.

7. 

Two cars, one white, one black are parked in the sun. After some time, one is much hotter to touch than the other. Which car is it? Explain your answer.

  • The black car feels hotter.
  • This is because black surfaces are better absorbers of infrared radiation than white surfaces.

8.

Two kettle, one silver, one white, contains water at 90 oC. After some time, one is cooler than the other. Which kettle is it? Explain your answer.

  • The black kettle feels cooler.
  • This is because black surfaces are better emitters of infrared radiation than silvered surfaces.

9.

Explain why heat loss from a cooking pan is reduced by fitting it with a lid.

  • The lid prevents hot air from rising, hence reducing heat loss due to convection.
  • It also reduces heat lost by evaporation (if a liquid is present in the pot).

10. 

*Explain why the filament reaches a constant temperature, even though heat is produced continually as current flows through the bulb.

  • As electricity passes through the filament, thermal energy is produced and the temperature of the filament increases.
  • At the same time, thermal energy is lost to the surrounding air by radiation as the temperature of the filament is higher than the surrounding.
  • The filament reaches a final constant temperature when the rate of increase of thermal energy by the filament is equal to the rate of thermal energy loss by the filament to the surrounding.
  • It is said that the filament have reached thermal equilibrium.

Chapter 11 - Thermal Properties of Matter

1.

When thermal energy is supplied to melt a solid, the temperature remained constant during the melting point. Explain what happened to the heat energy supplied.

  • During melting, thermal energy absorbed is used to break the intermolecular bonds between the particles, resulting in increase in potential energy of the substance.

  • Thermal energy is not used to cause an increase in kinetic energy of the particles, hence temperature remains constant.

2.

*Using the kinetic model of gases, explain why evaporation of a liquid causes a fall in temperature.

  • In a sample of water, the molecules move at different speeds, and therefore different kinetic energies.

  • At the water surface, molecules with sufficient energy to overcome the attractive forces of other molecules escape into the atmosphere.

  • When energetic molecules escapes from the surface of a liquid, its leaves behind the less energetic molecules.

  • As a result, the average kinetic energy of the remaining molecules decreases and therefore the temperature of liquid falls.

3.

Which will cause a more severe injury? Scalding by boiling water at 100 degree Celsius or steam at 100 degree Celsius?

  • Scalding by steam at 100 degree Celsius.

  • Although both are of the same temperature, the latent heat of vaporization of steam is greater than specific heat capacity of water.

  • A larger amount of latent heat of vaporization is released as steam condenses on the skin, as compare to thermal energy released as water contacts the skin.